Qt 5.13 | Qt Quick | QML Types | TapHandler QML Type

Qt 5.13.2 Reference Documentation

# TapHandler QML Type

Handler for taps and clicks. More...

Import Statement: | import QtQuick 2.13
Inherits: | SinglePointHandler

- List of all members, including inherited members

## Properties

- acceptedButtons : flags
- acceptedDevices : flags
- acceptedModifiers : flags
- acceptedPointerTypes : flags
- active : bool
- enabled : bool
- gesturePolicy : enumeration
- grabPermissions : flags
- longPressThreshold : real
- margin : real
- parent : Item
- point : HandlerPoint
- pressed : bool
- tapCount : int
- target : Item
- timeHeld : real

## Signals

- canceled(point)
- doubleTapped(eventPoint)
- grabChanged(transition, EventPoint point)
- longPressed()
- singleTapped(eventPoint)
- tapCountChanged()
- tapped(eventPoint)

## Detailed Description

TapHandler is a handler for taps on a touchscreen or clicks on a mouse.

Detection of a valid tap gesture depends on gesturePolicy. The default value is DragThreshold, which requires the press and release to be close together in both space and time. In this case, DragHandler is able to function using only a passive grab, and therefore does not interfere with event delivery to any other Items or Input Handlers. So the default gesturePolicy is useful when you want to modify behavior of an existing control or Item by adding a TapHandler with bindings and/or JavaScript callbacks.

Note that buttons (such as QPushButton) are often implemented not to care whether the press and release occur close together: if you press the button and then change your mind, you need to drag all the way off the edge of the button in order to cancel the click. For this use case, set the gesturePolicy to `TapHandler.ReleaseWithinBounds`.

For multi-tap gestures (double-tap, triple-tap etc.), the distance moved must not exceed QPlatformTheme::MouseDoubleClickDistance with mouse and QPlatformTheme::TouchDoubleTapDistance with touch, and the time between taps must not exceed QStyleHints::mouseDoubleClickInterval().

See also MouseArea.

## Property Documentation

acceptedButtons : flags

The mouse buttons which can activate this Pointer Handler.

By default, this property is set to Qt.LeftButton. It can be set to an OR combination of mouse buttons, and will ignore events from other buttons.

For example, a control could be made to respond to left and right clicks in different ways, with two handlers:

      Item {
          TapHandler {
              onTapped: console.log("left clicked")
          }
          TapHandler {
              acceptedButtons: Qt.RightButton
              onTapped: console.log("right clicked")
          }
      }

Note: Tapping on a touchscreen or tapping the stylus on a graphics tablet emulates clicking the left mouse button. This behavior can be altered via acceptedDevices or acceptedPointerTypes.

acceptedDevices : flags

The types of pointing devices that can activate this Pointer Handler.

By default, this property is set to PointerDevice.AllDevices. If you set it to an OR combination of device types, it will ignore events from non-matching devices.

For example, a control could be made to respond to mouse and stylus clicks in one way, and touchscreen taps in another way, with two handlers:

      Item {
         TapHandler {
             acceptedDevices: PointerDevice.Mouse | PointerDevice.Stylus
             onTapped: console.log("clicked")
         }
         TapHandler {
             acceptedDevices: PointerDevice.TouchScreen
             onTapped: console.log("tapped")
         }
      }

acceptedModifiers : flags

If this property is set, it will require the given keyboard modifiers to be pressed in order to react to pointer events, and otherwise ignore them.

If this property is set to `Qt.KeyboardModifierMask` (the default value), then the PointerHandler ignores the modifier keys.

For example, an Item could have two handlers of the same type, one of which is enabled only if the required keyboard modifiers are pressed:

      Item {
         TapHandler {
             acceptedModifiers: Qt.ControlModifier
             onTapped: console.log("control-tapped")
         }
         TapHandler {
             acceptedModifiers: Qt.NoModifier
             onTapped: console.log("tapped")
         }
      }

If you set `acceptedModifiers` to an OR combination of modifier keys, it means all of those modifiers must be pressed to activate the handler:

      Item {
         TapHandler {
             acceptedModifiers: Qt.ControlModifier | Qt.AltModifier | Qt.ShiftModifier
             onTapped: console.log("control-alt-shift-tapped")
         }
      }

The available modifiers are as follows:

Constant | Description
--- | ---
`NoModifier` | No modifier key is allowed.
`ShiftModifier` | A Shift key on the keyboard must be pressed.
`ControlModifier` | A Ctrl key on the keyboard must be pressed.
`AltModifier` | An Alt key on the keyboard must be pressed.
`MetaModifier` | A Meta key on the keyboard must be pressed.
`KeypadModifier` | A keypad button must be pressed.
`GroupSwitchModifier` | X11 only (unless activated on Windows by a command line argument). A Mode_switch key on the keyboard must be pressed.
`KeyboardModifierMask` | The handler does not care which modifiers are pressed.

If you need even more complex behavior than can be achieved with combinations of multiple handlers with multiple modifier flags, you can check the modifiers in JavaScript code:

      Item {
          TapHandler {
              onTapped:
                  switch (point.modifiers) {
                  case Qt.ControlModifier | Qt.AltModifier:
                      console.log("CTRL+ALT");
                      break;
                  case Qt.ControlModifier | Qt.AltModifier | Qt.MetaModifier:
                      console.log("CTRL+META+ALT");
                      break;
                  default:
                      console.log("other modifiers", point.modifiers);
                      break;
                  }
          }
      }

See also Qt::KeyboardModifier.

acceptedPointerTypes : flags

The types of pointing instruments (finger, stylus, eraser, etc.) that can activate this Pointer Handler.

By default, this property is set to PointerDevice.AllPointerTypes. If you set it to an OR combination of device types, it will ignore events from non-matching events.

For example, a control could be made to respond to mouse, touch, and stylus clicks in some way, but delete itself if tapped with an eraser tool on a graphics tablet, with two handlers:

      Rectangle {
         id: rect
         TapHandler {
             acceptedPointerTypes: PointerDevice.GenericPointer | PointerDevice.Finger | PointerDevice.Pen
             onTapped: console.log("clicked")
         }
         TapHandler {
             acceptedPointerTypes: PointerDevice.Eraser
             onTapped: rect.destroy()
         }
      }

[read-only] active : bool

This holds true whenever this Input Handler has taken sole responsibility for handing one or more EventPoints, by successfully taking an exclusive grab of those points. This means that it is keeping its properties up-to-date according to the movements of those Event Points and actively manipulating its target (if any).

enabled : bool

If a PointerHandler is disabled, it will reject all events and no signals will be emitted.

gesturePolicy : enumeration

The spatial constraint for a tap or long press gesture to be recognized, in addition to the constraint that the release must occur before longPressThreshold has elapsed. If these constraints are not satisfied, the tapped signal is not emitted, and tapCount is not incremented. If the spatial constraint is violated, pressed transitions immediately from true to false, regardless of the time held.

Constant | Description
--- | ---

`TapHandler.DragThreshold`

(the default value) The event point must not move significantly. If the mouse, finger or stylus moves past the system-wide drag threshold (QStyleHints::startDragDistance), the tap gesture is canceled, even if the button or finger is still pressed. This policy can be useful whenever TapHandler needs to cooperate with other input handlers (for example DragHandler) or event-handling Items (for example QtQuick Controls), because in this case TapHandler will not take the exclusive grab, but merely a passive grab.

`TapHandler.WithinBounds` | If the event point leaves the bounds of the `parent` Item, the tap gesture is canceled. The TapHandler will take the exclusive grab on press, but will release the grab as soon as the boundary constraint is no longer satisfied.

`TapHandler.ReleaseWithinBounds`

At the time of release (the mouse button is released or the finger is lifted), if the event point is outside the bounds of the `parent` Item, a tap gesture is not recognized. This corresponds to typical behavior for button widgets: you can cancel a click by dragging outside the button, and you can also change your mind by dragging back inside the button before release. Note that it's necessary for TapHandler take the exclusive grab on press and retain it until release in order to detect this gesture.

grabPermissions : flags

This property specifies the permissions when this handler's logic decides to take over the exclusive grab, or when it is asked to approve grab takeover or cancellation by another handler.

Constant | Description
--- | ---
`PointerHandler.TakeOverForbidden` | This handler neither takes from nor gives grab permission to any type of Item or Handler.
`PointerHandler.CanTakeOverFromHandlersOfSameType` | This handler can take the exclusive grab from another handler of the same class.
`PointerHandler.CanTakeOverFromHandlersOfDifferentType` | This handler can take the exclusive grab from any kind of handler.
`PointerHandler.CanTakeOverFromAnything` | This handler can take the exclusive grab from any type of Item or Handler.
`PointerHandler.ApprovesTakeOverByHandlersOfSameType` | This handler gives permission for another handler of the same class to take the grab.
`PointerHandler.ApprovesTakeOverByHandlersOfDifferentType` | This handler gives permission for any kind of handler to take the grab.
`PointerHandler.ApprovesTakeOverByItems` | This handler gives permission for any kind of Item to take the grab.
`PointerHandler.ApprovesCancellation` | This handler will allow its grab to be set to null.
`PointerHandler.ApprovesTakeOverByAnything` | This handler gives permission for any any type of Item or Handler to take the grab.

The default is `PointerHandler.CanTakeOverFromItems | PointerHandler.CanTakeOverFromHandlersOfDifferentType | PointerHandler.ApprovesTakeOverByAnything` which allows most takeover scenarios but avoids e.g. two PinchHandlers fighting over the same touchpoints.

longPressThreshold : real

The time in seconds that an event point must be pressed in order to trigger a long press gesture and emit the longPressed() signal. If the point is released before this time limit, a tap can be detected if the gesturePolicy constraint is satisfied. The default value is QStyleHints::mousePressAndHoldInterval() converted to seconds.

margin : real

The margin beyond the bounds of the parent item within which an event point can activate this handler. For example, on a PinchHandler where the target is also the `parent`, it's useful to set this to a distance at least half the width of a typical user's finger, so that if the `parent` has been scaled down to a very small size, the pinch gesture is still possible. Or, if a TapHandler-based button is placed near the screen edge, it can be used to comply with Fitts's Law: react to mouse clicks at the screen edge even though the button is visually spaced away from the edge by a few pixels.

The default value is 0.

[read-only] parent : Item

The Item which is the scope of the handler; the Item in which it was declared. The handler will handle events on behalf of this Item, which means a pointer event is relevant if at least one of its event points occurs within the Item's interior. Initially target() is the same, but it can be reassigned.

See also target and QObject::parent().

[read-only] point : HandlerPoint

The event point currently being handled. When no point is currently being handled, this object is reset to default values (all coordinates are 0).

[read-only] pressed : bool

Holds true whenever the mouse or touch point is pressed, and any movement since the press is compliant with the current gesturePolicy. When the event point is released or the policy is violated, pressed will change to false.

[read-only] tapCount : int

The number of taps which have occurred within the time and space constraints to be considered a single gesture. For example, to detect a triple-tap, you can write:

      Rectangle {
          width: 100; height: 30
          signal tripleTap
          TapHandler {
              acceptedButtons: Qt.AllButtons
              onTapped: if (tapCount == 3) tripleTap()
          }
      }

target : Item

The Item which this handler will manipulate.

By default, it is the same as the parent, the Item within which the handler is declared. However, it can sometimes be useful to set the target to a different Item, in order to handle events within one item but manipulate another; or to `null`, to disable the default behavior and do something else instead.

[read-only] timeHeld : real

The amount of time in seconds that a pressed point has been held, without moving beyond the drag threshold. It will be updated at least once per frame rendered, which enables rendering an animation showing the progress towards an action which will be triggered by a long-press. It is also possible to trigger one of a series of actions depending on how long the press is held.

A value of less than zero means no point is being held within this handler's Item.

## Signal Documentation

canceled(point)

If this handler has already grabbed the given point, this signal is emitted when the grab is stolen by a different Pointer Handler or Item.

doubleTapped(eventPoint)

This signal is emitted when the `parent` Item is tapped twice within a short span of time (QStyleHints::mouseDoubleClickInterval) and distance (QPlatformTheme::MouseDoubleClickDistance or QPlatformTheme::TouchDoubleTapDistance). This signal always occurs after singleTapped, tapped, and tapCountChanged. The eventPoint signal parameter contains information from the release event about the point that was tapped.

This signal was introduced in Qt 5.11.

grabChanged(transition, EventPoint point)

This signal is emitted when the grab has changed in some way which is relevant to this handler.

The transition (verb) tells what happened. The point (object) is the point that was grabbed or ungrabbed.

longPressed()

This signal is emitted when the `parent` Item is pressed and held for a time period greater than longPressThreshold. That is, if you press and hold a touchpoint or button, while any movement does not exceed the drag threshold, then the `longPressed` signal will be emitted at the time that timeHeld exceeds longPressThreshold.

singleTapped(eventPoint)

This signal is emitted when the `parent` Item is tapped once. After an amount of time greater than QStyleHints::mouseDoubleClickInterval, it can be tapped again; but if the time until the next tap is less, tapCount will increase. The eventPoint signal parameter contains information from the release event about the point that was tapped.

This signal was introduced in Qt 5.11.

tapCountChanged()

This signal is emitted when the `parent` Item is tapped once or more (within a specified time and distance span) and when the present `tapCount` differs from the previous `tapCount`.

tapped(eventPoint)

This signal is emitted each time the `parent` Item is tapped.

That is, if you press and release a touchpoint or button within a time period less than longPressThreshold, while any movement does not exceed the drag threshold, then the `tapped` signal will be emitted at the time of release. The eventPoint signal parameter contains information from the release event about the point that was tapped:

      import QtQuick 2.12

      Rectangle {
          width: 100
          height: 100

          TapHandler {
              acceptedButtons: Qt.LeftButton | Qt.RightButton
              onTapped: console.log("tapped", eventPoint.event.device.name,
                                    "button", eventPoint.event.button,
                                    "@", eventPoint.scenePosition)
          }
      }