Qt 5.8 | Qt Quick | QML Dynamic View Ordering Tutorial 3 - Moving Dragged Items

Qt 5.8.0 Reference Documentation

QML Dynamic View Ordering Tutorial 2 - Dragging View Items ◦ QML Dynamic View Ordering Tutorial 4 - Sorting Items

# QML Dynamic View Ordering Tutorial 3 - Moving Dragged Items

The next step in our application to move items within the list as they're dragged so that we can re-order the list. To achieve this we introduce three new types to our application; DelegateModel, Drag and DropArea.

      Rectangle {
          id: root

          width: 300; height: 400

          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
                      Drag.active: dragArea.held
                      Drag.source: dragArea
                      Drag.hotSpot.x: width / 2
                      Drag.hotSpot.y: height / 2
                  }
                  DropArea {
                      anchors { fill: parent; margins: 10 }

                      onEntered: {
                          visualModel.items.move(
                                  drag.source.DelegateModel.itemsIndex,
                                  dragArea.DelegateModel.itemsIndex)
                      }
                  }
              }
          }
      }

### Walkthrough

In order to re-order the view we need to determine when one item has been dragged over another. With the Drag attached property we can generate events that are sent to the scene graph whenever the item it is attached to moves.

                      Drag.active: dragArea.held
                      Drag.source: dragArea
                      Drag.hotSpot.x: width / 2
                      Drag.hotSpot.y: height / 2

Drag events are only sent while the active property is true, so in this example the first event would be sent when the delegate was held with additional event sents when dragging. The hotSpot property specifies the relative position of the drag events within the dragged item, the center of the item in this instance.

Then we use a DropArea in each view item to determine when the hot spot of the dragged item intersects another item, when a drag enters one of these DropAreas we can move the dragged item to the index of the item it was dragged over.

                  DropArea {
                      anchors { fill: parent; margins: 10 }

                      onEntered: {
                          visualModel.items.move(
                                  drag.source.DelegateModel.itemsIndex,
                                  dragArea.DelegateModel.itemsIndex)
                      }
                  }

To move the items within the view we use a DelegateModel. The DelegateModel type is used by the view types to instantiate delegate items from model data and when constructed explicitly can be used to filter and re-order the model items provided to ListView. The items property of DelegateModel provides access to the view's items and allows us to change the visible order without modifying the source model. To determine the current visible index of the items we use itemsIndex {itemsIndex} property on the DelegateModel attached property of the delegate item.

To utilize a DelegateModel with a ListView we bind it to the model property of the view and bind the model and delegate to the DelegateModel.

          DelegateModel {
              id: visualModel

              model: PetsModel {}
              delegate: dragDelegate
          }

          ListView {
              id: view

              anchors { fill: parent; margins: 2 }

              model: visualModel

              spacing: 4
              cacheBuffer: 50
          }

Files:

- tutorials/dynamicview/dynamicview3/PetsModel.qml
- tutorials/dynamicview/dynamicview3/dynamicview.qml
- tutorials/dynamicview/dynamicview3/dynamicview3.qmlproject