Subject：它既是Observable，又是observer。也就是既可以發送事件，也可以接收事件。
PublishSubject<Integer> source = PublishSubject.create(); //PublicSubject：接收到訂閱之后的所有數據。
// It will get 1, 2, 3, 4 and onComplete
source.subscribe(getFirstObserver());
source.onNext(1);
source.onNext(2);
source.onNext(3);
// It will get 4 and onComplete for second observer also.
source.subscribe(getSecondObserver());
source.onNext(4);
source.onComplete();
ReplaySubject<Integer> source = ReplaySubject.create(); //ReplaySubject：接收到所有的數據，包括訂閱之前的所有數據和訂閱之后的所有數據。
// It will get 1, 2, 3, 4
source.subscribe(getFirstObserver());
source.onNext(1);
source.onNext(2);
source.onNext(3);
source.onNext(4);
source.onComplete();
// It will also get 1, 2, 3, 4 as we have used replay Subject
source.subscribe(getSecondObserver());
BehaviorSubject<Integer> source = BehaviorSubject.create(); //BehaviorSubject：接收到訂閱前的最后一條數據和訂閱后的所有數據。
// It will get 1, 2, 3, 4 and onComplete
source.subscribe(getFirstObserver());
source.onNext(1);
source.onNext(2);
source.onNext(3);
// It will get 3(last emitted)and 4(subsequent item) and onComplete
source.subscribe(getSecondObserver());
source.onNext(4);
source.onComplete();
AsyncSubject<Integer> source = AsyncSubject.create(); //AsyncSubject：不管在什么位置訂閱，都只接接收到最后一條數據
// It will get only 4 and onComplete
source.subscribe(getFirstObserver());
source.onNext(1);
source.onNext(2);
source.onNext(3);
// It will also get only get 4 and onComplete
source.subscribe(getSecondObserver());
source.onNext(4);
source.onComplete();
