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doc: imagegestures example: document the pinch handler

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The swipe handler is simple, and also doesn't currently work on every
platform.  But the pinch handler is the one that needs explanation,
because the difference between incremental and absolute values of the
rotation and scale properties is tricky.

Change-Id: Ie3c7f4941d4a17734c9a920a8dd978f86fb03c4b
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>
This commit is contained in:
Shawn Rutledge 2015-02-25 10:53:31 +01:00
parent 2b5982aac8
commit a9515cd02f
2 changed files with 52 additions and 13 deletions
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52
examples/widgets/gestures/imagegestures/doc/src/imagegestures.qdoc
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@ -83,18 +83,50 @@
QGesture subclass.
To illustrate how a standard gesture can be interpreted by an application, we
show the implementation of the \c swipeTriggered() function, which handles the
gesture associated with a brushing or swiping motion on the user's display or
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 swipe function
\snippet gestures/imagegestures/imagewidget.cpp pinch function
The QSwipeGesture class provides specialized functions and defines a enum
to make it more convenient for developers to discover which direction, if
any, the user swiped the display. Here, we simply navigate to the previous
image in the collection if the user swiped upwards or to the left; otherwise
we navigate to the next image in the collection.
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 other gestures are also handled by special purpose functions, but use
the values of properties held by the QGesture object passed to them.
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.
*/

13
examples/widgets/gestures/imagegestures/imagewidget.cpp
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@ -77,6 +77,7 @@ bool ImageWidget::event(QEvent *event)
}
//! [event handler]
//! [paint method]
void ImageWidget::paintEvent(QPaintEvent*)
{
QPainter p(this);
@ -93,6 +94,7 @@ void ImageWidget::paintEvent(QPaintEvent*)
p.translate(-iw/2, -ih/2);
p.drawImage(0, 0, currentImage);
}
//! [paint method]
void ImageWidget::mouseDoubleClickEvent(QMouseEvent *)
{
@ -138,16 +140,20 @@ void ImageWidget::panTriggered(QPanGesture *gesture)
update();
}
//! [pinch function]
void ImageWidget::pinchTriggered(QPinchGesture *gesture)
{
QPinchGesture::ChangeFlags changeFlags = gesture->changeFlags();
if (changeFlags & QPinchGesture::RotationAngleChanged) {
rotationAngle += gesture->rotationAngle() - gesture->lastRotationAngle();
qCDebug(lcExample) << "pinchTriggered(): rotate to" << rotationAngle;
qreal rotationDelta = gesture->rotationAngle() - gesture->lastRotationAngle();
rotationAngle += rotationDelta;
qCDebug(lcExample) << "pinchTriggered(): rotate by" <<
rotationDelta << "->" << rotationAngle;
}
if (changeFlags & QPinchGesture::ScaleFactorChanged) {
currentStepScaleFactor = gesture->totalScaleFactor();
qCDebug(lcExample) << "pinchTriggered(): zoom by" << gesture->scaleFactor() << "->" << currentStepScaleFactor;
qCDebug(lcExample) << "pinchTriggered(): zoom by" <<
gesture->scaleFactor() << "->" << currentStepScaleFactor;
}
if (gesture->state() == Qt::GestureFinished) {
scaleFactor *= currentStepScaleFactor;
@ -155,6 +161,7 @@ void ImageWidget::pinchTriggered(QPinchGesture *gesture)
}
update();
}
//! [pinch function]
//! [swipe function]
void ImageWidget::swipeTriggered(QSwipeGesture *gesture)