Checking and changing the overlapping order of DisplayObjects in a container
getChildIndex method returns the index of the DisplayObject specified by the argument in the order of overlap.
The setChildIndex function repositions the DisplayObject specified in the first argument to the position specified in the second argument. This repositioning can be done in the same way with the addChildAt method.
The swapChildren function swaps the overlapping order (index position) of two DisplayObjects specified by the argument.
The swapChildrenAt function swaps the overlapping order (index position) of two DisplayObjects specified by the argument index. The addChild method is used to reposition a DisplayObject to the top level.