a.ts

class Delegator<T> {
    protected delegate: T;

    getDelegate() {
        return this.delegate;
    }

    setDelegate(delegate: T) {
        this.delegate = delegate;
    }
}

interface FooDelegate {
    barCalled(foo: Foo);
}

class AbstractFooDelegate implements FooDelegate {
    public static readonly defaultInstance: AbstractFooDelegate = new AbstractFooDelegate();
    barCalled(foo: Foo) {}
}

class Foo extends Delegator<FooDelegate> {
    private _name: string;
    constructor(name: string) { super(); this._name = name; this.delegate = AbstractFooDelegate.defaultInstance }
    name(): string { return this._name }
    bar(): void { this.delegate.barCalled(this) }
}

class ConcreteFooDelegate implements FooDelegate {
    barCalled(foo: Foo) { console.log('Bar called on ' + foo.name()); }
}

const d = new ConcreteFooDelegate();
const f = new Foo('baz');

f.setDelegate(d);

console.log('Calling bar');
f.bar();