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/*!

    \example widgets/wiggly

    \title Wiggly Example

    The Wiggly example shows how to animate a widget using

    QBasicTimer and \l{QObject::timerEvent()}{timerEvent()}. In

    addition, the example demonstrates how to use QFontMetrics to

    determine the size of text on screen.

    \image wiggly-example.png Screenshot of the Wiggly example

    QBasicTimer is a low-level class for timers. Unlike QTimer,

    QBasicTimer doesn't inherit from QObject; instead of emitting a

    \l{QTimer::timeout()}{timeout()} signal when a certain amount of

    time has passed, it sends a QTimerEvent to a QObject of our

    choice. This makes QBasicTimer a more lightweight alternative to

    QTimer. Qt's built-in widgets use it internally, and it is

    provided in Qt's API for highly-optimized applications (e.g.,

    \l{Qt for Embedded Linux} applications).

    The example consists of two classes:

    \list

    \o \c WigglyWidget is the custom widget displaying the text

        in a wiggly line.

    \o \c Dialog is the dialog widget allowing the user to enter a

        text. It combines a \c WigglyWidget and a \c QLineEdit.

    \endlist

    We will first take a quick look at the \c Dialog class, then we

    will review the \c WigglyWidget class.

    \section1 Dialog Class Definition

    \snippet examples/widgets/wiggly/dialog.h 0

    The \c Dialog class provides a dialog widget that allows the user

    to enter a text. The text is then rendered by \c WigglyWidget.

    \section1 Dialog Class Implementation

    \snippet examples/widgets/wiggly/dialog.cpp 0

    In the constructor we create a wiggly widget along with a

    \l{QLineEdit}{line edit}, and we put the two widgets in a

    vertical layout. We connect the line edit's \l

    {QLineEdit::textChanged()}{textChanged()} signal to the wiggly

    widget's \c setText() slot to obtain the real time interaction

    with the wiggly widget. The widget's default text is "Hello

    world!".

    \section1 WigglyWidget Class Definition

    \snippet examples/widgets/wiggly/wigglywidget.h 0

    The \c WigglyWidget class provides the wiggly line displaying the

    text. We subclass QWidget and reimplement the standard \l

    {QWidget::paintEvent()}{paintEvent()} and \l

    {QObject::timerEvent()}{timerEvent()} functions to draw and update

    the widget. In addition we implement a public \c setText() slot

    that sets the widget's text.

    The \c timer variable, of type QBasicTimer, is used to update the

    widget at regular intervals, making the widget move. The \c text

    variable is used to store the currently displayed text, and \c

    step to calculate position and color for each character on the

    wiggly line.

    \section1 WigglyWidget Class Implementation

    \snippet examples/widgets/wiggly/wigglywidget.cpp 0

    In the constructor, we make the widget's background slightly

    lighter than the usual background using the QPalette::Midlight

    color role. The background role defines the brush from the

    widget's palette that Qt uses to paint the background. Then we

    enlarge the widget's font with 20 points.

    Finally we start the timer; the call to QBasicTimer::start()

    makes sure that \e this particular wiggly widget will receive the

    timer events generated when the timer times out (every 60

    milliseconds).

    \snippet examples/widgets/wiggly/wigglywidget.cpp 1

    \snippet examples/widgets/wiggly/wigglywidget.cpp 2

    The \c paintEvent() function is called whenever a QPaintEvent is

    sent to the widget. Paint events are sent to widgets that need to

    update themselves, for instance when part of a widget is exposed

    because a covering widget was moved. For the wiggly widget, a

    paint event will also be generated every 60 milliseconds from

    the \c timerEvent() slot.

    The \c sineTable represents y-values of the sine curve,

    multiplied by 100. It is used to make the wiggly widget move

    along the sine curve.

    The QFontMetrics object provides information about the widget's

    font. The \c x variable is the horizontal position where we start

    drawing the text. The \c y variable is the vertical position of

    the text's base line. Both variables are computed so that the

    text is horizontally and vertically centered. To compute the base

    line, we take into account the font's ascent (the height of the

    font above the base line) and font's descent (the height of the

    font below the base line). If the descent equals the ascent, they

    cancel out each other and the base line is at \c height() / 2.

    \snippet examples/widgets/wiggly/wigglywidget.cpp 3

    \snippet examples/widgets/wiggly/wigglywidget.cpp 4

    Each time the \c paintEvent() function is called, we create a

    QPainter object \c painter to draw the contents of the widget.

    For each character in \c text, we determine the color and the

    position on the wiggly line based on \c step. In addition, \c x

    is incremented by the character's width.

    For simplicity, we assume that QFontMetrics::width(\c text)

    returns the sum of the individual character widths

    (QFontMetrics::width(\c text[i])). In practice, this is not

    always the case because QFontMetrics::width(\c text) also takes

    into account the kerning between certain letters (e.g., 'A' and

    'V'). The result is that the text isn't perfectly centered. You

    can verify this by typing "AVAVAVAVAVAV" in the line edit.

    \snippet examples/widgets/wiggly/wigglywidget.cpp 5

    \snippet examples/widgets/wiggly/wigglywidget.cpp 6

    The \c timerEvent() function receives all the timer events that

    are generated for this widget. If a timer event is sent from the

    widget's QBasicTimer, we increment \c step to make the text move,

    and call QWidget::update() to refresh the display. Any other

    timer event is passed on to the base class's implementation of

    the \l{QWidget::timerEvent()}{timerEvent()} function.

    The QWidget::update() slot does not cause an immediate repaint;

    instead the slot schedules a paint event for processing when Qt

    returns to the main event loop. The paint events are then handled

    by \c{WigglyWidget}'s \c paintEvent() function.

*/