Is there any way to embed an interface in another ...

# getting-started

[rob42]

Is there any way to embed an interface in another class? Put a slightly different way, a class that inherits from an implementation of an interface.

In Go, I could do something like

type MyInterface interface {
  DoSomething() int
}

type MyObject struct {
  MyInterface
}

bar := SomeImplementationOfMyInterface()
foo := MyObject { MyInterface: bar }
foo.DoSomething() // -> calls bar.DoSomething()

The closest I can get in Kotlin seems to be manually proxying methods to the wrapped implementation:

class MyObject(val underlying: MyInterface): MyInterface {
  override fun DoSomething() = underlying.DoSomething()
}

Which is tedious and breaks if MyInterface changes.
I want to end up with an object that delegates everything to the underlying interface, but adds additional methods.

[Joffrey]

Kotlin has interface delegation, I guess this is what you're looking for:

class MyObject(val underlying: MyInterface): MyInterface by underlying {
    // you may add or override methods here
}

With the MyInterface by underlying declaration, MyObject automatically implements all methods from MyInterace by delegating their implementation to underlying. You can override some of those if you want to do something different than just delegating. You can also of course add anything else as well in the body.

[rob42]

Ooh, that looks exactly what I was looking for!

[rob42]

Thank you!

[Joffrey]

My pleasure 🙂

[Larry Garfield]

What happens if the object/interface has a “return this” in it? Is it the wrapper that’s returned, or the underlying object?

[Joffrey]

I haven't checked, but I believe the implementation really is exactly equivalent to delegating every method call to the underlying object, so if that object returns this it will be the underlying object.

[Larry Garfield]

That’s what I’d expect. And why such delegation is tricky because it becomes very leaky. 🙂