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In this example, the custom style is called \c NorwegianWoodStyle and derives from QMotifStyle. Its main features are the wooden textures used for filling most of the widgets and its round buttons and comboboxes. To implement the style, we use some advanced features provided by QPainter, such as \l{QPainter::Antialiasing}{antialiasing} (to obtain smoother button edges), \l{QColor::alpha()}{alpha blending} (to make the buttons appeared raised or sunken), and \l{QPainterPath}{painter paths} (to fill the buttons and draw the outline). We also use many features of QBrush and QPalette. The example consists of the following classes: \list \o \c NorwegianWoodStyle inherits from QMotifStyle and implements the Norwegian Wood style. \o \c WidgetGallery is a \c QDialog subclass that shows the most common widgets and allows the user to switch style dynamically. \endlist \section1 NorwegianWoodStyle Class Definition Here's the definition of the \c NorwegianWoodStyle class: \snippet examples/widgets/styles/norwegianwoodstyle.h 0 The public functions are all declared in QStyle (QMotifStyle's grandparent class) and reimplemented here to override the Motif look and feel. The private functions are helper functions. \section1 NorwegianWoodStyle Class Implementation We will now review the implementation of the \c NorwegianWoodStyle class. \snippet examples/widgets/styles/norwegianwoodstyle.cpp 0 The \c polish() function is reimplemented from QStyle. It takes a QPalette as a reference and adapts the palette to fit the style. Most styles don't need to reimplement that function. The Norwegian Wood style reimplements it to set a "wooden" palette. We start by defining a few \l{QColor}s that we'll need. Then we load two PNG images. The \c : prefix in the file path indicates that the PNG files are \l{The Qt Resource System}{embedded resources}. \table \row \o \inlineimage widgets/styles/images/woodbackground.png \o \bold{woodbackground.png} This texture is used as the background of most widgets. The wood pattern is horizontal. \row \o \inlineimage widgets/styles/images/woodbutton.png \o \bold{woodbutton.png} This texture is used for filling push buttons and comboboxes. The wood pattern is vertical and more reddish than the texture used for the background. \endtable The \c midImage variable is initialized to be the same as \c buttonImage, but then we use a QPainter and fill it with a 25% opaque black color (a black with an \l{QColor::alpha()}{alpha channel} of 63). The result is a somewhat darker image than \c buttonImage. This image will be used for filling buttons that the user is holding down. \snippet examples/widgets/styles/norwegianwoodstyle.cpp 1 We initialize the palette. Palettes have various \l{QPalette::ColorRole}{color roles}, such as QPalette::Base (used for filling text editors, item views, etc.), QPalette::Text (used for foreground text), and QPalette::Background (used for the background of most widgets). Each role has its own QBrush, which usually is a plain color but can also be a brush pattern or even a texture (a QPixmap). In addition to the roles, palettes have several \l{QPalette::ColorGroup}{color groups}: active, disabled, and inactive. The active color group is used for painting widgets in the active window. The disabled group is used for disabled widgets. The inactive group is used for all other widgets. Most palettes have identical active and inactive groups, while the disabled group uses darker shades. We initialize the QPalette object with a brown color. Qt automatically derivates all color roles for all color groups from that single color. We then override some of the default values. For example, we use Qt::darkGreen instead of the default (Qt::darkBlue) for the QPalette::Highlight role. The QPalette::setBrush() overload that we use here sets the same color or brush for all three color groups. The \c setTexture() function is a private function that sets the texture for a certain color role, while preserving the existing color in the QBrush. A QBrush can hold both a solid color and a texture at the same time. The solid color is used for drawing text and other graphical elements where textures don't look good. At the end, we set the brush for the disabled color group of the palette. We use \c woodbackground.png as the texture for all disabled widgets, including buttons, and use a darker color to accompany the texture. \image styles-disabledwood.png The Norwegian Wood style with disabled widgets Let's move on to the other functions reimplemented from QMotifStyle: \snippet examples/widgets/styles/norwegianwoodstyle.cpp 3 \snippet examples/widgets/styles/norwegianwoodstyle.cpp 4 This QStyle::polish() overload is called once on every widget drawn using the style. We reimplement it to set the Qt::WA_Hover attribute on \l{QPushButton}s and \l{QComboBox}es. When this attribute is set, Qt generates paint events when the mouse pointer enters or leaves the widget. This makes it possible to render push buttons and comboboxes differently when the mouse pointer is over them. \snippet examples/widgets/styles/norwegianwoodstyle.cpp 5 \snippet examples/widgets/styles/norwegianwoodstyle.cpp 6 This QStyle::unpolish() overload is called to undo any modification done to the widget in \c polish(). For simplicity, we assume that the flag wasn't set before \c polish() was called. In an ideal world, we would remember the original state for each widgets (e.g., using a QMap) and restore it in \c unpolish(). \snippet examples/widgets/styles/norwegianwoodstyle.cpp 7 \snippet examples/widgets/styles/norwegianwoodstyle.cpp 8 The \l{QStyle::pixelMetric()}{pixelMetric()} function returns the size in pixels for a certain user interface element. By reimplementing this function, we can affect the way certain widgets are drawn and their size hint. Here, we return 8 as the width around a shown in a QComboBox, ensuring that there is enough place around the text and the arrow for the Norwegian Wood round corners. The default value for this setting in the Motif style is 2. We also change the extent of \l{QScrollBar}s, i.e., the height for a horizontal scroll bar and the width for a vertical scroll bar, to be 4 pixels more than in the Motif style. This makes the style a bit more distinctive. For all other QStyle::PixelMetric elements, we use the Motif settings. \snippet examples/widgets/styles/norwegianwoodstyle.cpp 9 \snippet examples/widgets/styles/norwegianwoodstyle.cpp 10 The \l{QStyle::styleHint()}{styleHint()} function returns some hints to widgets or to the base style (in our case QMotifStyle) about how to draw the widgets. The Motif style returns \c true for the QStyle::SH_DitherDisabledText hint, resulting in a most unpleasing visual effect. We override this behavior and return \c false instead. We also return \c true for the QStyle::SH_EtchDisabledText hint, meaning that disabled text is rendered with an embossed look (as QWindowsStyle does). \snippet examples/widgets/styles/norwegianwoodstyle.cpp 11 \snippet examples/widgets/styles/norwegianwoodstyle.cpp 12 The \l{QStyle::drawPrimitive()}{drawPrimitive()} function is called by Qt widgets to draw various fundamental graphical elements. Here we reimplement it to draw QPushButton and QComboBox with round corners. The button part of these widgets is drawn using the QStyle::PE_PanelButtonCommand primitive element. The \c option parameter, of type QStyleOption, contains everything we need to know about the widget we want to draw on. In particular, \c option->rect gives the rectangle within which to draw the primitive element. The \c painter parameter is a QPainter object that we can use to draw on the widget. The \c widget parameter is the widget itself. Normally, all the information we need is available in \c option and \c painter, so we don't need \c widget. We can use it to perform special effects; for example, QMacStyle uses it to animate default buttons. If you use it, be aware that the caller is allowed to pass a null pointer. We start by defining three \l{QColor}s that we'll need later on. We also put the x, y, width, and height components of the widget's rectangle in local variables. The value used for the \c semiTransparentWhite and for the \c semiTransparentBlack color's alpha channel depends on whether the mouse cursor is over the widget or not. Since we set the Qt::WA_Hover attribute on \l{QPushButton}s and \l{QComboBox}es, we can rely on the QStyle::State_MouseOver flag to be set when the mouse is over the widget. \snippet examples/widgets/styles/norwegianwoodstyle.cpp 13 \snippet examples/widgets/styles/norwegianwoodstyle.cpp 14 The \c roundRect variable is a QPainterPath. A QPainterPath is is a vectorial specification of a shape. Any shape (rectangle, ellipse, spline, etc.) or combination of shapes can be expressed as a path. We will use \c roundRect both for filling the button background with a wooden texture and for drawing the outline. The \c roundRectPath() function is a private function; we will come back to it later. \snippet examples/widgets/styles/norwegianwoodstyle.cpp 15 \snippet examples/widgets/styles/norwegianwoodstyle.cpp 16 \snippet examples/widgets/styles/norwegianwoodstyle.cpp 17 \snippet examples/widgets/styles/norwegianwoodstyle.cpp 18 We define two variables, \c brush and \c darker, and initialize them based on the state of the button: \list \o If the button is a \l{QPushButton::flat}{flat button}, we use the \l{QPalette::Background}{Background} brush. We set \c darker to \c true if the button is \l{QAbstractButton::down}{down} or \l{QAbstractButton::checked}{checked}. \o If the button is currently held down by the user or in the \l{QAbstractButton::checked}{checked} state, we use the \l{QPalette::Mid}{Mid} component of the palette. We set \c darker to \c true if the button is \l{QAbstractButton::checked}{checked}. \o Otherwise, we use the \l{QPalette::Button}{Button} component of the palette. \endlist The screenshot below illustrates how \l{QPushButton}s are rendered based on their state: \image styles-woodbuttons.png Norwegian Wood buttons in different states To discover whether the button is flat or not, we need to cast the \c option parameter to QStyleOptionButton and check if the \l{QStyleOptionButton::features}{features} member specifies the QStyleOptionButton::Flat flag. The qstyleoption_cast() function performs a dynamic cast; if \c option is not a QStyleOptionButton, qstyleoption_cast() returns a null pointer. \snippet examples/widgets/styles/norwegianwoodstyle.cpp 19 \snippet examples/widgets/styles/norwegianwoodstyle.cpp 20 \snippet examples/widgets/styles/norwegianwoodstyle.cpp 21 \snippet examples/widgets/styles/norwegianwoodstyle.cpp 22 \snippet examples/widgets/styles/norwegianwoodstyle.cpp 23 We turn on antialiasing on QPainter. Antialiasing is a technique that reduces the visual distortion that occurs when the edges of a shape are converted into pixels. For the Norwegian Wood style, we use it to obtain smoother edges for the round buttons. \image styles-aliasing.png Norwegian wood buttons with and without antialiasing The first call to QPainter::fillPath() draws the background of the button with a wooden texture. The second call to \l{QPainter::fillPath()}{fillPath()} paints the same area with a semi-transparent black color (a black color with an alpha channel of 63) to make the area darker if \c darker is true. \snippet examples/widgets/styles/norwegianwoodstyle.cpp 24 \snippet examples/widgets/styles/norwegianwoodstyle.cpp 25 Next, we draw the outline. The top-left half of the outline and the bottom-right half of the outline are drawn using different \l{QPen}s to produce a 3D effect. Normally, the top-left half of the outline is drawn lighter whereas the bottom-right half is drawn darker, but if the button is \l{QAbstractButton::down}{down} or \l{QAbstractButton::checked}{checked}, we invert the two \l{QPen}s to give a sunken look to the button. \snippet examples/widgets/styles/norwegianwoodstyle.cpp 26 We draw the top-left part of the outline by calling QPainter::drawPath() with an appropriate \l{QPainter::setClipRegion()}{clip region}. If the \l{QStyleOption::direction}{layout direction} is right-to-left instead of left-to-right, we swap the \c x1, \c x2, \c x3, and \c x4 variables to obtain correct results. On right-to-left desktop, the "light" comes from the top-right corner of the screen instead of the top-left corner; raised and sunken widgets must be drawn accordingly. The diagram below illustrates how 3D effects are drawn according to the layout direction. The area in red on the diagram corresponds to the \c topHalf polygon: \image styles-3d.png An easy way to test how a style looks in right-to-left mode is to pass the \c -reverse command-line option to the application. This option is recognized by the QApplication constructor. \snippet examples/widgets/styles/norwegianwoodstyle.cpp 32 \snippet examples/widgets/styles/norwegianwoodstyle.cpp 33 \snippet examples/widgets/styles/norwegianwoodstyle.cpp 34 The bottom-right part of the outline is drawn in a similar fashion. Then we draw a one-pixel wide outline around the entire button, using the \l{QPalette::Foreground}{Foreground} component of the QPalette. This completes the QStyle::PE_PanelButtonCommand case of the \c switch statement. Other primitive elements are handled by the base style. Let's now turn to the other \c NorwegianWoodStyle member functions: \snippet examples/widgets/styles/norwegianwoodstyle.cpp 35 \snippet examples/widgets/styles/norwegianwoodstyle.cpp 36 We reimplement QStyle::drawControl() to draw the text on a QPushButton in a bright color when the button is \l{QAbstractButton::down}{down} or \l{QAbstractButton::checked}{checked}. If the \c option parameter points to a QStyleOptionButton object (it normally should), we take a copy of the object and modify its \l{QStyleOption::palette}{palette} member to make the QPalette::ButtonText be the same as the QPalette::BrightText component (unless the widget is disabled). \snippet examples/widgets/styles/norwegianwoodstyle.cpp 37 \snippet examples/widgets/styles/norwegianwoodstyle.cpp 38 The \c setTexture() function is a private function that sets the \l{QBrush::texture()}{texture} component of the \l{QBrush}es for a certain \l{QPalette::ColorRole}{color role}, for all three \l{QPalette::ColorGroup}{color groups} (active, disabled, inactive). We used it to initialize the Norwegian Wood palette in \c polish(QPalette &). \snippet examples/widgets/styles/norwegianwoodstyle.cpp 39 \snippet examples/widgets/styles/norwegianwoodstyle.cpp 40 The \c roundRectPath() function is a private function that constructs a QPainterPath object for round buttons. The path consists of eight segments: four arc segments for the corners and four lines for the sides. With around 250 lines of code, we have a fully functional custom style based on one of the predefined styles. Custom styles can be used to provide a distinct look to an application or family of applications. \section1 WidgetGallery Class For completeness, we will quickly review the \c WidgetGallery class, which contains the most common Qt widgets and allows the user to change style dynamically. Here's the class definition: \snippet examples/widgets/styles/widgetgallery.h 0 \dots \snippet examples/widgets/styles/widgetgallery.h 1 Here's the \c WidgetGallery constructor: \snippet examples/widgets/styles/widgetgallery.cpp 0 We start by creating child widgets. The \gui Style combobox is initialized with all the styles known to QStyleFactory, in addition to \c NorwegianWood. The \c create...() functions are private functions that set up the various parts of the \c WidgetGallery. \snippet examples/widgets/styles/widgetgallery.cpp 1 \snippet examples/widgets/styles/widgetgallery.cpp 2 We connect the \gui Style combobox to the \c changeStyle() private slot, the \gui{Use style's standard palette} check box to the \c changePalette() slot, and the \gui{Disable widgets} check box to the child widgets' \l{QWidget::setDisabled()}{setDisabled()} slot. \snippet examples/widgets/styles/widgetgallery.cpp 3 \snippet examples/widgets/styles/widgetgallery.cpp 4 Finally, we put the child widgets in layouts. \snippet examples/widgets/styles/widgetgallery.cpp 5 \snippet examples/widgets/styles/widgetgallery.cpp 6 When the user changes the style in the combobox, we call QApplication::setStyle() to dynamically change the style of the application. \snippet examples/widgets/styles/widgetgallery.cpp 7 \snippet examples/widgets/styles/widgetgallery.cpp 8 If the user turns the \gui{Use style's standard palette} on, the current style's \l{QStyle::standardPalette()}{standard palette} is used; otherwise, the system's default palette is honored. For the Norwegian Wood style, this makes no difference because we always override the palette with our own palette in \c NorwegianWoodStyle::polish(). \snippet examples/widgets/styles/widgetgallery.cpp 9 \snippet examples/widgets/styles/widgetgallery.cpp 10 The \c advanceProgressBar() slot is called at regular intervals to advance the progress bar. Since we don't know how long the user will keep the Styles application running, we use a logarithmic formula: The closer the progress bar gets to 100%, the slower it advances. We will review \c createProgressBar() in a moment. \snippet examples/widgets/styles/widgetgallery.cpp 11 \snippet examples/widgets/styles/widgetgallery.cpp 12 The \c createTopLeftGroupBox() function creates the QGroupBox that occupies the top-left corner of the \c WidgetGallery. We skip the \c createTopRightGroupBox(), \c createBottomLeftTabWidget(), and \c createBottomRightGroupBox() functions, which are very similar. \snippet examples/widgets/styles/widgetgallery.cpp 13 In \c createProgressBar(), we create a QProgressBar at the bottom of the \c WidgetGallery and connect its \l{QTimer::timeout()}{timeout()} signal to the \c advanceProgressBar() slot. */