[flexcomponents] updateDisplayList, what's the right way to do this...

[Tom Bray]

So, I have this component that extends Canvas so that when I scale it's child, the scrollbars of the Canvas adjust to keep the child in the center (in other words, it lets me zoom to center).

It works except for the first time I call my moveToCenter function because the maxHorizontalScrollPosition is still 0 (see the comments below):

    public function moveToCenter():void
    {
        trace( "moveToCenter()" );
        var x:Number = useFixedCenter ? content.width/2 : contentCenter.x;
        var y:Number = useFixedCenter ? content.height/2 : contentCenter.y;
        var sx:Number = useFixedCenter ? 1 : content.scaleX;
        var sy:Number = useFixedCenter ? 1 : content.scaleY;
        horizontalScrollPosition = (x * sx) - (width/2);
        trace( "horizontalScrollPosition: " + horizontalScrollPosition ); //this gives me the right number, but...
        trace( "max hsp: " + this.maxHorizontalScrollPosition ); // the max horizontal scroll position is still zero the first time this function is called, so the scrollbar doesn't actually move
        verticalScrollPosition = (y * sy) - (height/2);
    }

If I store the new horizonatlScrollPosition in an instance variable and then set the horizontalScrollPosition property in updateDisplayList, it works but I know that's not the right way to do this.  The following code shows what I mean:

    public function moveToCenter():void
    {
        trace( "moveToCenter()" );
        var x:Number = useFixedCenter ? content.width/2 : contentCenter.x;
        var y:Number = useFixedCenter ? content.height/2 : contentCenter.y;
        var sx:Number = useFixedCenter ? 1 : content.scaleX;
        var sy:Number = useFixedCenter ? 1 : content.scaleY;
        trace( "x: " + x );   
        horizontalScrollPosition = (x * sx) - (width/2);
        trace( "horizontalScrollPosition: " + horizontalScrollPosition );

      //I added this to hack it so it works the first time.  see updateDisplayList below
        _hsp = horizontalScrollPosition;
        trace( "max hsp: " + this.maxHorizontalScrollPosition );
        verticalScrollPosition = (y * sy) - (height/2);
    }


    override protected function updateDisplayList( unscaledWidth:Number, unscaledHeight:Number ):void
    {
        super.updateDisplayList( unscaledWidth, unscaledHeight );
        trace( "update display list" );
        trace( maxHorizontalScrollPosition + ", " + horizontalScrollPosition );
        horizontalScrollPosition = _hsp;
    }

What's the right way to do this?

Thanks,

Tom
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