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/*!

    \example widgets/shapedclock

    \title Shaped Clock Example

    The Shaped Clock example shows how to apply a widget mask to a top-level

    widget to produce a shaped window.

    \image shapedclock-example.png

    Widget masks are used to customize the shapes of top-level widgets by restricting

    the available area for painting. On some 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 \gui File menu or a close

    button, we provide a context menu with an \gui 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 examples/widgets/shapedclock/shapedclock.h 0

    The \l{QWidget::paintEvent()}{paintEvent()} implementation is the same as that found

    in the \c AnalogClock class. 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 examples/widgets/shapedclock/shapedclock.cpp 0

    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 examples/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 examples/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 given for completeness. See the

    \l{Analog Clock Example}{Analog Clock} example for a description of the process used

    to render the clock.

    \snippet examples/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:

    \snippet examples/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 examples/widgets/shapedclock/shapedclock.cpp 5

    \section1 Notes on Widget Masks

    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().

*/