Difference in QML between Window and Item in parent-children relationship

问题

I'm wondering why for Item this works:

Item {
    id: root
    width: 640
    height: 480
    MouseArea {
        anchors.fill: (root or parent. It doesn't matter)
        onClicked: console.log("clicked")
    }
}

But for Window it doesn't. Only anchoring by parent will work, but for anchoring by id it will fail.

回答1:

According to the documentation, anchors.fill requires the argument either to be or to identify an Item-derived object.

From here you can follow the inheritance chain of Window and see that it is not actually an Item.
Moreover, from here you can see that:

If you assign an Item to the data list, it becomes a child of the Window's contentItem, so that it appears inside the window.

where for the contentItem we have that:

This attached property holds the invisible root item of the scene or null if the item is not in a window. 

Because of that, it makes sense what you are observing:

• the id of the Window does not identify an Item
  → anchoring by id results in an error
• the parent is actually the hidden Item-derived contentItem to which each child of Window is automatically parented
  → anchoring by parent correctly works

来源：https://stackoverflow.com/questions/34527083/difference-in-qml-between-window-and-item-in-parent-children-relationship