qtbase/examples/widgets/doc/src/shapedclock.qdoc

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/*!
    \example widgets/shapedclock
    \title Shaped Clock Example
    \ingroup examples-widgets
    \brief The Shaped Clock example shows how to apply a translucent background
    and a widget mask to a top-level widget to produce a shaped window.

    \borderedimage shapedclock-example.png

    Widget masks are used to customize the shapes of top-level widgets by
    restricting the area available for painting and mouse input. Using a
    translucent background facilitates partially transparent windows and smooth
    edges. On most window systems, setting certain window flags will cause the
    window decoration (title bar, window frame, buttons) to be disabled,
    allowing specially-shaped windows to be created. In this example, we use
    this feature to create a circular window containing an analog clock.

    Since this example's window does not provide a \uicontrol File menu or a close
    button, we provide a context menu with an \uicontrol Exit entry so that the example
    can be closed. Click the right mouse button over the window to open this menu.

    \section1 ShapedClock Class Definition

    The \c ShapedClock class is based on the \c AnalogClock class defined in the
    \l{Analog Clock Example}{Analog Clock} example. The whole class definition is
    presented below:

    \snippet widgets/shapedclock/shapedclock.h 0

    The \l{QWidget::paintEvent()}{paintEvent()} implementation is the same as
    that found in the \c AnalogClock class, with one important exception: we
    now must also draw background (the clock face) ourselves, since the widget
    background is just transparent. We implement \l{QWidget::sizeHint()}{sizeHint()}
    so that we don't have to resize the widget explicitly. We also provide an event
    handler for resize events. This allows us to update the mask if the clock is resized.

    Since the window containing the clock widget will have no title bar, we provide
    implementations for \l{QWidget::mouseMoveEvent()}{mouseMoveEvent()} and
    \l{QWidget::mousePressEvent()}{mousePressEvent()} to allow the clock to be dragged
    around the screen. The \c dragPosition variable lets us keep track of where the user
    last clicked on the widget.

    \section1 ShapedClock Class Implementation

    The \c ShapedClock constructor performs many of the same tasks as the \c AnalogClock
    constructor. We set up a timer and connect it to the widget's update() slot:

    \snippet widgets/shapedclock/shapedclock.cpp 0

    We request a transparent window by setting the Qt::WA_TranslucentBackground
    widget attribute. We inform the window manager that the widget is not to be
    decorated with a window frame by setting the Qt::FramelessWindowHint flag
    on the widget. As a result, we need to provide a way for the user to move
    the clock around the screen.

    Mouse button events are delivered to the \c mousePressEvent() handler:

    \snippet widgets/shapedclock/shapedclock.cpp 1

    If the left mouse button is pressed over the widget, we record the displacement in
    global (screen) coordinates between the top-left position of the widget's frame (even
    when hidden) and the point where the mouse click occurred. This displacement will be
    used if the user moves the mouse while holding down the left button. Since we acted
    on the event, we accept it by calling its \l{QEvent::accept()}{accept()} function.

    \image shapedclock-dragging.png

    The \c mouseMoveEvent() handler is called if the mouse is moved over the widget.

    \snippet widgets/shapedclock/shapedclock.cpp 2

    If the left button is held down while the mouse is moved, the top-left corner of the
    widget is moved to the point given by subtracting the \c dragPosition from the current
    cursor position in global coordinates. If we drag the widget, we also accept the event.

    The \c paintEvent() function is mainly the same as described in the
    \l{Analog Clock Example}{Analog Clock} example. The one addition is that we
    use QPainter::drawEllipse() to draw a round clock face with the current
    palette's default background color. We make the clock face a bit smaller
    than the widget mask, so that the anti-aliased, semi-transparent pixels on
    the edge are not clipped away by the widget mask. This gives the shaped
    window smooth edges on the screen.

    \snippet widgets/shapedclock/shapedclock.cpp 3

    In the \c resizeEvent() handler, we re-use some of the code from the \c
    paintEvent() to determine the region of the widget that is visible to the
    user. This tells the system the area where mouse clicks should go to us,
    and not to whatever window is behind us:

    \snippet widgets/shapedclock/shapedclock.cpp 4

    Since the clock face is a circle drawn in the center of the widget, this is the region
    we use as the mask.

    Although the lack of a window frame may make it difficult for the user to resize the
    widget on some platforms, it will not necessarily be impossible. The \c resizeEvent()
    function ensures that the widget mask will always be updated if the widget's dimensions
    change, and additionally ensures that it will be set up correctly when the widget is
    first displayed.

    Finally, we implement the \c sizeHint() for the widget so that it is given a reasonable
    default size when it is first shown:

    \snippet widgets/shapedclock/shapedclock.cpp 5

    \section1 Notes on Widget Masks

    Widget masks are used to hint to the window system that the application
    does not want mouse events for areas outside the mask. On most systems,
    they also result in coarse visual clipping. To get smooth window edges, one
    should use translucent background and anti-aliased painting, as shown in
    this example.

    Since QRegion allows arbitrarily complex regions to be created, widget masks can be
    made to suit the most unconventionally-shaped windows, and even allow widgets to be
    displayed with holes in them.

    Widget masks can also be constructed by using the contents of pixmap to define the
    opaque part of the widget. For a pixmap with an alpha channel, a suitable mask can be
    obtained with QPixmap::mask().
*/