How do you work around the need to cast an interfaced object back to its base class?

Answer 1

Okay, I'm anaswering generally, because you said, "Is this really just a phenomenon in Flex/AS3?".

In your init method, obviously you're always calling addChild with foo. That means foo must always be an instance of DisplayObject. You also want it to be an instance of IFooable (though it's not clear here why). Since DisplayObject is a class, you would consider using a subclass of DisplayObject (e.g. FooableDisplayObject), that implemented IFooable. In Java, this would the below. I'm not familiar with AS, but I think this shows there's not any general flaw in interfaces here.

interface IFooable
{
    public void runFoo();
}

class DisplayObject
{

}

abstract class FooableDisplayObject extends DisplayObject implements IFooable
{

}

class Foo extends FooableDisplayObject
{
    public void runFoo()
    {

    }
}

public void init()
{
    FooableDisplayObject foo = new Foo();
    foo.percentHeight = 100;

    addChild(foo);
}

Answer 2

I think this is a place where Flex's/Flash's API is not correct. I think that addChild should take an interface not a class. However since that is not the case you have to cast it. Another option would be to monkey patch UIComponent so that it takes an interface or maybe add another method like addIChild(IUIComponent). But that's messy. So I recommend you file a bug.

Answer 3

@James Ward, That's definitely something I wish was in the language, probably a interface IDisplayObject. That would solve a lot of issues in OOP display programing in AS3.

In regards the the original question, something I've used in the past, and have seen mentioned on www.as3dp.com is to include a getDisplay():DisplayObject method in the interface, which would typically return "this" by its implementor. It's less than ideal, but works.

@Matthew Flaschen, While we don't have Abstarct Classes native to AS3, common practice is to name the class with the word Abstract ie: AbstarctMyObject, and then just treat it like the abstarct objects in Java and other languages. Our want for true abstarct classes is something the Flash player team is well aware of, and we'll likly see it in the next version of the ActionScript language.