let behaviorSubject = BehaviorSubject(value:0)
_ = behaviorSubject.subscribe {
    print ("first subscriber \($0)")
}
behaviorSubject.onNext(1)

_ = behaviorSubject.subscribe {
    print ("second subscriber \($0)")
}
behaviorSubject.onNext(2)
behaviorSubject.onNext(3)
_ = behaviorSubject.subscribe {
    print ("third subscriber \($0)")
}
behaviorSubject.onNext(4)