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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<Int>()
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<Int>(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<Int>()
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<Int>.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
