// Copyright (C) 2016 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only
/*!
    \page style-reference.html
    \title Styles and Style Aware Widgets
    \ingroup qt-gui-concepts
    \brief Styles and the styling of widgets.

    Styles (classes that inherit QStyle) draw on behalf of widgets
    and encapsulate the look and feel of a GUI. The QStyle class is
    an abstract base class that encapsulates the look and feel of a
    GUI. Qt's built-in widgets use it to perform nearly all of their
    drawing, ensuring that they look exactly like the equivalent
    native widgets.

    Qt comes with a selection of built-in styles. Certain styles are only
    available on specific platforms. Custom styles are made available as
    plugins or by creating an instance of a specific style class with
    QStyleFactory::create() and setting it with QApplication::setStyle().

    \section1 Customizing a Style

    In order to customize an existing style, inherit QProxyStyle and
    reimplement the desired virtual methods. QProxyStyle allows one
    to specify a certain base style, or it will automatically use the
    application style when the base style is left unspecified. The
    former gives a full control on the base style and works best if
    the customization expects a certain style behavior, whereas the
    latter provides a platform agnostic way to customize the application
    style that defaults to the native platform style.

    \section1 Implementing a Custom Style

    QCommonStyle provides a convenient base for full custom style
    implementations. The approach is same than with QProxyStyle, but
    inherit QCommonStyle instead and reimplement the appropriate virtual
    methods. Implementing a full custom style is somewhat involved, and
    we therefore provide this overview. We give a step-by-step walkthrough
    of how to style individual Qt widgets. We will examine the QStyle
    virtual functions, member variables, and enumerations.

    The part of this document that does not concern the styling of
    individual widgets is meant to be read sequentially because later
    sections tend to depend on earlier ones. The description of the
    widgets can be used for reference while implementing a style.
    However, you may need to consult the Qt source code in some cases.
    The sequence in the styling process should become clear after
    reading this document, which will aid you in locating relevant code.

    To develop style aware widgets (i.e., widgets that conform to
    the style in which they are drawn), you need to draw them using the
    current style. This document shows how widgets draw themselves
    and which possibilities the style gives them.

    \section1 Classes for Widget Styling

    These classes are used to customize an application's appearance and
    style.

    \annotatedlist appearance

    \section1 The QStyle Implementation

    The API of QStyle contains functions that draw the widgets, static
    helper functions to do common and difficult tasks (e.g.,
    calculating the position of slider handles) and functions to do
    the various calculations necessary while drawing (e.g., for the
    widgets to calculate their size hints). The style also helps some
    widgets with the layout of their contents. In addition, it creates
    a QPalette that contains \l{QBrush}es to draw with.

    QStyle draws graphical elements; an element is a widget or a
    widget part like a push button bevel, a window frame, or a scroll
    bar. Most draw functions now take four arguments:

    \list
    \li an enum value specifying which graphical element to draw
    \li a QStyleOption specifying how and where to render that element
    \li a QPainter that should be used to draw the element
    \li a QWidget on which the drawing is performed (optional)
    \endlist

    When a widget asks a style to draw an element, it provides the style
    with a QStyleOption, which is a class that contains the information
    necessary for drawing. Thanks to QStyleOption, it is possible to make
    QStyle draw widgets without linking in any code for the widget. This
    makes it possible to use \l{QStyle}'s draw functions on any paint
    device, i.e., you can draw a combobox on any widget, not just on a
    QComboBox.

    The widget is passed as the last argument in case the style needs
    it to perform special effects (such as animated default buttons on
    \macos), but it isn't mandatory.

    In the course of this section, we will look at the style elements,
    the style options, and the functions of QStyle. Finally, we describe
    how the palette is used.

    Items in item views are drawn by \l{Delegate Classes}{delegates} in
    Qt. The item view headers are still drawn by the style. Qt's
    default delegate, QStyledItemDelegate, draws its items partially
    through the current style; it draws the check box indicators and
    calculates bounding rectangles for the elements of which the item
    consists. In this document, we only describe how to implement a
    QStyle subclass. If you wish to add support for other datatypes
    than those supported by the QStyledItemDelegate, you need to
    implement a custom delegate. Note that delegates must be set
    programmatically for each individual widget (i.e., default
    delegates cannot be provided as plugins).

    \section2 The Style Elements

    A style element is a graphical part of a GUI. A widget consists
    of a hierarchy (or tree) of style elements. For instance, when a
    style receives a request to draw a push button (from QPushButton,
    for example), it draws a label (text and icon), a button bevel,
    and a focus frame. The button bevel, in turn, consists of a frame
    around the bevel and two other elements, which we will look at
    later. Below is a conceptual illustration of the push button
    element tree. We will see the actual tree for QPushButton when we
    go through the individual widgets.

    \image javastyle/conceptualpushbuttontree.png

    Widgets are not necessarily drawn by asking the style to draw
    only one element. Widgets can make several calls to the style to
    draw different elements. An example is QTabWidget, which draws its
    tabs and frame individually.

    There are three element types: primitive elements, control
    elements, and complex control elements. The elements are defined
    by the \l{QStyle::}{ComplexControl}, \l{QStyle::}{ControlElement},
    and \l{QStyle::}{PrimitiveElement} enums. The values of
    each element enum has a prefix to identify their type: \c{CC_} for
    complex elements, \c{CE_} for control elements, and \c{PE_} for
    primitive elements. We will in the following three sections see what
    defines the different elements and see examples of widgets that use
    them.

    The QStyle class description contains a list of these elements and
    their roles in styling widgets. We will see how they are used when
    we style individual widgets.

    \section3 Primitive Elements

    Primitive elements are GUI elements that are common and often used
    by several widgets. Examples of these are frames, button bevels,
    and arrows for spin boxes, scroll bars, and combo boxes.
    Primitive elements cannot exist on their own: they are always part
    of a larger construct. They take no part in the interaction with
    the user, but are passive decorations in the GUI.

    \section3 Control Elements

    A control element performs an action or displays information
    to the user. Examples of control elements are push buttons, check
    boxes, and header sections in tables and tree views. Control
    elements are not necessarily complete widgets such as push
    buttons, but can also be widget parts such as tab bar tabs and
    scroll bar sliders. They differ from primitive elements in that
    they are not passive, but fill a function in the interaction with
    the user. Controls that consist of several elements often use the
    style to calculate the bounding rectangles of the elements. The
    available sub elements are defined by the \l{QStyle::}{SubElement}
    enum. This enum is only used for calculating bounding rectangles;
    sub elements are not graphical elements to be drawn like primitive,
    control, and complex elements.

    \section3 Complex Control Elements

    Complex control elements contain sub controls. Complex controls
    behave differently depending on where the user handles them with
    the mouse and which keyboard keys are pressed. This is dependent
    on which sub control (if any) the mouse is over or pressed on.
    Examples of complex controls are scroll bars and
    combo boxes. With a scroll bar, you can use the mouse to move the
    slider and press the line up and line down buttons. The available
    sub controls are defined by the \l{QStyle}{SubControl} enum.

    In addition to drawing, the style needs to provide the widgets
    with information on which sub control (if any) a mouse press was
    made on. For instance, a QScrollBar needs to know if the user
    pressed the slider, the slider groove, or one of the buttons.

    Note that sub controls are not the same as the control elements
    described in the previous section. You cannot use the style to
    draw a sub control; the style will only calculate the bounding
    rectangle in which the sub control should be drawn. It is common,
    though, that complex elements use control and primitive elements
    to draw their sub controls, which is an approach that is
    frequently used by the built-in styles in Qt and also the Java
    style. For instance, the Java style uses PE_IndicatorCheckBox to
    draw the check box in group boxes (which is a sub control of
    \c CC_GroupBox). Some sub controls have an equivalent control element,
    e.g., the scroll bar slider (\c SC_SCrollBarSlider and
    \c CE_ScrollBarSlider).

    \section3 Other QStyle Tasks

    The style elements and widgets, as mentioned, use the style to
    calculate bounding rectangles of sub elements and sub controls.
    Pixel metrics, which are style-dependent sizes in screen
    pixels, are also used for measurements when drawing. The available
    rectangles and pixel metrics are represented by three enums in
    QStyle: \l{QStyle::}{SubElement}, \l{QStyle::}{SubControl}, and
    \l{QStyle::}{PixelMetric}. Values of the enums can easily by
    identified as they start with SE_, SC_ and PM_.

    The style also contains a set of style hints, which is
    represented as values in the \l{QStyle::}{StyleHint} enum. All
    widgets do not have the same functionality and look in the
    different styles. For instance, when the menu items in a menu do not
    fit in a single column on the screen, some styles support
    scrolling while others draw more than one column to fit all items.

    A style usually has a set of standard images (such as a warning, a
    question, and an error image) for message boxes, file dialogs,
    etc. QStyle provides the \l{QStyle::}{StandardPixmap} enum. Its
    values represent the standard images. Qt's widgets use these, so
    when you implement a custom style you should supply the images
    used by the style that is being implemented.

    The style calculates the spacing between widgets in layouts. There
    are two ways the style can handle these calculations. You can set
    the \c PM_LayoutHorizontalSpacing and \c PM_LayoutVerticalSpacing, which
    is the way the Java style does it (through QCommonStyle).
    Alternatively, you can implement QStyle::layoutSpacing() and
    QStyle::layoutSpacingImplementation() if you need more control over
    this part of the layout. In these functions you can calculate the
    spacing based on control types (QSizePolicy::ControlType) for
    different size policies (QSizePolicy::Policy) and also the style
    option for the widget in question.

    \section2 Style Options

    The sub-classes of QStyleOption contain all information necessary
    to style the individual elements. Style options are instantiated -
    usually on the stack - and filled out by the caller of the QStyle
    function. Depending on what is drawn the style will expect
    different a different style option class. For example, the
    \c QStyle::PE_FrameFocusRect element expects a QStyleOptionFocusRect
    argument, and it's possible to create custom subclasses that a
    custom style can use. The style options keep public variables
    for performance reasons.

    The widgets can be in a number of different states, which are
    defined by the \l{QStyle::}{State} enum. Some of the state flags have
    different meanings depending on the widget, but others are common
    for all widgets like \c State_Disabled. It is QStyleOption that sets
    the common states with QStyleOption::initFrom(); the rest of the
    states are set by the individual widgets.

    Most notably, the style options contain the palette and bounding
    rectangles of the widgets to be drawn. Most widgets have
    specialized style options. QPushButton and QCheckBox, for
    instance, use QStyleOptionButton as their style option, which contains
    the text, icon, and the size of their icon. The exact contents of
    all options are described when we go through the individual widgets.

    When reimplementing QStyle functions that take a
    QStyleOption parameter, you often need to cast the
    QStyleOption to a subclass (e.g., QStyleOptionFocusRect). For
    safety, you can use qstyleoption_cast() to ensure that the
    pointer type is correct. If the object isn't of the right type,
    qstyleoption_cast() returns \nullptr. For example:

    \snippet code/doc_src_qt4-styles.cpp 0

    The following code snippet illustrates how to use QStyle to
    draw the focus rectangle from a custom widget's paintEvent():

    \snippet code/doc_src_qt4-styles.cpp 1

    The next example shows how to derive from an existing style to
    customize the look of a graphical element:

    \snippet customstyle/customstyle.h 0
    \codeline
    \snippet customstyle/customstyle.cpp 2

    \section2 QStyle Functions

    The QStyle class defines three functions for drawing the primitive,
    control, and complex elements:
    \l{QStyle::}{drawPrimitive()},
    \l{QStyle::}{drawControl()}, and
    \l{QStyle::}{drawComplexControl()}. The functions takes the
    following parameters:

    \list
        \li the enum value of the element to draw.
        \li a QStyleOption which contains the information needed to
           draw the element.
        \li a QPainter with which to draw the element.
        \li a pointer to a QWidget, typically the widget
           that the element is painted on.
    \endlist

    Not all widgets send a pointer to themselves. If the style
    option sent to the function does not contain the information you
    need, you should check the widget implementation to see if it
    sends a pointer to itself.

    The QStyle class also provides helper functions that are used
    when drawing the elements. The \l{QStyle::}{drawItemText()}
    function draws text within a specified rectangle, taking a
    QPalette as a parameter. The \l{QStyle::}{drawItemPixmap()}
    function helps to align a pixmap within a specified bounding
    rectangle.

    Other QStyle functions do various calculations for the
    functions that do drawing. The widgets also use these functions for
    calculating size hints and bounding rectangles if they draw several
    style elements themselves. As with the functions that draw elements,
    the helper functions typically take the same arguments.

    \list
        \li The \l{QStyle::}{subElementRect()} function takes a
        \l{QStyle::}{SubElement} enum value and calculates a bounding
        rectangle for a sub element. The style uses this function to
        know where to draw the different parts of an element. This is
        mainly done for reuse; if you create a new style, you can use
        the same location of sub elements as the super class.

        \li The \l{QStyle::}{subControlRect()} function is used to
        calculate bounding rectangles for sub controls in complex
        controls. When you implement a new style, you reimplement \c
        subControlRect() and calculate the rectangles that are different
        from the super class.

        \li The \l{QStyle::}{pixelMetric()} function returns a pixel
        metric, which is a style-dependent size given in screen
        pixels. It takes a value of the \l{QStyle::}{PixelMetric} enum
        and returns the correct measurement. Note that pixel metrics do
        not necessarily have to be static measurements, but can be
        calculated with, for example, the style option.

        \li The \l{QStyle::}{hitTestComplexControl()} function returns the
        sub control that the mouse pointer is over in a complex control.
        Usually, this is simply a matter of using
        \l{QStyle::}{subControlRect()} to get the bounding rectangles of
        the sub controls, and then seeing which rectangle contains the
        position of the cursor.
    \endlist

    QStyle also has the functions \l{QStyle::}{polish()} and
    \l{QStyle::}{unpolish()}. All widgets are sent to the \c polish()
    function before being shown and to \c unpolish() when they
    are hidden. You can use these functions to set attributes on the
    widgets or do other work that is required by your style. For
    instance, if you need to know when the mouse is hovering over the
    widget, you need to set the \l{Qt::}{WA_Hover} widget attribute.
    The \c State_MouseOver state flag will then be set in the widget's
    style options.

    QStyle has a few static helper functions that do some common and
    difficult tasks. They can calculate the position of a slider
    handle from the value of the slider and transform rectangles
    and draw text considering reverse layouts; see the QStyle
    class documentation for more details.

    The usual approach when one reimplements QStyle virtual
    functions is to do work on elements that are different from the
    super class; for all other elements, you can simply use the super
    class implementation.

    \section2 The Palette

    Each style provides a color - that is, QBrush - palette that
    should be used for drawing the widgets. There is one set of colors
    for the different widget states (QPalette::ColorGroup): active
    (widgets in the window that have keyboard focus), inactive (widgets
    used for other windows), and disabled (widgets that are disabled).
    The states can be found by querying the \c State_Active
    and \c State_Enabled state flags. Each set contains certain color
    roles given by the QPalette::ColorRole enum. The roles describe in
    which situations the colors should be used (e.g., for painting
    widget backgrounds, text, or buttons).

    How the color roles are used is up to the style. For instance, if
    the style uses gradients, one can use a palette color and make it
    darker or lighter with QColor::darker() and QColor::lighter() to
    create the gradient. In general, if you need a brush that is not
    provided by the palette, you should try to derive it from one.

    QPalette, which provides the palette, stores colors for
    different widget states and color roles. The palette for a style
    is returned by \l{QStyle::}{standardPalette()}. The standard
    palette is not installed automatically when a new style is set
    on the application (QApplication::setStyle()) or widget
    (QWidget::setStyle()), so you must set the palette yourself
    with (QApplication::setPalette()) or (QWidget::setPalette()).

    It is not recommended to hard-code colors, as applications and
    individual widgets can set their own palette and also use their
    style's palette for drawing. Note that none of Qt's widgets set
    their own palette. The Java style does hard-code some colors, but
    only as a decision of the author; it is not advised. Of course,
    it is not intended that the style should look good with any palette.

    \section2 Implementation Issues

    When you implement styles, there are several issues to
    consider. We will give some hints and advice on implementation
    here.

    When implementing styles, it is necessary to look through the
    code of the widgets and code of the base class and its ancestors.
    This is because the widgets use the style differently, because the
    implementation in the different styles' virtual functions can
    affect the state of the drawing (e.g., by altering the QPainter
    state without restoring it and drawing some elements without using
    the appropriate pixel metrics and sub elements).

    It is recommended that the styles do not alter the proposed size
    of widgets with the QStyle::sizeFromContents() function, but let
    the QCommonStyle implementation handle it instead. If changes need
    to be made, you should try to keep them small; application development
    may be difficult if the layout of widgets looks considerably
    different in the various styles.

    \section1 Java Style

    We have implemented a style that resembles the Java default look
    and feel (previously known as Metal). We have done this as it is
    relatively simple to implement and we wanted to build a style for
    this overview document. To keep it simple and not too extensive, we
    have simplified the style somewhat, but Qt is perfectly able to
    make an exact copy of the style. However, there are no concrete
    plans to implement the style as a part of Qt.

    In this section we will have a look at some implementation
    issues. Finally, we will see a complete example on the styling of
    a Java widget. We will continue to use the Java style
    throughout the document for examples and widget images. The
    implementation itself is somewhat involved, and it is not
    intended that you should read through it.

    \section2 Design and Implementation

    The first step in designing the style was to select the base
    class. We chose to subclass QCommonStyle. This class implements
    most of the functionality we need, other than performing the actual
    drawing.

    The style is implemented in one class. We have done this
    because we find it convenient to keep all code in one file. Also,
    it is an advantage with regards to optimization as we instantiate
    less objects. We also keep the number of functions at a minimum by
    using switches to identify which element to draw in the functions.
    This results in large functions, but since we divide the code for
    each element in the switches, the code should still be easy to
    read.

    \section2 Limitations and Differences from Java

    We have not fully implemented every element in the Java style.
    This way, we have reduced the amount and complexity of the code.
    In general, the style was intended as a practical example for
    this style overview document, and not to be a part of Qt
    itself.

    Not all widgets have every state implemented. This goes for
    states that are common, e.g., \c State_Disabled. Each state is,
    however, implemented for at least one widget.

    We have only implemented ticks below the slider. Flat push
    buttons are also left out. We do not handle the case where the
    title bars and dock window titles grow too small for their
    contents, but simply draw sub controls over each other.

    We have not tried to emulate the Java fonts. Java and Qt use very
    different font engines, so we don't consider it worth the effort
    as we only use the style as an example for this overview.

    We have hard-coded the colors (we don't use the QPalette) for
    the linear gradients, which are used, for example, for button
    bevels, tool bars, and check boxes. This is because the Java
    palette cannot produce these colors. Java does not change these
    colors based on widget color group or role anyway (they are not
    dependent on the palette), so it does not present a problem in any
    case.

    It is Qt's widgets that are styled. Some widgets do not exist
    at all in Java, e.g., QToolBox. Others contain elements that the
    Java widgets don't. The tree widget is an example of the latter in
    which Java's JTree does not have a header.

    The style does not handle reverse layouts. We assume that the
    layout direction is left to right. QCommonStyle handles reverse
    widgets; if we implemented reverse layouts, widgets that we change
    the position of sub elements, or handle text alignment in labels
    ourselves would need to be updated.

    \section2 Styling Java Check Boxes

    As an example, we will examine the styling of check boxes in the
    Java style. We describe the complete process and print all code in
    both the Java style and Qt classes involved. In the rest of this
    document, we will not examine the source code of the individual
    widgets. Hopefully, this will give you an idea on how to search
    through the code if you need to check specific implementation
    details; most widgets follow the same structure as the check
    boxes. We have edited the QCommonStyle code somewhat to remove
    code that is not directly relevant for check box styling.

    We start with a look at how QCheckBox builds its style option,
    which is QStyleOptionButton for checkboxes:

    \snippet code/doc_src_styles.cpp 0

    First we let QStyleOption set up the option with the information
    that is common for all widgets with \c initFrom().  We will look at
    this shortly.

    QStyleOption's \c down variable is \c true when the user presses the box down;
    this is true of the checkbox whether the box is checked or not. The
    \c State_NoChange state is set when we have a tri-state checkbox and
    it is partially checked. It has \c State_On if the box is checked and
    \c State_Off if it is unchecked. \c State_MouseOver is set if the mouse
    hovers over the checkbox and the widget has attribute \c Qt::WA_Hover
    set - you set this in QStyle::polish(). In addition, the style
    option also contains the text, icon, and icon size of the button.

    \l{QStyleOption::}{initFrom()} sets up the style option with the
    attributes that are common for all widgets. We print its
    implementation here:

    \snippet code/doc_src_styles.cpp 1

    The \c State_Enabled is set when the widget is enabled. When the
    widget has focus the \c State_HasFocus flag is set. Equally, the
    \c State_Active flag is set when the widget is a child of the active
    window. The \c State_MouseOver will only be set if the widget has
    the \c WA_HoverEnabled windows flag set. Notice that keypad
    navigation must be enabled in Qt for the \c State_HasEditFocus to
    be included; it is not included by default.

    In addition to setting state flags the QStyleOption contains
    other information about the widget: \c direction is the layout
    direction of the layout, \c rect is the bounding rectangle of the
    widget (the area in which to draw), \c palette is the QPalette
    that should be used for drawing the widget, and \c fontMetrics is
    the metrics of the font that is used by the widget.

    We give an image of a checkbox and the style option to match
    it.

    \image javastyle/checkboxexample.png A Java style checkbox

    The above checkbox will have the following state flags in its
    style option:

    \table 90%
    \header
        \li State flag
        \li Set
    \row
        \li \c State_Sunken
        \li Yes
    \row
        \li \c State_NoChange
        \li No
    \row
        \li \c State_On
        \li Yes
    \row
        \li \c State_Off
        \li No
    \row
        \li \c State_MouseOver
        \li Yes
    \row
        \li \c State_Enabled
        \li Yes
    \row
        \li \c State_HasFocus
        \li Yes
    \row
        \li \c State_KeyboardFocusChange
        \li No
    \row
        \li \c State_Active
        \li Yes
    \endtable

    The QCheckBox paints itself in QWidget::paintEvent() with
    style option \c opt and QStylePainter \c p. The QStylePainter
    class is a convenience class to draw style elements. Most
    notably, it wraps the methods in QStyle used for painting. The
    QCheckBox draws itself as follows:

    \snippet code/doc_src_styles.cpp 2

    QCommonStyle handles the CE_CheckBox element. The QCheckBox
    has two sub elements: SE_CheckBoxIndicator (the checked indicator)
    and SE_CheckBoxContents (the contents, which are used for the
    checkbox label). QCommonStyle also implements these sub element
    bounding rectangles. Next, we'll have a look at the QCommonStyle code:

    \snippet code/doc_src_styles.cpp 3

    As can be seen from the code extract, the common style gets
    the bounding rectangles of the two sub elements of
    CE_CheckBox, and then draws them. If the checkbox has focus,
    the focus frame is also drawn.

    The Java style draws CE_CheckBoxIndicator, while QCommonStyle
    handles CE_CheckboxLabel. We will examine each implementation and
    start with CE_CheckBoxLabel:

    \snippet code/doc_src_styles.cpp 4

    \l{QStyle::}{visualAlignment()} adjusts the alignment of text
    according to the layout direction. We then draw an icon if it
    exists, and adjust the space left for the text.
    \l{QStyle::}{drawItemText()} draws the text, taking alignment,
    layout direction, and the mnemonic into account. It also uses the
    palette to draw the text in the right color.

    The drawing of labels often gets somewhat involved. Luckily, it
    can usually be handled by the base class. The Java style
    implements its own push button label, since Java centers button
    contents also when the button has an icon. You can examine that
    implementation if you need an example of reimplementing label
    drawing.

    We'll now take a look at the Java implementation
    of CE_CheckBoxIndicator in \c drawControl():

    \snippet javastyle.cpp 0

    We first save the state of the painter. This is not always
    necessary, but in this case the QCommonStyle needs the painter in
    the same state as it was when PE_IndicatorCheckBox was called (We
    could also set the state with function calls, of course). We then
    use \c drawButtonBackground() to draw the background of the check
    box indicator. This is a helper function that draws the background
    and also the frame of push buttons and check boxes. We take a look
    at that function below. We then check if the mouse is hovering
    over the checkbox. If it is, we draw the frame that Java checkboxes
    have when the box is not pressed down and the mouse is over it.
    You may note that Java does not handle tri-state boxes, so we have
    not implemented it.

    Here we use a PNG image for our indicator. We could also check
    if the widget is disabled. We would then have to use
    another image with the indicator in the disabled color.

    \snippet javastyle.cpp 1

    We have seen how check boxes are styled in the Java style from the
    time the widget gets a paint request to the time the style is
    finished painting. To learn in detail how each widget is painted,
    you need to go through the code step-by-step as we have done here.
    However, it is usually enough to know which style elements the
    widgets draw. The widget builds a style option and calls on the
    style one or more times to draw the style elements of which it
    consists. Usually, it is also sufficient to know the states a widget
    can be in and the other contents of the style option, i.e., what we
    list in the next section.

    \section1 Widget Walkthrough

    In this section, we will examine how most of Qt's widgets are
    styled. Hopefully, this will save you some time and effort while
    developing your own styles and widgets. You will not find
    information here that is not attainable elsewhere (i.e., by
    examining the source code or the class descriptions for the style
    related classes).

    We mostly use Java style widgets as examples. The Java style does not
    draw every element in the element trees. This is because they are
    not visible for that widget in the Java style. We still make sure
    that all elements are implemented in a way that conforms with the
    Java style, as custom widgets might need them (this does not
    exclude leaving implementations to QCommonStyle though).

    The following is given for each widget:

    \list
        \li A table with the members (variables, etc.) of its style option.
        \li A table of the state flags (QStyle::StateFlag) that
           can be set on the widget and when the states are set.
        \li Its element tree (see section \l{The Style Elements}).
        \li An image of the widget in which the elements are outlined.
        \omit This is not written yet - probably never will be
              either
        \li A list of style hints that should be checked for the
           widget.
        \li A list of standard pixmaps that could be used by the
           elements.
        \endomit
    \endlist

    The element tree contains the primitive, control, and complex
    style elements. By doing a top-down traversal of the element tree,
    you get the sequence in which the elements should be drawn. In the
    nodes, we have written the sub element rectangles, sub control
    elements, and pixel metrics that should be considered when drawing
    the element of the node.

    Our approach on styling centers on the drawing of the widgets. The
    calculations of sub elements rectangles, sub controls, and pixel
    metrics used \b during drawing is only listed as contents in
    the element trees. Note that there are rectangles and pixel
    metrics that are only used by widgets. This leaves these
    calculations untreated in the walkthrough. For instance, the
    \l{QStyle::}{subControlRect()} and
    \l{QStyle::}{sizeFromContents()} functions often call
    \l{QStyle::}{subElementRect()} to calculate their bounding
    rectangles. We could draw trees for this as well. However, how
    these calculations are done is completely up to the individual
    styles, and they do not have to follow a specific structure (Qt
    does not impose a specific structure). You should still make sure
    that you use the appropriate pixel metrics, though. To limit the
    size of the document, we have therefore chosen not to include
    trees or describe the calculations made by the Java (or any other)
    style.

    You may be confused about how the different pixel metrics, sub
    element rectangles, and sub control rectangles should be used when
    examining the trees. If you are in doubt after reading the QStyle
    enum descriptions, we suggest that you examine the QCommonStyle
    implementation.

    Some of the bounding rectangles that we outline in the widget
    images are equal. Reasons for this are that some elements draw
    backgrounds while others draw frames and labels. If in doubt,
    check the description of each element in QStyle. Also, some
    elements are there to layout, i.e., decide where to draw, other
    elements.

    \section2 Common Widget Properties

    Some states and variables are common for all widgets. These are
    set with QStyleOption::initFrom(). Not all elements use this function;
    it is the widgets that create the style options, and for some
    elements the information from \l{QStyleOption::}{initFrom()} is not
    necessary.

    A table with the common states follows:

    \table 90%
        \header
            \li State
            \li State Set When
        \row
            \li \c State_Enabled
            \li Set if the widget is not disabled (see
               QWidget::setEnabled())
        \row
            \li \c State_Focus
            \li Set if the widget has focus (see
               QWidget::hasFocus())
        \row
            \li \c State_KeyboardFocusChange
            \li Set when the user changes focus with the keyboard
               (see Qt::WA_KeyboardFocusChange)
        \row
            \li \c State_MouseOver
            \li Set if the mouse cursor is over the widget
        \row
            \li \c State_Active
            \li Set if the widget is a child of the active window.
        \row
            \li \c State_HasEditFocus
            \li Set if the widget has the edit focus
    \endtable

    The other common members for widgets are:

    \table 90%
        \header
            \li Member
            \li Content
        \row
            \li rect
            \li The bounding rectangle of the element to draw. This
               is set to the widget bounding rectangle
               (QWidget::rect()).
        \row
            \li direction
            \li The layout direction; a value of the
               Qt::LayoutDirection enum.
        \row
            \li palette
            \li The QPalette to use when drawing the element. This
               is set to the widgets palette (QWidget::palette()).
        \row
            \li fontMetrics
            \li The QFontMetrics to use when drawing text on the
               widget.
    \endtable

    The complex style options (classes that inherit
    QStyleOptionComplex) used for complex style elements share two
    variables: \l{QStyleOptionComplex::}{subControls} and
    \l{QStyleOptionComplex::}{activeSubControls}. Both variables are
    an OR'ed combination of QStyle::SubControl enum values. They
    indicate which sub controls the complex control consists of and
    which of these controls are currently active.

    As mentioned, the style calculates the size of the widget's
    contents, which the widgets calculate their size hints from. In
    addition, complex controls also use the style to test which
    sub-controls the mouse is over.

    \section2 Widget Reference

    Without further delay, we present the widget walkthrough; each
    widget has its own sub-section.

    \section3 Push Buttons

    The style structure for push buttons is shown below. By doing a
    top-down traversal of the tree, you get the sequence in which the
    elements should be drawn.

    \image javastyle/pushbutton.png The style structure for push buttons

    The layout of the buttons, with regard to element bounds, varies from
    style to style. This makes it difficult to show conceptual images
    of this. Also, elements may - even be intended to - have the same
    bounds; the \c PE_PushButtonBevel, for instance, is used in
    QCommonStyle to draw the elements that it contains:
    \c PE_FrameDefaultButton, \c PE_FrameButtonBevel, and
    \c PE_PanelButtonCommand, all of which have the same bounds in common
    style. \c PE_PushButtonBevel is also responsible for drawing the menu
    indicator (QCommonStyle draws \c PE_IndicatorArrowDown).

    An image of a push button in the Java style that shows the bounding
    rectangles of the elements is given below. Colors are used to
    separate the bounding rectangles in the image; they do not fill
    any other purpose. This is also true for similar images for the
    other widgets.

    \image javastyle/button.png

    The Java style, as well as all other styles implemented in Qt,
    does not use \c PE_FrameButtonBevel. It is usual that a button
    with a \c PE_DefaultFrame adjusts the \c PE_PanelButtonCommand's
    rectangle by \c PM_ButtonDefaultIndicator. The \c CE_PushButtonLabel
    is found by adjusting the rect by \c PM_DefaultFrameWidth.

    We will now examine the style option for push
    buttons - QStyleOptionButton. A table for the states that
    QPushButton can set on the style option follows:

    \table 90%
        \header
            \li State
            \li State Set When
        \row
            \li \c State_Sunken
            \li Button is down or menu is pressed shown
        \row
            \li \c State_On
            \li Button is checked
        \row
            \li \c State_Raised
            \li Button is not flat and not pressed down
    \endtable

    Other members of QStyleOptionButton are:

    \table 90%
        \header
            \li Member
            \li Content
        \row
            \li features
            \li Flags of the QStyleOptionButton::ButtonFeatures enum,
               which describes various button properties (see enum)
        \row
            \li icon
            \li The buttons QIcon (if any)
        \row
            \li iconSize
            \li The QSize of the icon
        \row
            \li text
            \li a QString with the buttons text
    \endtable

    \section3 Check and Radio Buttons

    The structures for radio and check buttons are identical.
    We show the structure using QCheckBox element and pixel
    metric names:

    \image javastyle/checkbox.png

    QStyleOptionButton is used as the style option for both check
    and radio buttons. We first give a table of the states that
    can be set in the option:

    \table 90%
        \header
            \li State
            \li State Set When
        \row
            \li \c State_sunken
            \li The box is pressed down
        \row
            \li \c State_NoChange
            \li The box is partially checked (for tri-state
               checkboxes.)
        \row
            \li \c State_On
            \li The box is checked
        \row
            \li \c State_Off
            \li The box is unchecked
    \endtable

    See \l{Push Buttons} for a table over other members in the
    QStyleOptionButton class.

    \section3 Tabs

    In Qt, QTabBar uses the style to draw its tabs. Tabs exist either
    in a QTabWidget, which contains a QTabBar, or as a separate bar.
    If the bar is not part of a tab widget, it draws its own base.

    QTabBar lays out the tabs, so the style does not have control over
    tab placement. However, while laying out its tabs, the bar asks
    the style for \c PM_TabBarTabHSpace and \c PM_TabBarTabVSpace, which is
    extra width and height over the minimum size of the tab bar tab
    label (icon and text). The style can also further influence the
    tab size before it is laid out, as the tab bar asks for
    \c CT_TabBarTab. The bounding rectangle of the bar is decided by the
    tab widget when it is part of the widget (still considering
    \c CT_TabBarTab).

    The tab bar is responsible for drawing the buttons that appear on
    the tab bar when all tabs do not fit. Their placement is not
    controlled by the style, but the buttons are \l{QToolButton}s
    and are therefore drawn by the style.

    Here is the style structure for QTabWidget and QTabBar:

    \image javastyle/tab.png

    The dotted lines indicate that the QTabWidget contains a tab bar,
    but does not draw the tab bar itself. QTabBar only draws its base line
    when not part of a tab widget, and it keeps two tool
    buttons that scroll the bar when all tabs do not fit; see \l{Tool
    Buttons} for their element tree. Also note that since the buttons
    are children of the tab bar, they are drawn after the bar. The
    tabs' bounding rectangles overlap the base by \c PM_TabBarBaseOverlap.

    Here is a tab widget in the Java style:

    \image javastyle/tabwidget.png

    In the Java style, the tab bar shape and label have the same bounding
    rectangle as \c CE_TabBarTab. Notice that the tabs overlap with the
    tab widget frame. The base of the tab bar (if drawn) is the area where
    the tabs and frame overlap.

    The style option for tabs (QStyleOptionTab) contains the necessary
    information for drawing tabs. The option contains the position of
    the tab in the tab bar, the position of the selected tab, the
    shape of the tab, the text, the icon, and the icon's size.

    As the Java style tabs don't overlap, we also present an image of
    a tab widget in the common style. Note that if you want the tabs
    to overlap horizontally, you do that when drawing the tabs in
    \c CE_TabBarTabShape; the tabs bounding rectangles will not be
    altered by the tab bar. The tabs are drawn from left to right in a
    north tab bar shape, top to bottom in an east tab bar shape, etc.
    The selected tab is drawn last, so that it is easy to draw it over
    the other tabs (if it is to be bigger).

    \image javastyle/windowstabimage.png

    A table of the states a tab bar can set on its tabs follows:

    \table 90%
        \header
            \li State
            \li State Set When
        \row
            \li \c State_Sunken
            \li The tab is pressed on with the mouse.
        \row
            \li \c State_Selected
            \li If it is the current tab.
        \row
            \li \c State_HasFocus
            \li The tab bar has focus and the tab is selected.
    \endtable

    Note that individual tabs may be disabled even if the tab bar
    is not. The tab will be active if the tab bar is active.

    Here follows a table of QStyleOptionTab's members:

    \table 90%
        \header
            \li Member
            \li Content
        \row
            \li cornerWidgets
            \li Flags of the CornerWidget enum, which indicate
               if and which corner widgets the tab bar has.
        \row
            \li icon
            \li The QIcon of the tab.
        \row
            \li iconSize
            \li The QSize of the icon.
        \row
            \li position
            \li A TabPosition enum value that indicates the tab's
               position on the bar relative to the other tabs.
        \row
            \li row
            \li Holds which row the tab is in.
        \row
            \li selectedPosition
            \li A value of the SelectedPosition enum that indicates
               whether the selected tab is adjacent to or is the
               tab.
        \row
            \li shape
            \li A value of the QTabBar::Shape enum indicating
               whether the tab has rounded or triangular corners
               and the orientation of the tab.
        \row
            \li text
            \li The tab text.
    \endtable

    The frame for tab widgets use QStyleOptionTabWidgetFrame as
    style option. We list its members here. It does not have
    states set besides the common flags.

    \table 90%
        \header
            \li Member
            \li content
        \row
            \li leftCornerWidgetSize
            \li The QSize of the left corner widget (if any).
        \row
            \li rightCornerWidgetSize
            \li The QSize of the right corner widget (if any).
        \row
            \li lineWidth
            \li Holds the line with for drawing the panel.
        \row
            \li midLineWith
            \li This value is currently always 0.
        \row
            \li shape
            \li The shape of the tabs on the tab bar.
        \row
            \li tabBarSize
            \li The QSize of the tab bar.
    \endtable

    \section3 Scroll Bars

    Here is the style structure for scrollbars:

    \image javastyle/scrollbar.png

    QScrollBar simply creates its style option and then draws
    \c CC_ScrollBar. Some styles draw the background of add page and sub
    page with \c PE_PanelButtonBevel, and also use indicator arrows to
    draw the arrows in the next and previous line indicators; we have
    not included these in the tree as their use is up to the
    individual style. The style's \c PM_MaximumDragDistance is the
    maximum distance in pixels the mouse can move from the bounds
    of the scroll bar and still move the handle.

    Here is an image of a scrollbar in the Java style:

    \image javastyle/scrollbarimage.png

    You may notice that the scrollbar is slightly different from
    Java's, as it has two line up indicators. We have done this to show
    that you can have two separate bounding rectangles for a
    single sub control. The scroll bar is an example of a widget that
    is entirely implemented by the Java style - QCommonStyle is not
    involved in the drawing.

    We have a look at the different states a scroll bar can set on
    the style option:

    \table 90%
        \header
            \li State
            \li State Set When
        \row
            \li \c State_Horizontal
            \li The scroll bar is horizontal.
    \endtable

    The style option of QScrollBar is QStyleOptionSlider. Its
    members are listed in the following table. The option is used
    by all \l{QAbstractSlider}s; we only describe the members
    relevant for scroll bars here.

    \table 90%
        \header
            \li Member
            \li Content
        \row
            \li maximum
            \li The maximum value of the scroll bar.
        \row
            \li minimum
            \li The minimum value of the scroll bar.
        \row
            \li notchTarget
            \li The number of pixels between notches.
        \row
            \li orientation
            \li A value of the Qt::Orientation enum that specifies
               whether the scroll bar is vertical or horizontal.
        \row
            \li pageStep
            \li The number by which to increase or decrease the slider's
               value (relative to the size of the slider and its value
               range) on page steps.
        \row
            \li singleStep
            \li The number by which to increase or decrease the slider's
               value on single (or line) steps.
        \row
            \li sliderValue
            \li The value of the slider.
        \row
            \li sliderPosition
            \li The position of the slider handle. This is the same
               as \c sliderValue if the scroll bar is
               QAbstractSlider::tracking. If not, the scroll
               bar does not update its value before the mouse
               releases the handle.
        \row
            \li upsideDown
            \li Holds the direction in which the scroll bar
               increases its value. This is used instead of
               QStyleOption::direction for all abstract sliders.
    \endtable

    \section3 Sliders

    When calculating the slider's size hint, \c PM_SliderThickness and
    \c PM_SliderLength are queried from the style. As with scroll bars,
    the QSlider only lets the user move the handle if the mouse is
    within \c PM_MaximumDragDistance from the slider bounds. When it
    draws itself, it creates the style option and calls \c
    drawComplexControl() with \c CC_Slider:

    \image javastyle/slider.png

    We also show a picture of a slider in the Java style. We show
    the bounding rectangles of the sub elements, as all drawing is done
    in \c CC_Slider.

    \image javastyle/sliderimage.png

    QSlider uses QStyleOptionSlider as all \l{QAbstractSlider}s do. We
    present a table with the members that affect QSlider:

    \table 90%
        \header
            \li Member
            \li Content
        \row
            \li maximum
            \li The maximum value of the slider.
        \row
            \li minimum
            \li The minimum value of the slider.
        \row
            \li notchTarget
            \li This is the number of pixels between each notch.
        \row
            \li orientation
            \li A Qt::Orientation enum value that indicates whether the
               slider is vertical or horizontal.
        \row
            \li pageStep
            \li The number by which to increase or decrease the slider's
               value on page steps.
        \row
            \li singleStep
            \li The number by which to increase or decrease the slider's
               value on single (or line) steps.
        \row
            \li sliderValue
            \li The value of the slider.
        \row
            \li sliderPosition
            \li The position of the slider given as a slider value.
               This  will be equal to the \c sliderValue if the
               slider is \l{QAbstractSlider::}{tracking}; if
               not, the slider's value will not change until the handle is
               released with the mouse.
        \row
            \li upsideDown
            \li This member is used instead of QStyleOption::direction
                for all abstract sliders.
    \endtable

    You should note that the slider does not use direction for
    reverse layouts; it uses \c upsideDown.

    \section3 Spin Boxes

    When QSpinBox paints itself, it creates a QStyleOptionSpinBox and
    asks the style to draw \c CC_SpinBox. The edit field is a line
    edit that is a child of the spin box. The dimensions of the
    field are calculated by the style with \c SC_SpinBoxEditField.

    Here follows the style tree for spin boxes. It is not
    required that a style uses the button panel primitive to paint
    the indicator backgrounds. You can see an image below the tree
    showing the sub elements in QSpinBox in the Java style.

    \image javastyle/spinbox.png

    \image javastyle/spinboximage.png

    The QStyleOptionSpinBox, which is the style option for spin
    boxes. It can set the following states on the spin box:

    \table 90%
        \header
            \li State
            \li State Set When
        \row
            \li \c State_Sunken
            \li Is set if one of the sub controls \c CC_SpinUp or
               \c CC_SpinDown is pressed on with the mouse.
    \endtable

    The rest of the members in the spin box style options are:

    \table 90%
        \header
            \li Property
            \li Function
        \row
            \li frame
            \li Boolean that is \c true if the spin box is to draw a
               frame.
        \row
            \li buttonSymbols
            \li Value of the ButtonSymbols enum that decides the
               symbol on the up/down buttons.
        \row
            \li stepEnabled
            \li A value of the StepEnabled enum, indicating which of the
               spin box buttons are pressed down.
    \endtable

    \section3 Title Bar

    The title bar complex control, \c CC_TitleBar, is used to draw
    the title bars of internal windows in QMdiArea. It typically
    consists of a window title, and close, minimize, system menu, and
    maximize buttons. Some styles also provide buttons for shading
    the window, as well as a button for context sensitive help.

    The bar is drawn in \c CC_TitleBar without using any sub elements.
    How the individual styles draw their buttons is up to them, but
    there are standard pixmaps for the buttons that the style should
    provide.

    \image javastyle/titlebar.png

    In an image over a title bar in the Java style, we show the
    bounding rectangles of the sub elements supported by the Java style
    (all of which are drawn with standard pixmaps). It is usual to
    draw the button backgrounds using \c PE_PanelButtonTool, but it's not
    mandatory.

    \image javastyle/titlebarimage.png

    The style option for title bars is QStyleOptionTitleBar. Its
    members are:

    \table 90%
        \header
            \li Member
            \li Content
        \row
            \li icon
            \li The title bar's icon.
        \row
            \li text
            \li The text for the title bar's label.
        \row
            \li windowFlags
            \li Flags of the Qt::WindowFlag enum. The window flags
               used by QMdiArea for window management.
        \row
            \li titleBarState
            \li This is the QWidget::windowState() of the window
               that contains the title bar.
    \endtable

    \section3 Combo Box

    A QComboBox uses the style to draw the button and label of
    non-editable boxes with \c CC_ComboBox and \c CE_ComboBoxLabel.

    The list that pops up when the user clicks on the combo box is
    drawn by a \l{Delegate Classes}{delegate}, which we do not cover
    in this overview. You can, however, use the style to control the
    list's size and position with the sub element
    \c SC_ComboBoxListBoxPopup. The style also decides where the edit
    field for editable boxes should be with \c SC_ComboBoxEditField; the
    field itself is a QLineEdit that is a child of the combo box.

    \image javastyle/combobox.png

    We show an image over a Java style combo box in which we have
    outlined its sub elements and sub element rectangles:

    \image javastyle/comboboximage.png

    Java combo boxes do not use the focus rect; it changes its
    background color when it has focus. The \c SC_ComboBoxEdit field is
    used both by QComboBox to calculate the size of the edit field and
    the style for calculating the size of the combo box label.

    The style option for combo boxes is QStyleOptionComboBox. It
    can set the following states:

    \table 90%
        \header
            \li State
            \li Set When
        \row
            \li \c State_Selected
            \li The box is not editable and has focus.
        \row
            \li \c State_Sunken
            \li \c SC_ComboBoxArrow is active.
        \row
            \li \c State_on
            \li The container (list) of the box is visible.
    \endtable

    The style options other members are:

    \table
        \header
            \li Member
            \li Content
        \row
            \li currentIcon
            \li The icon of the current (selected) item of the
               combo box.
        \row
            \li currentText
            \li The text of the current item in the box.
        \row
            \li editable
            \li Holds whether the combo box is editable or not.
        \row
            \li frame
            \li Holds whether the combo box has a frame or not.
        \row
            \li iconSize
            \li The size of the current items icon.
        \row
            \li popupRect
            \li The bounding rectangle of the combo box's popup
               list.
    \endtable

    \section3 Group Boxes

    When calculating the size hint, QGroupBox fetches three pixel
    metrics from the style: \c PM_IndicatorWidth,
    \c PM_CheckBoxLabelSpacing, and \c PM_IndicatorHeight. QGroupBox has
    the following style element tree:

    \image javastyle/groupbox.png

    Qt does not impose restrictions on how the check box is drawn; the
    Java style draws it with \c CE_IndicatorCheckBox. See \l{Check and
    Radio Buttons} for the complete tree.

    We also give an image of the widget with the sub controls and
    sub control rectangles drawn:

    \image javastyle/groupboximage.png

    The style option for group boxes is QStyleOptionGroupBox. The
    following states can be set on it:

    \table 90%
        \header
            \li State
            \li Set When
        \row
            \li \c State_On
            \li The check box is checked.
        \row
            \li \c State_Sunken
            \li The check box is pressed down.
        \row
            \li \c State_Off
            \li The check box is unchecked (or there is no check box).
    \endtable

    The remaining members of QStyleOptionGroupBox are:

    \table
        \header
            \li Member
            \li Content
        \row
            \li features
            \li Flags of the QStyleOptionFrame::FrameFeatures
               enum describing the frame of the group box.
        \row
            \li lineWidth
            \li The line width with which to draw the panel. This
               is always 1.
        \row
            \li text
            \li The text of the group box.
        \row
            \li textAlignment
            \li The alignment of the group box title.
        \row
            \li textColor
            \li The QColor of the text.
    \endtable

    \section3 Splitters

    As the structure of splitters are simple and do not contain any
    sub elements, we do not include any images of splitters. \c CE_Splitter
    does not use any other elements or metrics.

    For its style option, the splitter uses the base class QStyleOption.
    It can set the following state flags on it:

    \table 90%
        \header
            \li State
            \li Set When
        \row
            \li \c State_Horizontal
            \li Set if it is a horizontal splitter.
    \endtable

    QSplitter does not use \l{QStyleOption::}{initFrom()} to set up its
    option; it sets the \c State_MouseOver and \c State_Disabled flags
    itself.

    \section3 Progress Bar

    The \c CE_ProgressBar element is used by QProgressBar, and it is the
    only element used by this widget. We start with the style structure:

    \image javastyle/progressbar.png

    Here is a progress bar in the common style (the Java style
    bounding rectangles are equal):

    \image javastyle/progressbarimage.png

    The style option for QProgressBar is QStyleOptionProgressBar.
    The bar does not set any state flags, but the other members of the
    option are:

    \table 90%
        \header
            \li Member
            \li Content
        \row
            \li minimum
            \li The minimum value of the bar.
        \row
            \li maximum
            \li The maximum value of the bar.
        \row
            \li progress
            \li The current value of the bar.
        \row
            \li textAlignment
            \li How the text is aligned in the label.
        \row
            \li textVisible
            \li Whether the label is drawn.
        \row
            \li text
            \li The label text.
        \row
            \li orientation
            \li Progress bars can be vertical or horizontal.
        \row
            \li invertedAppearance
            \li The progress is inverted (i.e., right to left in a
               horizontal bar).
        \row
            \li bottomToTop
            \li Boolean, that, if \c true, turns the label of vertical
               progress bars 90 degrees.
    \endtable

    \section3 Tool Buttons

    Tool buttons exist either independently or as part of tool bars.
    They are drawn equally either way. The QToolButton draws only one
    style element: \c CC_ToolButton.

    Below is a tree of the widget's style structure:

    \image javastyle/toolbutton.png

    Note that \c PE_FrameButtonTool and \c PE_IndicatorArrowDown are
    included in the tree as the Java style draws them, but they can
    safely be omitted if you prefer it. The structure may also be
    different. QCommonStyle, for instance, draws both
    \c PE_IndicatorButtonDropDown and \c PE_IndicatorArrowDown in
    \c CE_ToolButton.

    We also have an image of a tool button where we have outlined
    the sub element bounding rectangles and sub controls.

    \image javastyle/toolbuttonimage.png

    Here is the states table for tool buttons:

    \table 90%
        \header
            \li State
            \li Set When
        \row
            \li \c State_AutoRise
            \li The tool button has the autoRise property set.
        \row
            \li \c State_Raised
            \li The button is not sunken (i.e., by being checked or
               pressed on with the mouse).
        \row
            \li \c State_Sunken
            \li The button is down.
        \row
            \li \c State_On
            \li The button is checkable and checked.
    \endtable

    QStyleOptionToolButton also contains the following members:

    \table
        \header
            \li Member
            \li Content
        \row
            \li arrowType
            \li A Qt::ArrowType enum value, which contains the
                 direction of the buttons arrow (if an arrow is to
                 be used in place of an icon).
        \row
            \li features
            \li Flags of the QStyleOptionToolButton::ButtonFeature
               enum describing if the button has an arrow, a menu,
               and/or has a popup-delay.
        \row
            \li font
            \li The QFont of the buttons label.
        \row
            \li icon
            \li The QIcon of the tool button.
        \row
            \li iconSize
            \li The icon size of the button's icon.
        \row
            \li pos
            \li The position of the button, as given by
               QWidget::pos()
        \row
            \li text
            \li The text of the button.
        \row
            \li toolButtonStyle
            \li A Qt::ToolButtonStyle enum value which decides
               whether the button shows the icon, the text, or both.
    \endtable

    \section3 Toolbars

    Toolbars are part of the \l{QMainWindow}{main window framework},
    and cooperate with the QMainWindow to which they belong while it
    builds its style option. A main window has 4 areas that toolbars
    can be placed in. They are positioned next to the four sides of
    the window (i.e., north, south, east and west). Within each area
    there can be more than one line of toolbars; a line consists of
    toolbars with equal orientation (vertical or horizontal) placed
    next to each other.

    \l{QToolBar}{Toolbars} in Qt consist of three elements:
    \c CE_ToolBar, \c PE_IndicatorToolBarHandle, and
    \c PE_IndicatorToolBarSeparator. It is QMainWindowLayout that
    calculates the bounding rectangles (i.e., position and size of the
    toolbars and their contents. The main window also uses the \c
    sizeHint() of the items in the toolbars when calculating the size
    of the bars.

    Here is the element tree for QToolBar:

    \image javastyle/toolbar.png

    The dotted lines indicate that the QToolBar keeps an instance of
    QToolBarLayout and that QToolBarSeparators are kept by
    QToolBarLayout. When the toolbar is floating (i.e., has its own
    window) the \c PE_FrameMenu element is drawn, else QToolBar draws
    \c CE_ToolBar.

    Here is an image of a toolbar in the Java style:

    \image javastyle/toolbarimage.png

    QToolBarSaparator uses QStyleOption for its style option. It
    sets the \c State_Horizontal flag if the toolbar it lives in is
    horizontal. Other than that, they use \l{QStyleOption::}{initFrom()}.

    The style option for QToolBar is QStyleOptionToolBar. The only
    state flag set (besides the common flags) is \c State_Horizontal
    if the bar is horizontal (i.e., in the north or south toolbar area).
    The member variables of the style option are:

    \table 90%
        \header
            \li Member
            \li Content
        \row
            \li features
            \li Holds whether the bar is movable in a value of the
               ToolBarFeature, which is either Movable or None.
        \row
            \li lineWidth
            \li The width of the tool bar frame.
        \row
            \li midLineWidth
            \li This variable is currently not used and is always 0.
        \row
            \li positionOfLine
            \li The position of the toolbar line within the toolbar
               area to which it belongs.
        \row
            \li positionWithinLine
            \li The position of the toolbar within the toolbar line.
        \row
            \li toolBarArea
            \li The toolbar area in which the toolbar lives.
    \endtable

    \section3 Menus

    Menus in Qt are implemented in QMenu. The QMenu keeps a list of
    actions, which it draws as menu items. When QMenu receives paint
    events, it calculates the size of each menu item and draws them
    individually with \c CE_MenuItem. Menu items do not have a separate
    element for their label (contents), so all drawing is done in
    \c CE_MenuItem. The menu also draws the frame of the menu with
    \c PE_FrameMenu. It also draws \c CE_MenuScroller if the style supports
    scrolling. \c CE_MenuTearOff is drawn if the menu is too large for its
    bounding rectangle.

    In the style structure tree, we also include QMenu as it also does
    styling related work. The bounding rectangles of menu items are
    calculated for the menu's size hint and when the menu is displayed
    or resized.

    \image javastyle/menu.png

    The \c CE_MenuScroller and \c CE_MenuTearOff elements are handled by
    QCommonStyle and are not shown unless the menu is too large to fit
    on the screen. \c PE_FrameMenu is only drawn for pop-up menus.

    QMenu calculates rectangles based on its actions and calls
    \c CE_MenuItem and \c CE_MenuScroller if the style supports that.

    It is also usual to use \c PE_IndicatorCheckBox (instead of using
    \c PE_IndicatorMenuCheckMark) and \c PE_IndicatorRadioButton for drawing
    checkable menu items; we have not included them in the style tree
    as this is optional and varies from style to style.

    \image javastyle/menuimage.png

    The style option for menu items is QStyleOptionMenuItem. The
    following tables describe its state flags and other members.

    \table 90%
        \header
            \li State
            \li Set When
        \row
            \li \c State_Selected
            \li The mouse is over the action and the action is not
               a separator.
        \row
            \li \c State_Sunken
            \li The mouse is pressed down on the menu item.
        \row
            \li \c State_DownArrow
            \li Set if the menu item is a menu scroller and it scrolls
               the menu downwards.
    \endtable

    \table 90%
        \header
            \li Member
            \li Content
        \row
            \li checkType
            \li A value of the \l{QStyleOptionMenuItem::}{CheckType} enum,
               which is either NotCheckable, Exclusive, or
               NonExclusive.
        \row
            \li checked
            \li Boolean that is \c true if the menu item is checked.
        \row
            \li font
            \li The QFont to use for the menu item's text.
        \row
            \li icon
            \li The QIcon of the menu item.
        \row
            \li maxIconWidth
            \li The maximum width allowed for the icon.
        \row
            \li menuHasCheckableItems
            \li Boolean which is \c true if at least one item in the
               menu is checkable.
        \row
            \li menuItemType
            \li The type of the menu item. This a value of the
               \l{QStyleOptionMenuItem::}{MenuItemType}.
        \row
            \li menuRect
            \li The bounding rectangle for the QMenu that the menu
               item lives in.
        \row
            \li tabWidth
            \li This is the distance between the text of the menu
               item and the shortcut.
        \row
            \li text
            \li The text of the menu item.
    \endtable

    The setup of the style option for \c CE_MenuTearOff and
    \c CE_MenuScroller also uses QStyleOptionMenuItem; they only set the
    \c menuRect variable in addition to the common settings with
    QStyleOption's \l{QStyleOption::}{initFrom()}.

    \section3 Menu Bar

    QMenuBar uses the style to draw each menu bar item and the empty
    area of the menu bar. The pull-down menus themselves are
    \l{QMenu}s (see \l{Menus}). The style element tree for the menu
    bar follows:

    \image javastyle/menubar.png

    The panel and empty area are drawn after the menu items. The
    QPainter that the QMenuBar sends to the style has the bounding
    rectangles of the items clipped out (i.e., clip region), so you
    don't need to worry about drawing over the items. The pixel
    metrics in QMenuBar are used when the bounding rectangles of the
    menu bar items are calculated.

    \image javastyle/menubarimage.png

    QStyleOptionMenuItem is used for menu bar items. The members that
    are used by QMenuBar are described in the following table:

    \table
        \header
            \li Member
            \li Content
        \row
            \li menuRect
            \li The bounding rectangle of the entire menu bar to
               which the item belongs.
        \row
            \li text
            \li The text of the item.
        \row
            \li icon
            \li The icon of the menu item (it is not common that
               styles draw this icon).
    \endtable

    QStyleOptionMenuItem is also used for drawing \c CE_EmptyMenuBarArea.

    QStyleOptionFrame is used for drawing the panel frame. The
    \l{QStyleOptionFrame::}{lineWidth} is set to \c PM_MenuBarPanelWidth.
    The \l{QStyleOptionFrame::}{midLineWidth} is currently always set
    to 0.

    \section3 Item View Headers

    It is the style that draws the headers of Qt's item views. The
    item views keep the dimensions on individual sections. Also
    note that the delegates may use the style to paint decorations
    and frames around items. QItemDelegate, for instance, draws
    \c PE_FrameFocusRect and \c PE_IndicatorItemViewItemCheck.

    \image javastyle/header.png

    Here is a QTableWidget showing the bounding rects of a Java
    header:

    \image javastyle/headerimage.png

    The QHeaderView uses \c CT_HeaderSection, \c PM_HeaderMargin and
    \c PM_HeaderGripMargin for size and hit test calculations. The
    \c PM_HeaderMarkSize is currently not used by Qt. QTableView draws
    the button in the top-left corner (i.e., the area where the
    vertical and horizontal headers intersect) as a \c CE_Header.

    The style option for header views is QStyleOptionHeader. The view
    paints one header section at a time, so the data is for the
    section being drawn. Its contents are:

    \table 90%
        \header
            \li Member
            \li Content
        \row
            \li icon
            \li The icon of the header (for section that is being drawn).
        \row
            \li iconAlignment
            \li The alignment (Qt::Alignment) of the icon in the header.
        \row
            \li orientation
            \li A Qt::Orientation value deciding whether the header
               is the horizontal header above the view or the
               vertical header on the left.
        \row
            \li position
            \li A QStyleOptionHeader::SectionPosition value
               giving the header section's position relative to
               the other sections.
        \row
            \li section
            \li Holds the section that is being drawn.
        \row
            \li selectedPosition
            \li A QStyleOptionHeader::SelectedPosition value giving
               the selected section's position relative to the
               section that is being painted.
        \row
            \li sortIndicator
            \li A QStyleOptionHeader::SortIndicator value that
               describes the direction in which the section's sort
               indicator should be drawn.
        \row
            \li text
            \li The text of the currently drawn section.
        \row
            \li textAlignment
            \li The Qt::Alignment of the text within the header section.
    \endtable

    \section3 Tree Branch Indicators

    The branch indicators in a tree view are drawn by the style with
    \c PE_IndicatorBranch. We think of indicators here as the indicators
    that describe the relationship of the nodes in the tree. The
    generic QStyleOption is sent to the style for drawing these
    elements. The various branch types are described by states. Since
    there is no specific style option, we simply present the states
    table:

    \table 90%
        \header
            \li State
            \li Set When
        \row
            \li \c State_Sibling
            \li The node in the tree has a sibling (i.e., there is
               another node in the same column).
        \row
            \li \c State_Item
            \li This branch indicator has an item.
        \row
            \li \c State_Children
            \li The branch has children (i.e., a new sub-tree can
               be opened at the branch).
        \row
            \li \c State_Open
            \li The branch indicator has an opened sub-tree.
    \endtable

    The tree view (and tree widget) use the style to draw the branches
    (nodes) of the tree.

    QStyleOption is used as the style for \c PE_IndicatorBranch has state
    flags set depending on what type of branch it is.

    Since there is no tree structure for branch indicators, we only
    present an image of a tree in the Java style. Each state is marked
    in the image with a rectangle in a specific color (i.e., the
    rectangles are not bounding rectangles). All combinations of
    states you must be aware of are represented in the image.

    \image javastyle/branchindicatorimage.png

    \section3 Tool Boxes

    \c PM_SmallIconSize for sizeHints.

    QToolBox is a container that keeps a collection of widgets. It has
    one tab for each widget and displays one of them at a time. The
    tool box lays the components it displays (the tool box buttons
    and selected widget) in a QVBoxLayout. The style tree for tool
    boxes looks like this:

    \image javastyle/toolbox.png

    We show an image of a tool box in the Plastique style:

    \image javastyle/toolboximage.png

    All elements have the same bounding rectangles in the
    Plastique style as well as the other built-in Qt styles.

    The style option for tool boxes is QStyleOptionToolBox. It
    contains the text and icon of the tool box contents. The only
    state set by QToolBox is \c State_Sunken, which is set when the user
    presses a tab down with the mouse. The rest of the
    QStyleOptionToolBox members are:

    \table 90%
        \header
            \li Member
            \li Content
        \row
            \li icon
            \li The icon on the toolbox tab.
        \row
            \li text
            \li The text on the toolbox tab.
    \endtable

    \section3 Size Grip

    The size grip calculates its size hint with \c CT_SizeGrip. The pixel
    metric \c PM_SizeGripSize is currently unused by Qt. The element tree
    for an image in the Plastique style of QSizeGrip follows:

    \image javastyle/sizegrip.png

    \image javastyle/sizegripimage.png

    We show the size grip in \l{QMainWindow}'s bottom right
    corner.

    The size grip style option, QStyleOptionSizeGrip, has one
    member besides the common members from QStyleOption:

    \table 90%
        \header
            \li Member
            \li Content
        \row
            \li corner
            \li A Qt::Corner value that describes which corner in a
               window (or equivalent) the grip is located.
    \endtable

    \section3 Rubber Band

    The \l{QRubberBand}'s style tree consists of two nodes.

    \image javastyle/rubberband.png

    We present an image of a Java style window being moved in a
    QMdiArea with a rubber band:

    \image javastyle/rubberbandimage.png

    The style option for rubber bands is QStyleOptionRubberBand.
    Its members are:

    \table
        \header
            \li Member
            \li Content
        \row
            \li opaque
            \li Boolean that is \c true if the rubber band must be
               drawn in an opaque style (i.e., color).
        \row
            \li shape
            \li A QRubberBand::Shape enum value that holds the
               shape of the band (which is either a rectangle or a
               line).
    \endtable

    \section3 Dock Widgets

    When the dock widget lays out its contents it asks the style for
    these pixel metrics: \c PM_DockWidgetSeparatorExtent,
    \c PM_DockWidgetTitleBarButtonMargin, \c PM_DockWidgetFrameWidth, and
    \c PM_DockWidgetTitleMargin. It also calculates the bounding
    rectangles of the float and close buttons with
    \c SE_DockWidgetCloseButton and \c SE_DockWidgetFloatButton.

    \image javastyle/dockwidget.png

    The dotted lines indicate that the sender keeps instances of the
    recipient of the arrow (i.e., it is not a style element to draw).
    The dock widget only draws \c PE_frameDockWidget when it is detached
    from its main window (i.e., it is a top level window). If it is
    docked it draws the indicator dock widget resize handle. We show a
    dock widget in both docked and floating state in the plastique
    style:

    \image javastyle/dockwidgetimage.png

    The style option is QStyleOptionDockWidget:

    \table 90%
        \header
            \li Member
            \li Content
        \row
            \li closeable
            \li Boolean that holds whether the dock window can be
               closed.
        \row
            \li floatable
            \li Boolean that holds whether the dock window can
               float (i.e., detach from the main window in which
               it lives).
        \row
            \li movable
            \li Boolean that holds whether the window is movable
               (i.e., can move to other dock widget areas).
        \row
            \li title
            \li The title text of the dock window.
    \endtable

    For the buttons, QStyleOptionButton is used (see \l{Tool Buttons}
    for content description). The dock widget resize handle has a
    plain QStyleOption.
*/
