How to replace the default property in qt quick?

[Accelerated]

Hi everyone,
I want to replace the property like header&ToolBar.
For example, there is a property in ApplicationWindow called header. It can be replaced by ToolBar. like this:

ApplicationWindow {
    visible:true

  header: ToolBar {
      RowLayout {
          anchors.fill: parent
          Item { Layout.fillWidth: true }
          Switch {
              checked: true
              text: qsTr("Notifications")
         }
      }
   }
}

The new ToolBar replace the ApplicationWindow‘s header, and it always at the top of ApplicationWindow.

And now, i have some codes:

Bar{
    id: titleBar
    property alias title: titleText
    //left controls

    property Item leftControls:
        ControlsList{
        parent: titleBar
        anchors.left: parent.left
        anchors.verticalCenter: parent.verticalCenter
    }

    //right controls
    property Item rightControls:
        ControlsList{
            parent: titleBar
            anchors.right: parent.right
            anchors.verticalCenter: parent.verticalCenter
    }

How could i replace the leftControls or rightControls and don't change their layout? (。・・)ノ