QTouchEvent Class

Detailed Description

Enabling Touch Events

Touch events occur when pressing, releasing, or moving one or more touch points on a touch device (such as a touch-screen or track-pad). To receive touch events, widgets have to have the Qt::WA_AcceptTouchEvents attribute set and graphics items need to have the acceptTouchEvents attribute set to true.

When using QAbstractScrollArea based widgets, you should enable the Qt::WA_AcceptTouchEvents attribute on the scroll area's viewport.

Similarly to QMouseEvent, Qt automatically grabs each touch point on the first press inside a widget, and the widget will receive all updates for the touch point until it is released. Note that it is possible for a widget to receive events for numerous touch points, and that multiple widgets may be receiving touch events at the same time.

Event Handling

All touch events are of type QEvent::TouchBegin, QEvent::TouchUpdate, QEvent::TouchEnd or QEvent::TouchCancel. Reimplement QWidget::event() or QAbstractScrollArea::viewportEvent() for widgets and QGraphicsItem::sceneEvent() for items in a graphics view to receive touch events.

Unlike widgets, QWindows receive touch events always, there is no need to opt in. When working directly with a QWindow, it is enough to reimplement QWindow::touchEvent().

The QEvent::TouchUpdate and QEvent::TouchEnd events are sent to the widget or item that accepted the QEvent::TouchBegin event. If the QEvent::TouchBegin event is not accepted and not filtered by an event filter, then no further touch events are sent until the next QEvent::TouchBegin.

Some systems may send an event of type QEvent::TouchCancel. Upon receiving this event applications are requested to ignore the entire active touch sequence. For example in a composited system the compositor may decide to treat certain gestures as system-wide gestures. Whenever such a decision is made (the gesture is recognized), the clients will be notified with a QEvent::TouchCancel event so they can update their state accordingly.

The touchPoints() function returns a list of all touch points contained in the event. Note that this list may be empty, for example in case of a QEvent::TouchCancel event. Information about each touch point can be retrieved using the QTouchEvent::TouchPoint class. The Qt::TouchPointState enum describes the different states that a touch point may have.

Event Delivery and Propagation

By default, QGuiApplication translates the first touch point in a QTouchEvent into a QMouseEvent. This makes it possible to enable touch events on existing widgets that do not normally handle QTouchEvent. See below for information on some special considerations needed when doing this.

QEvent::TouchBegin is the first touch event sent to a widget. The QEvent::TouchBegin event contains a special accept flag that indicates whether the receiver wants the event. By default, the event is accepted. You should call ignore() if the touch event is not handled by your widget. The QEvent::TouchBegin event is propagated up the parent widget chain until a widget accepts it with accept(), or an event filter consumes it. For QGraphicsItems, the QEvent::TouchBegin event is propagated to items under the mouse (similar to mouse event propagation for QGraphicsItems).

Touch Point Grouping

As mentioned above, it is possible that several widgets can be receiving QTouchEvents at the same time. However, Qt makes sure to never send duplicate QEvent::TouchBegin events to the same widget, which could theoretically happen during propagation if, for example, the user touched 2 separate widgets in a QGroupBox and both widgets ignored the QEvent::TouchBegin event.

To avoid this, Qt will group new touch points together using the following rules:

• When the first touch point is detected, the destination widget is determined firstly by the location on screen and secondly by the propagation rules.
• When additional touch points are detected, Qt first looks to see if there are any active touch points on any ancestor or descendent of the widget under the new touch point. If there are, the new touch point is grouped with the first, and the new touch point will be sent in a single QTouchEvent to the widget that handled the first touch point. (The widget under the new touch point will not receive an event).

This makes it possible for sibling widgets to handle touch events independently while making sure that the sequence of QTouchEvents is always correct.

Mouse Events and Touch Event Synthesizing

QTouchEvent delivery is independent from that of QMouseEvent. The application flags Qt::AA_SynthesizeTouchForUnhandledMouseEvents and Qt::AA_SynthesizeMouseForUnhandledTouchEvents can be used to enable or disable automatic synthesizing of touch events to mouse events and mouse events to touch events.

Caveats

• As mentioned above, enabling touch events means multiple widgets can be receiving touch events simultaneously. Combined with the default QWidget::event() handling for QTouchEvents, this gives you great flexibility in designing touch user interfaces. Be aware of the implications. For example, it is possible that the user is moving a QSlider with one finger and pressing a QPushButton with another. The signals emitted by these widgets will be interleaved.
• Recursion into the event loop using one of the exec() methods (e.g., QDialog::exec() or QMenu::exec()) in a QTouchEvent event handler is not supported. Since there are multiple event recipients, recursion may cause problems, including but not limited to lost events and unexpected infinite recursion.
• QTouchEvents are not affected by a mouse grab or an active pop-up widget. The behavior of QTouchEvents is undefined when opening a pop-up or grabbing the mouse while there are more than one active touch points.