QML Dynamic View Ordering Tutorial 2 - Dragging View Items

Now that we have a visible list of items we want to be able to interact with them. We’ll start by extending the delegate so the visible content can be dragged up and down the screen. The updated delegate looks like this:

Component {
    id: dragDelegate


    MouseArea {
        id: dragArea

        property bool held: false

        anchors { left: parent.left; right: parent.right }
        height: content.height

        drag.target: held ? content : undefined
        drag.axis: Drag.YAxis

        onPressAndHold: held = true
        onReleased: held = false

        Rectangle {
            id: content

            anchors {
                horizontalCenter: parent.horizontalCenter
                verticalCenter: parent.verticalCenter
            }
            width: dragArea.width; height: column.implicitHeight + 4

            border.width: 1
            border.color: "lightsteelblue"

            color: dragArea.held ? "lightsteelblue" : "white"
            Behavior on color { ColorAnimation { duration: 100 } }

            radius: 2

            states: State {
                when: dragArea.held

                ParentChange { target: content; parent: root }
                AnchorChanges {
                    target: content
                    anchors { horizontalCenter: undefined; verticalCenter: undefined }
                }
            }

            Column {
                id: column
                anchors { fill: parent; margins: 2 }

                Text { text: 'Name: ' + name }
                Text { text: 'Type: ' + type }
                Text { text: 'Age: ' + age }
                Text { text: 'Size: ' + size }
            }

        }
    }

}

Walkthrough

The major change here is the root item of the delegate is now a MouseArea which provides handlers for mouse events and will allow us to drag the delegate’s content item. It also acts as a container for the content item which is important as a delegate’s root item is positioned by the view and cannot be moved by other means.

MouseArea {
    id: dragArea

    property bool held: false

    anchors { left: parent.left; right: parent.right }
    height: content.height

    drag.target: held ? content : undefined
    drag.axis: Drag.YAxis

    onPressAndHold: held = true
    onReleased: held = false

    Rectangle {
        id: content                    }
}

Dragging the content item is enabled by binding it to the MouseArea ‘s drag.target property. Because we still want the view to be flickable we wait until the MouseArea ‘s pressAndHold signal is emitted before binding the drag target. This way when mouse moves before the hold timeout has expired it is interpreted as moving the list and if it moves after it is interpreted as dragging an item. To make it more obvious to the user when an item can be dragged we’ll change the background color of the content item when the timeout has expired.

color: dragArea.held ? "lightsteelblue" : "white"
Behavior on color { ColorAnimation { duration: 100 } }

The other thing we’ll need to do before an item can be dragged is to unset any anchors on the content item so it can be freely moved around. We do this in a state change that is triggered when the delegate item is held, at the same time we can reparent the content item to the root item so that is above other items in the stacking order and isn’t obscured as it is dragged around.

states: State {
    when: dragArea.held

    ParentChange { target: content; parent: root }
    AnchorChanges {
        target: content
        anchors { horizontalCenter: undefined; verticalCenter: undefined }
    }
}

Example project @ code.qt.io