Contents

DragHandler QML Type

Handler for dragging. More...

Import Statement: import QtQuick 2.15
Inherits:

MultiPointHandler

Properties

Signals

Detailed Description

DragHandler is a handler that is used to interactively move an Item. Like other Input Handlers, by default it is fully functional, and manipulates its target.

 import QtQuick 2.12

 Rectangle {
     width: 100
     height: 100
     color: "lightsteelblue"
     DragHandler { }
 }

It has properties to restrict the range of dragging.

If it is declared within one Item but is assigned a different target, then it handles events within the bounds of the parent Item but manipulates the target Item instead:

 import QtQuick 2.12

 Item {
     width: 640
     height: 480

     Rectangle {
         id: feedback
         border.color: "red"
         width: Math.max(10, handler.centroid.ellipseDiameters.width)
         height: Math.max(10, handler.centroid.ellipseDiameters.height)
         radius: Math.max(width, height) / 2
         visible: handler.active
     }

     DragHandler {
         id: handler
         target: feedback
     }
 }

A third way to use it is to set target to null and react to property changes in some other way:

 import QtQuick 2.12

 Item {
     width: 640
     height: 480

     DragHandler {
         id: handler
         target: null
     }

     Text {
         color: handler.active ? "darkgreen" : "black"
         text: handler.centroid.position.x.toFixed(1) + "," + handler.centroid.position.y.toFixed(1)
         x: handler.centroid.position.x - width / 2
         y: handler.centroid.position.y - height
     }
 }

If minimumPointCount and maximumPointCount are set to values larger than 1, the user will need to drag that many fingers in the same direction to start dragging. A multi-finger drag gesture can be detected independently of both a (default) single-finger DragHandler and a PinchHandler on the same Item, and thus can be used to adjust some other feature independently of the usual pinch behavior: for example adjust a tilt transformation, or adjust some other numeric value, if the target is set to null. But if the target is an Item, centroid is the point at which the drag begins and to which the target will be moved (subject to constraints).

At this time, drag-and-drop is not yet supported.

See also Drag and 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:

ConstantDescription
NoModifierNo modifier key is allowed.
ShiftModifierA Shift key on the keyboard must be pressed.
ControlModifierA Ctrl key on the keyboard must be pressed.
AltModifierAn Alt key on the keyboard must be pressed.
MetaModifierA Meta key on the keyboard must be pressed.
KeypadModifierA keypad button must be pressed.
GroupSwitchModifierX11 only (unless activated on Windows by a command line argument). A Mode_switch key on the keyboard must be pressed.
KeyboardModifierMaskThe 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).


cursorShape : Qt::CursorShape

This property holds the cursor shape that will appear whenever the mouse is hovering over the parentItem while active is true.

The available cursor shapes are:

  • Qt.ArrowCursor
  • Qt.UpArrowCursor
  • Qt.CrossCursor
  • Qt.WaitCursor
  • Qt.IBeamCursor
  • Qt.SizeVerCursor
  • Qt.SizeHorCursor
  • Qt.SizeBDiagCursor
  • Qt.SizeFDiagCursor
  • Qt.SizeAllCursor
  • Qt.BlankCursor
  • Qt.SplitVCursor
  • Qt.SplitHCursor
  • Qt.PointingHandCursor
  • Qt.ForbiddenCursor
  • Qt.WhatsThisCursor
  • Qt.BusyCursor
  • Qt.OpenHandCursor
  • Qt.ClosedHandCursor
  • Qt.DragCopyCursor
  • Qt.DragMoveCursor
  • Qt.DragLinkCursor

The default value is not set, which allows the cursor of parentItem to appear. This property can be reset to the same initial condition by setting it to undefined.

Note: When this property has not been set, or has been set to undefined, if you read the value it will return Qt.ArrowCursor.

This property was introduced in Qt 5.15.

See also Qt::CursorShape, QQuickItem::cursor(), and HoverHandler::cursorShape.


dragThreshold : int

The distance in pixels that the user must drag an event point in order to have it treated as a drag gesture.

The default value depends on the platform and screen resolution. It can be reset back to the default value by setting it to undefined. The behavior when a drag gesture begins varies in different handlers.

This property was introduced in Qt 5.15.


enabled : bool

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


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.

ConstantDescription
PointerHandler.TakeOverForbiddenThis handler neither takes from nor gives grab permission to any type of Item or Handler.
PointerHandler.CanTakeOverFromHandlersOfSameTypeThis handler can take the exclusive grab from another handler of the same class.
PointerHandler.CanTakeOverFromHandlersOfDifferentTypeThis handler can take the exclusive grab from any kind of handler.
PointerHandler.CanTakeOverFromAnythingThis handler can take the exclusive grab from any type of Item or Handler.
PointerHandler.ApprovesTakeOverByHandlersOfSameTypeThis handler gives permission for another handler of the same class to take the grab.
PointerHandler.ApprovesTakeOverByHandlersOfDifferentTypeThis handler gives permission for any kind of handler to take the grab.
PointerHandler.ApprovesTakeOverByItemsThis handler gives permission for any kind of Item to take the grab.
PointerHandler.ApprovesCancellationThis handler will allow its grab to be set to null.
PointerHandler.ApprovesTakeOverByAnythingThis 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.


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().


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] translation : QVector2D

The translation since the gesture began.


xAxis group

xAxis.enabled : bool

xAxis.maximum : real

xAxis.minimum : real

xAxis controls the constraints for horizontal dragging.

minimum is the minimum acceptable value of x to be applied to the target. maximum is the maximum acceptable value of x to be applied to the target. If enabled is true, horizontal dragging is allowed.


yAxis group

yAxis.enabled : bool

yAxis.maximum : real

yAxis.minimum : real

yAxis controls the constraints for vertical dragging.

minimum is the minimum acceptable value of y to be applied to the target. maximum is the maximum acceptable value of y to be applied to the target. If enabled is true, vertical dragging is allowed.


Signal Documentation

canceled(EventPoint 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.

Note: The corresponding handler is onCanceled.


grabChanged(GrabTransition 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.

Note: The corresponding handler is onGrabChanged.


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