observableObject

int subscribeCount = 0;
class TestObservable : Observable!int
{
    Disposable subscribe(Observer!int observer)
    {
        subscribeCount++;
        return NopDisposable.instance;
    }
}

auto test = new TestObservable;
auto observable = observableObject!int(test);
assert(observable is test);
assert(subscribeCount == 0);
auto d = observable.subscribe(null);
assert(subscribeCount == 1);