doc/src/examples/shapedclock.qdoc
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    41 
       
    42 /*!
       
    43     \example widgets/shapedclock
       
    44     \title Shaped Clock Example
       
    45 
       
    46     The Shaped Clock example shows how to apply a widget mask to a top-level
       
    47     widget to produce a shaped window.
       
    48 
       
    49     \image shapedclock-example.png
       
    50 
       
    51     Widget masks are used to customize the shapes of top-level widgets by restricting
       
    52     the available area for painting. On some window systems, setting certain window flags
       
    53     will cause the window decoration (title bar, window frame, buttons) to be disabled,
       
    54     allowing specially-shaped windows to be created. In this example, we use this feature
       
    55     to create a circular window containing an analog clock.
       
    56 
       
    57     Since this example's window does not provide a \gui File menu or a close
       
    58     button, we provide a context menu with an \gui Exit entry so that the example
       
    59     can be closed. Click the right mouse button over the window to open this menu.
       
    60 
       
    61     \section1 ShapedClock Class Definition
       
    62 
       
    63     The \c ShapedClock class is based on the \c AnalogClock class defined in the
       
    64     \l{Analog Clock Example}{Analog Clock} example. The whole class definition is
       
    65     presented below:
       
    66 
       
    67     \snippet examples/widgets/shapedclock/shapedclock.h 0
       
    68 
       
    69     The \l{QWidget::paintEvent()}{paintEvent()} implementation is the same as that found
       
    70     in the \c AnalogClock class. We implement \l{QWidget::sizeHint()}{sizeHint()}
       
    71     so that we don't have to resize the widget explicitly. We also provide an event
       
    72     handler for resize events. This allows us to update the mask if the clock is resized.
       
    73 
       
    74     Since the window containing the clock widget will have no title bar, we provide
       
    75     implementations for \l{QWidget::mouseMoveEvent()}{mouseMoveEvent()} and
       
    76     \l{QWidget::mousePressEvent()}{mousePressEvent()} to allow the clock to be dragged
       
    77     around the screen. The \c dragPosition variable lets us keep track of where the user
       
    78     last clicked on the widget.
       
    79 
       
    80     \section1 ShapedClock Class Implementation
       
    81 
       
    82     The \c ShapedClock constructor performs many of the same tasks as the \c AnalogClock
       
    83     constructor. We set up a timer and connect it to the widget's update() slot:
       
    84 
       
    85     \snippet examples/widgets/shapedclock/shapedclock.cpp 0
       
    86 
       
    87     We inform the window manager that the widget is not to be decorated with a window
       
    88     frame by setting the Qt::FramelessWindowHint flag on the widget. As a result, we need
       
    89     to provide a way for the user to move the clock around the screen.
       
    90 
       
    91     Mouse button events are delivered to the \c mousePressEvent() handler:
       
    92 
       
    93     \snippet examples/widgets/shapedclock/shapedclock.cpp 1
       
    94 
       
    95     If the left mouse button is pressed over the widget, we record the displacement in
       
    96     global (screen) coordinates between the top-left position of the widget's frame (even
       
    97     when hidden) and the point where the mouse click occurred. This displacement will be
       
    98     used if the user moves the mouse while holding down the left button. Since we acted
       
    99     on the event, we accept it by calling its \l{QEvent::accept()}{accept()} function.
       
   100 
       
   101     \image shapedclock-dragging.png
       
   102 
       
   103     The \c mouseMoveEvent() handler is called if the mouse is moved over the widget.
       
   104 
       
   105     \snippet examples/widgets/shapedclock/shapedclock.cpp 2
       
   106 
       
   107     If the left button is held down while the mouse is moved, the top-left corner of the
       
   108     widget is moved to the point given by subtracting the \c dragPosition from the current
       
   109     cursor position in global coordinates. If we drag the widget, we also accept the event.
       
   110 
       
   111     The \c paintEvent() function is given for completeness. See the
       
   112     \l{Analog Clock Example}{Analog Clock} example for a description of the process used
       
   113     to render the clock.
       
   114 
       
   115     \snippet examples/widgets/shapedclock/shapedclock.cpp 3
       
   116 
       
   117     In the \c resizeEvent() handler, we re-use some of the code from the \c paintEvent()
       
   118     to determine the region of the widget that is visible to the user:
       
   119 
       
   120     \snippet examples/widgets/shapedclock/shapedclock.cpp 4
       
   121 
       
   122     Since the clock face is a circle drawn in the center of the widget, this is the region
       
   123     we use as the mask.
       
   124 
       
   125     Although the lack of a window frame may make it difficult for the user to resize the
       
   126     widget on some platforms, it will not necessarily be impossible. The \c resizeEvent()
       
   127     function ensures that the widget mask will always be updated if the widget's dimensions
       
   128     change, and additionally ensures that it will be set up correctly when the widget is
       
   129     first displayed.
       
   130 
       
   131     Finally, we implement the \c sizeHint() for the widget so that it is given a reasonable
       
   132     default size when it is first shown:
       
   133 
       
   134     \snippet examples/widgets/shapedclock/shapedclock.cpp 5
       
   135 
       
   136     \section1 Notes on Widget Masks
       
   137 
       
   138     Since QRegion allows arbitrarily complex regions to be created, widget masks can be
       
   139     made to suit the most unconventionally-shaped windows, and even allow widgets to be
       
   140     displayed with holes in them.
       
   141 
       
   142     Widget masks can also be constructed by using the contents of pixmap to define the
       
   143     opaque part of the widget. For a pixmap with an alpha channel, a suitable mask can be
       
   144     obtained with QPixmap::mask().
       
   145 */