val source: ReplaySubject<Int> = ReplaySubject.create()
// subscribe with FirstObserver
source.subscribe { number -> Log.d("ReplaySubject", "FirstObserver onNext: $number") }
source.onNext(1)
source.onNext(2)
source.onNext(3)
// subscribe with SecondObserver
source.subscribe{ number -> Log.d("ReplaySubject", "SecondObserver onNext: $number") }
source.onNext(4)
source.onComplete()

// OUTPUT:
FirstObserver onNext: 1
FirstObserver onNext: 2
FirstObserver onNext: 3
SecondObserver onNext: 1
SecondObserver onNext: 2
SecondObserver onNext: 3
FirstObserver onNext: 4
SecondObserver onNext: 4