qt-everywhere-src-5.15.1/qtbase/examples/widgets/gestures/imagegestures/doc/src/imagegestures.qdoc

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/*!
    \example gestures/imagegestures
    \title Image Gestures Example
    \brief Demonstrates the use of simple gestures in a widget.

    This example shows how to enable gestures for a widget and use gesture input
    to perform actions.

    \image imagegestures-example.jpg

    We use two classes to create the user interface for the application: \c MainWidget
    and \c ImageWidget. The \c MainWidget class is simply used as a container for the
    \c ImageWidget class, which we will configure to accept gesture input. Since we
    are interested in the way gestures are used, we will concentrate on the
    implementation of the \c ImageWidget class.

    \section1 ImageWidget Class Definition

    The \c ImageWidget class is a simple QWidget subclass that reimplements the general
    QWidget::event() handler function in addition to several more specific event handlers:

    \snippet gestures/imagegestures/imagewidget.h class definition begin
    \dots
    \snippet gestures/imagegestures/imagewidget.h class definition end

    We also implement a private helper function, \c gestureEvent(), to help manage
    gesture events delivered to the widget, and three functions to perform actions
    based on gestures: \c panTriggered(), \c pinchTriggered() and \c swipeTriggered().

    \section1 ImageWidget Class Implementation

    In the widget's constructor, we begin by setting up various parameters that will
    be used to control the way images are displayed.

    \snippet gestures/imagegestures/imagewidget.cpp constructor

    We enable three of the standard gestures for the widget by calling QWidget::grabGesture()
    with the types of gesture we need. These will be recognized by the application's
    default gesture recognizer, and events will be delivered to our widget.

    Since QWidget does not define a specific event handler for gestures, the widget
    needs to reimplement the general QWidget::event() to receive gesture events.

    \snippet gestures/imagegestures/imagewidget.cpp event handler

    We implement the event handler to delegate gesture events to a private function
    specifically written for the task, and pass all other events to QWidget's
    implementation.

    The \c gestureHandler() function examines the gestures supplied by the
    newly-delivered QGestureEvent. Since only one gesture of a given type can be
    used on a widget at any particular time, we can check for each gesture type
    using the QGestureEvent::gesture() function:

    \snippet gestures/imagegestures/imagewidget.cpp gesture event handler

    If a QGesture object is supplied for a certain type of gesture, we call a special
    purpose function to deal with it, casting the gesture object to the appropriate
    QGesture subclass.

    To illustrate how a standard gesture can be interpreted by an application, we
    show the implementation of the \c pinchTriggered() function, which handles the
    pinch gesture when the user moves two fingers around on the display or
    input device:

    \snippet gestures/imagegestures/imagewidget.cpp pinch function

    The QPinchGesture class provides properties to interpret the changing
    distance between the two touch points as a zoom factor, and the angle delta
    as a rotation to be applied to the image. The center point between the
    touch points could be used to drag the image, but in this example we use
    the pan gesture for that purpose.

    The \c scaleFactor() is a relative value representing how much the zoom
    should change from one event to the next, whereas \c totalScaleFactor()
    provides the amount of zoom that has been expressed since the gesture
    began. When the touch points are released and another gesture begins,
    \c totalScaleFactor() will begin again at 1.0. In this case we store
    \c totalScaleFactor() into the \c currentStepScaleFactor variable so that
    it can be used in \c paintEvent() to scale the image. Alternatively it would
    be possible to simply multiply the stored total scale factor by
    \c scaleFactor() here in the pinch handler.

    In contrast, \c rotationAngle() represents the amount of rotation since the
    pinch gesture began, while \c lastRotationAngle() provides the previous
    value. So it is necessary to subtract in order to get an incremental delta.
    When the user begins a new pinch gesture, \c rotationAngle() will start
    from zero, and we want the image to begin to rotate from its current angle.
    This is achieved by adding the delta to the stored \c rotationAngle (which
    will be applied in \c paintEvent()). If we simply assigned
    \c totalRotationAngle() to the stored \c rotationAngle, a new gesture would
    cause the image to reset to a right-side-up orientation before beginning to
    rotate again. But it would be possible to store the rotation angle since the
    gesture began and add it to \c rotationAngle in \c paintEvent(), just as
    we store the amount of zoom since the gesture began.

    The pan and swipe gestures in this example are also handled in separate
    functions, and use the values of properties from the QGesture objects
    passed to them.

    \snippet gestures/imagegestures/imagewidget.cpp paint method

    In \c paintEvent(), scaleFactor represents the zoom level before the pinch
    gesture began, while currentStepScaleFactor represents the additional zoom
    factor while a pinch gesture is in progress. But for rotation, only the
    current rotationAngle is stored. The horizontal and vertical offsets
    represent the distance that the image has been dragged by the pan gesture.
*/