So if you've got any sort of programming background, you'd probably notice the omission of the abstract  keyword in AS3. As it relates to methods, an abstract  method in a class is one that should never be called directly, and should only be overwritten by a class which extends the original class.

Here's a technique for replicating that in AS3.

The root class:

package {
    public class Popup extends Sprite {       
        public function reveal():void {
            // to be defined in subclasses
            throw new Error("Abstract method reveal must be overridden");
        }
    }
}

And the class that extends it:

package {
    public class VideoPopup extends Popup {       
        public override function reveal():void {
            this.visible = true;
        }
    }
}

So if the base reveal  method is called, an error is thrown.