Question

How would one create a custom MXML component in flex which is based on an existing component but draws an overlay over this existing component in certain situations.

Ideally, the new component should be based on (derive from) the exiting component so that occurrences of the existing component could just be swapped out with the new one.

I tried to override updateDisplayList() in the new component and to paint the overlay using this.graphics. This resulted in the overlay being drawn underneath the children of the existing component. I also tried to do the drawing upon receiving a render-event which lead to similar results.

When the external condition which should trigger the display of the overlay changes, I call invalidateDisplayList() on my new component. That works to trigger the drawing for both cases described above. The remaining problem seems to be to figure out how to draw on top of all the other components once they are added.

The following example should illustrate what I tried to do; when overlayEnabled was set and the component’s invalidateDisplayList() method was called, the red rectangle would get painted in the background….

// NewComponent.mxml
<ExistingComponent ...>
    <mx:Script>
    ...

        public var overlayEnabled:Boolean;

        override protected updateDisplayList(...) {
            super.updateDisplayList(...)
            if (overlayEnabled) {
                var g:Graphics = this.graphics;     
                g.beginFill(0xFF0000, 0.5);
                g.drawRect(0, 0, width, height);
                g.endFill();
            }
        }
    ...
    </mx:Script>
</ExistingComponent>

Also, feel free to suggest different approaches.

Answer 1

You will have to add a DisplayObject for you overlay and insure when you call updateDisplayList that it is place on the top of the other.

        public var overlayEnabled:Boolean;

        public overlayHolder:(whatever display object you want to use)

        override protected updateDisplayList(...) {
            super.updateDisplayList(...)
            if (overlayEnabled) {
                if (overlayHolder.parent != this){
                 addChild(overlayHolder);
                } else {
                  if (numChildren > 0)
                     setChildIndex(overlayHolder, numChildren-1);
                }
                var g:Graphics = overlayHolder.graphics;     
                g.beginFill(0xFF0000, 0.5);
                g.drawRect(0, 0, width, height);
                g.endFill();
            } else if (overlayHolder.parent == this) {
              removeChild(overlayHolder);
            }
        }

Edit:
One property you can use to add your overlay to the display list can be the rawchildren:

package {
    import flash.display.Graphics;
    import flash.display.Sprite;

    import mx.containers.VBox;

    public class MyVBox extends VBox {
        public var overlayEnabled : Boolean = true;
        public var overlay : Sprite = new Sprite();

        public function MyVBox() {
            super();
        }

        protected override function updateDisplayList(unscaledWidth : Number, unscaledHeight : Number) : void {
            super.updateDisplayList(unscaledWidth, unscaledHeight);
            if (overlayEnabled) {
                if (overlay.parent != this) {
                    rawChildren.addChild(overlay);
                } else {
                    if (rawChildren.numChildren > 0)
                        rawChildren.setChildIndex(overlay, rawChildren.numChildren - 1);
                }
                var g : Graphics = overlay.graphics;
                g.beginFill(0xFF0000, 0.5);
                g.drawRect(0, 0, width, height);
                g.endFill();
            } else if (overlay.parent == this) {
                rawChildren.removeChild(overlay);
            }
        }
    }
}