2009/6/3 j2me_soul <j2me_s...@163.com>:
>
>
> Why the label can't be displaied on the srceen?
> public class rectElement extends UIComponent
> {
> private var lal:Label;
> public function rectElement()
> {
> super();
> }
You don't have to write a constructor if all you're going to do it
call the superclass constructor. Just so you know. You can still do it
if you consider it a good practice.
> override protected function createChildren():void{
> if( lal == null ){
> lal = new Label;
> lal.text = "Label";
> addChild(lal);
> this.explicitWidth = lal.width;
> this.explicitHeight = lal.height;
> }
> }
Normally a component should not set its own
explicitWidth/explicitHeight. These properties are to be set by the
Flex framework. A component should only calculate its measuredWidth
and measuredHeight, based on its contents. The right place to do that
is the measure() implementation.
> override protected function measure():void{
> this.explicitMinWidth = this.explicitMinHeight = 50;
> }
> }
Here you should set the meauredWidth, measuredHeight, and optionally
measuredMinWidth and measuredMinHeight (they can be set to the same as
measuredWidth and measuredHeight respectively).
> <mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"; layout="absolute"
> mlns:local="*">
>
> <local:rectElement >
> /* there is nothing on the screen. Where is the Label ? */
> </local:rectElement>
> </mx:Application>
Okay, you haven't implemented updateDisplayList(). That's where you
have do the layout. For example, in the following way:
function updateDisplayList(unscaledWidth:Number,
unscaledHeight:Number):void
{
super.updateDisplayList(unscaledWidth, unscaledHeight);
lal.setActualSize(unscaledWidth, unscaledHeight);
}
Basically your component object is first measured by the framework,
and then it is assigned a size. You have to lay out the contents of
the component based on the allocated size (unscaledWidth and
unscaledHeight).
It is also a good idea to always call the superclass's
createChildren() method at the end of your implementation.
http://manishjethani.com/blog/why-the-superclasss-createchildren-should-be-called-last/
Manish
