Ans:

Invalidation life-cycle:

*Flex imposes deferred validation on the Flash API.

Goal: defer screen updates until all properties have been set

*Three main method pairs to be aware of:

1. invalidateProperties() -> commitProperties: Any property changes
2. invalidateSize() -> measure(): Changes to width or height
3. invalidateDisplayList() -> updateDisplayList(): Changes to child component size or position.

for example:

 public class BadList extends VBox
    {
    private var _dataprovider : ArrayCollection
    public function set dataprovider( arr: ArrayCollection ):void{
    this._dataprovider = arr;
    arr.addEventListner( CollectionEvent COLLECTION_CHANGE,
 dataProviderChangeHandler);
    }
    private function dataProviderChangeHandler(e:Event):void{
    this.removeAllChildren();
    for each(var n:Number in this._dataProvider){
    var l:Label = new Label();
    l.text = n.toString();
    this.addChild(l);
    }
    }
    public function BadList(){
    }
    }