QTouchEvent Class▲

  • Header: QTouchEvent

  • CMake:

    find_package(Qt6 REQUIRED COMPONENTS Gui)

    target_link_libraries(mytarget PRIVATE Qt6::Gui)

  • qmake: QT += gui

  • Inherits: QPointerEvent

  • Inherited By:

  • Group: QTouchEvent is part of events, touch

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 pointCount() and point() functions can be used to access and iterate individual touch points.

The points() 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. Each point is an instance of the QEventPoint class. The QEventPoint::State enum describes the different states that a touch point may have.

The list of points() will never be partial: A touch event will always contain a touch point for each existing physical touch contacts targeting the window or widget to which the event is sent. For instance, assuming that all touches target the same window or widget, an event with a condition of points().count()==2 is guaranteed to imply that the number of fingers touching the touchscreen or touchpad is exactly two.

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(