Qt 4.8: QML Mouse Events

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- Qt 4.8
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# QML Mouse Events

## Mouse Elements

- MouseArea Element
- MouseEvent Object

## Mouse Event Handling

QML uses signals and handlers to deliver mouse interactions. Specifically, the MouseArea and MouseEvent elements provide QML components with signal handlers to accept mouse events within a defined area.

## Defining a Mouse Area

The MouseArea element receives events within a defined area. One quick way to define this area is to anchor the `MouseArea` to its parent's area using the `anchors.fill` property. If the parent is a Rectangle (or any Item component), then the MouseArea will fill the area defined by the parent's dimensions. Alternatively, an area smaller or larger than the parent is definable.

     Rectangle {
         id: button1
         width: 100; height: 100

         MouseArea {
             anchors.fill: parent
             onClicked: console.log("button clicked")
         }
         MouseArea {
             width:150; height: 75
             onClicked: console.log("irregular area clicked")
         }
     }

## Receiving Events

The MouseArea element provides signals and handlers to detect different mouse events. The MouseArea element documentation describes these gestures in greater detail:

- canceled
- clicked
- doubleClicked
- entered
- exited
- positionChanged
- pressAndHold
- pressed
- released

These signals have signal handlers that are invoked when the signals are emitted.

         MouseArea {
             anchors.fill: parent
             onClicked: console.log("area clicked")
             onDoubleClicked: console.log("area double clicked")
             onEntered: console.log("mouse entered the area")
             onExited: console.log("mouse left the area")
         }

## Enabling Gestures

Some mouse gestures and button clicks need to be enabled before they send or receive events. Certain MouseArea and MouseEvent properties enable these gestures.

To listen to (or explicitly ignore) a certain mouse button, set the appropriate mouse button to the acceptedButtons property.

Naturally, the mouse events, such as button presses and mouse positions, are sent during a mouse click. For example, the `containsMouse` property will only retrieve its correct value during a mouse press. The hoverEnabled will enable mouse events and positioning even when there are no mouse button presses. Setting the `hoverEnabled` property to `true`, in turn will enable the `entered`, `exited`, and `positionChanged` signal and their respective signal handlers.

         MouseArea {
             hoverEnabled: true
             acceptedButtons: Qt.LeftButton | Qt.RightButton
             onEntered: console.log("mouse entered the area")
             onExited: console.log("mouse left the area")
         }

Additionally, to disable the whole mouse area, set the `MouseArea` element's `enabled` property to `false`.

## MouseEvent Object

Signals and their handlers receive a MouseEvent object as a parameter. The `mouse` object contain information about the mouse event. For example, the mouse button that started the event is queried through the mouse.button property.

The `MouseEvent` object can also ignore a mouse event using its `accepted` property.

### Accepting Further Signals

Many of the signals are sent multiple times to reflect various mouse events such as double clicking. To facilitate the classification of mouse clicks, the MouseEvent object has an `accepted` property to disable the event propagation.

To learn more about QML's event system, please read the QML Signal and Handler Event System document.

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