Interface that is also a Display Object - actionscript-3

Question

I have some classes that implement an interface, but also extend the Sprite class:
package {
public interface IState {
function addMonster():void;
}
}
package {
public class Fuzzy extends Sprite implements IState {
public function addMonster():void {
}
}
}
package {
public class LizardSkin extends Sprite implements IState {
public function addMonster():void {
}
}
}
// Document class
package {
public class Main extends MovieClip {
private var state:IState;
public function Main():void {
state = new Fuzzy();
addChild(state);
}
}
}
When I try to addChild(state) I keep getting an error message 1067: Implicit coercion of a value of type IState to an unrelated type flash.display:DisplayObject.
Now I know I've seen examples where a class extends MovieClip / Sprite and implements an interface...what can I do to make it so I can add "state" to the stage but also implement the methods I want??

Answer 1

A simple cast should do it most of the time:
addChild(state as DisplayObject);
The compiler raises that error otherwise because it doesn't assume IState is always implemented by something that is a DisplayObject, but if you can guarantee that condition you can always cast.

Answer 2

What I do in situations like this is use an IDisplayable interface, that looks like this:
public interface IDisplayable
{
function get displayObject():DisplayObject;
}
and its implementation in a Sprite or MovieClip simply looks like this:
public function get displayObject():DisplayObject
{
return this;
}
Your IState objects could then be added to the stage using addChild(state.displayObject);, when your IState interface extends IDisplayable.