Предположим, что это класс документа в b.swf:
public Class B extends MovieClip {
protected var _theVar:String;
public function get theVar():String {
return _theVar;
}
public function set theVar(value:String):void {
if (value != _theVar) {
_theVar=value;
//do something here with the new value
}
}
public function B () {
super();
}
}
в документе класса а:
//this will be inside a function that detects that the content has been loaded
var bContent = yourLoader.loaderInfo.content as B;
bContent.theVar = 'some value';