User Guide

116 Creating Advanced MXML Components

Methods that are implemented in the custom component must have the same return type as

their corresponding methods in the interface. If no return type is specified in the interface, the

implementing methods can declare any return type.

About implementing IMXMLObject

You cannot define a constructor for an MXML component. If you do, the Flex compiler

issues an error message that specifies that you defined a duplicate function.

For many types of Flex components, you can use an event listener instead of a constructor. For

example, depending on what you want to do, you can write an event listener for the

preinitialize, initialize, or creationComplete event to replace the constructor.

These events are all defined by the UIComponent class, and inherited by all of its subclasses.

If you create an MXML component that is not a subclass of UIComponent, you cannot take

advantage of these events. You can instead implement the IMXMLObject interface in your

MXML component, and then implement the

IMXMLObject.initialized() method, as the

following example shows:

<?xml version="1.0"?>

<mx:Object xmlns:mx="http://www.adobe.com/2006/mxml"

implements="mx.core.IMXMLObject">

<mx:Script>

<![CDATA[

// Implement the IMXMLObject.initialized() method.

public function initialized(document:Object, id:String):void {

trace("initialized, x = " + x);

}

]]>

</mx:Script>

<mx:Number id="y"/>

<mx:Number id="z"/>

<mx:Number id="x"/>

</mx:Object>