Changing Child Index of MovieClips based on their Y Position?

Question

I'm writing a game with Flash CS5/AS 3.0 that tries to simulate depth of field by drawing all the relevant Movie Clips based on their Y position in ascending order, i.e. things lower on the stage overlap things higher on the stage. A MovieClip with a Y position of 10 would therefore need to have a lower index compared to a MovieClip with a Y position of 20, so the second one gets drawn on top of the first.

I wrote a quick and dirty function just to test this. During the trace, I've noticed that the truck's index hits 0 when I go near the top of the stage, but if I go too far up it will completely disappear from the stage. Trace then starts generating this error:

ArgumentError: Error #2025: The supplied DisplayObject must be a child of the caller.
        at flash.display::DisplayObjectContainer/getChildIndex()
        at EICT::Game/ReorganizeDisplayIndexes()
        at EICT::Game/loop()

theTruck is a MovieClip of the player controlled vehicle Enemies, Cones, Rocks are all arrays which contain MovieClips

None of them have event listeners.

    private function ReorganizeDisplayIndexes(): void
    {
        var drawableObjects:Array = new Array();
        drawableObjects.push(theTruck);
        drawableObjects = drawableObjects.concat(Enemies, Rocks, Bushes);
        drawableObjects.sortOn("y", Array.DESCENDING | Array.NUMERIC);
        drawableObjects.reverse();
        trace(collisionLayer.getChildIndex(theTruck));
        for (var a:int = collisionLayer.numChildren - 1; a >= 0; a--)
        {
            collisionLayer.removeChildAt(a);
        }
        for (var i:int = 0; i < drawableObjects.length; i++)
        {
            collisionLayer.addChild(drawableObjects[i]);
        }
    }

Answer 1

Hint: You don't need to remove the child first. When you use addChild() on an object, it is automatically re-added to the next highest depth.

That said, you'll just need to do something like this:

drawableObjects.sortOn("y");

for each(var i:DisplayObject in drawableObjects)
{
    if(i.parent)
        i.parent.addChild(i);
}

Answer 2

Use setChildIndex instead of removing and re-adding:

for (var a:int = collisionLayer.numChildren - 1; i >= 0; i--)
{
    collisionLayer.setChildIndex(drawableObjects[i], i);
}

Also, it's a bit wasteful to first order the sort in descending and then reversing the array. Sort it ascending to begin with!