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/*!

    \example widgets/analogclock

    \title Analog Clock Example

    The Analog Clock example shows how to draw the contents of a custom

    widget.

    \image analogclock-example.png Screenshot of the Analog Clock example

    This example also demonstrates how the transformation and scaling

    features of QPainter can be used to make drawing custom widgets

    easier.

    \section1 AnalogClock Class Definition

    The \c AnalogClock class provides a clock widget with hour and minute

    hands that is automatically updated every few seconds.

    We subclass \l QWidget and reimplement the standard

    \l{QWidget::paintEvent()}{paintEvent()} function to draw the clock face:

    \snippet examples/widgets/analogclock/analogclock.h 0

    \section1 AnalogClock Class Implementation

    \snippet examples/widgets/analogclock/analogclock.cpp 1

    When the widget is constructed, we set up a one-second timer to

    keep track of the current time, and we connect it to the standard

    \l{QWidget::update()}{update()} slot so that the clock face is

    updated when the timer emits the \l{QTimer::timeout()}{timeout()}

    signal.

    Finally, we resize the widget so that it is displayed at a

    reasonable size.

    \snippet examples/widgets/analogclock/analogclock.cpp 8

    \snippet examples/widgets/analogclock/analogclock.cpp 10

    The \c paintEvent() function is called whenever the widget's

    contents need to be updated. This happens when the widget is

    first shown, and when it is covered then exposed, but it is also

    executed when the  widget's \l{QWidget::update()}{update()} slot

    is called. Since we connected the timer's

    \l{QTimer::timeout()}{timeout()} signal to this slot, it will be

    called at least once every five seconds.

    Before we set up the painter and draw the clock, we first define

    two lists of \l {QPoint}s and two \l{QColor}s that will be used

    for the hour and minute hands. The minute hand's color has an

    alpha component of 191, meaning that it's 75% opaque.

    We also determine the length of the widget's shortest side so that we

    can fit the clock face inside the widget. It is also useful to determine

    the current time before we start drawing.

    \snippet examples/widgets/analogclock/analogclock.cpp 11

    \snippet examples/widgets/analogclock/analogclock.cpp 12

    \snippet examples/widgets/analogclock/analogclock.cpp 13

    \snippet examples/widgets/analogclock/analogclock.cpp 14

    The contents of custom widgets are drawn with a QPainter.

    Painters can be used to draw on any QPaintDevice, but they are

    usually used with widgets, so we pass the widget instance to the

    painter's constructor.

    We call QPainter::setRenderHint() with QPainter::Antialiasing to

    turn on antialiasing. This makes drawing of diagonal lines much

    smoother.

    The translation moves the origin to the center of the widget, and

    the scale operation ensures that the following drawing operations

    are scaled to fit within the widget. We use a scale factor that

    let's us use x and y coordinates between -100 and 100, and that

    ensures that these lie within the length of the widget's shortest

    side.

    To make our code simpler, we will draw a fixed size clock face that will

    be positioned and scaled so that it lies in the center of the widget.

    The painter takes care of all the transformations made during the

    paint event, and ensures that everything is drawn correctly. Letting

    the painter handle transformations is often easier than performing

    manual calculations just to draw the contents of a custom widget.

    \img analogclock-viewport.png

    We draw the hour hand first, using a formula that rotates the coordinate

    system counterclockwise by a number of degrees determined by the current

    hour and minute. This means that the hand will be shown rotated clockwise

    by the required amount.

    \snippet examples/widgets/analogclock/analogclock.cpp 15

    \snippet examples/widgets/analogclock/analogclock.cpp 16

    We set the pen to be Qt::NoPen because we don't want any outline,

    and we use a solid brush with the color appropriate for

    displaying hours. Brushes are used when filling in polygons and

    other geometric shapes.

    \snippet examples/widgets/analogclock/analogclock.cpp 17

    \snippet examples/widgets/analogclock/analogclock.cpp 19

    We save and restore the transformation matrix before and after the

    rotation because we want to place the minute hand without having to

    take into account any previous rotations.

    \snippet examples/widgets/analogclock/analogclock.cpp 20

    \codeline

    \snippet examples/widgets/analogclock/analogclock.cpp 21

    We draw markers around the edge of the clock for each hour. We

    draw each marker then rotate the coordinate system so that the

    painter is ready for the next one.

    \snippet examples/widgets/analogclock/analogclock.cpp 22

    \snippet examples/widgets/analogclock/analogclock.cpp 23

    The minute hand is rotated in a similar way to the hour hand.

    \snippet examples/widgets/analogclock/analogclock.cpp 25

    \codeline

    \snippet examples/widgets/analogclock/analogclock.cpp 26

    Again, we draw markers around the edge of the clock, but this

    time to indicate minutes. We skip multiples of 5 to avoid drawing

    minute markers on top of hour markers.

*/