AsyncSubject

一个AsyncSubject只在原始Observable完成后，发射来自原始Observable的最后一个值。它会把这最后一个值发射给任何后续的观察者。

  let disposeBag = DisposeBag()

  let subject = AsyncSubject()

  subject.onNext(1)

  subject.subscribe(onNext: { int in
print("observerA: \(int)")
  }, onCompleted: {
print("observerA: onCompleted")
  }).disposed(by: disposeBag)

  subject.onNext(2)

  subject.subscribe(onNext: { int in
print("observerB: \(int)")
  }, onCompleted: {
print("observerB: onCompleted")
  }).disposed(by: disposeBag)

  subject.onNext(3)

  subject.subscribe(onNext: { int in
print("observerC: \(int)")
  }, onCompleted: {
print("observerC: onCompleted")
  }).disposed(by: disposeBag)

  subject.onCompleted()

  subject.onNext(4)

  subject.subscribe(onNext: { int in
print("observerD: \(int)")
  }, onCompleted: {
print("observerD: onCompleted")
  }).disposed(by: disposeBag)

输出：

observerA: 3
observerB: 3
observerC: 3
observerA: onCompleted
observerB: onCompleted
observerC: onCompleted
observerD: 3
observerD: onCompleted

BehaviorSubject

当观察者订阅BehaviorSubject时，它开始发射原始Observable最近发射的数据，然后继续发射其它任何来自原始Observable的数据。

  let disposeBag = DisposeBag()

  let subject = BehaviorSubject(value: 0)

  subject.onNext(1)

  subject.subscribe(onNext: { int in
print("observerA: \(int)")
  }, onCompleted: {
print("observerA: onCompleted")
  }).disposed(by: disposeBag)

  subject.onNext(2)

  subject.subscribe(onNext: { int in
print("observerB: \(int)")
  }, onCompleted: {
print("observerB: onCompleted")
  }).disposed(by: disposeBag)

  subject.onNext(3)

输出：

observerA: 1
observerA: 2
observerB: 2
observerA: 3
observerB: 3

PublishSubject

PublishSubject只会把在订阅发生的时间点之后来自原始Observable的数据发射给观察者。

  let disposeBag = DisposeBag()

  let subject = PublishSubject()

  subject.onNext(0)

  subject.subscribe(onNext: { int in
print("observerA: \(int)")
  }, onCompleted: {
print("observerA: onCompleted")
  }).disposed(by: disposeBag)

  subject.onNext(1)

  subject.subscribe(onNext: { int in
print("observerB: \(int)")
  }, onCompleted: {
print("observerB: onCompleted")
  }).disposed(by: disposeBag)

  subject.onNext(2)

输出：

observerA: 1
observerA: 2
observerB: 2

ReplaySubject

ReplaySubject在缓存增长到一定大小后会丢弃旧的数据，不然会发射所有来自原始Observable的数据给观察者，无论它们是何时订阅的。

  let disposeBag = DisposeBag()

  let subject = ReplaySubject.create(bufferSize: 3)

  subject.onNext(1)
  subject.onNext(2)
  subject.onNext(3)

  subject.subscribe(onNext: { int in
print("observerA: \(int)")
  }, onCompleted: {
print("observerA: onCompleted")
  }).disposed(by: disposeBag)

  subject.onNext(4)

  subject.subscribe(onNext: { int in
print("observerB: \(int)")
  }, onCompleted: {
print("observerB: onCompleted")
  }).disposed(by: disposeBag)

  subject.onCompleted()

  subject.subscribe(onNext: { int in
print("observerC: \(int)")
  }, onCompleted: {
print("observerC: onCompleted")
  }).disposed(by: disposeBag)

输出：

observerA: 1
observerA: 2
observerA: 3
observerA: 4
observerB: 2
observerB: 3
observerB: 4
observerA: onCompleted
observerB: onCompleted
observerC: 2
observerC: 3
observerC: 4
observerC: onCompleted