什么是Subjects

Subjet是observable和Observer之間的橋梁，一個Subject既是一個Obserable也是一個Observer，他既可以發出事件，也可以監聽事件。

PublishSubject

當你訂閱PublishSubject的時候，你只能接收到訂閱他之后發生的事件。subject.onNext()發出onNext事件，對應的還有onError()和onCompleted()事件

let disposeBag = DisposeBag()
let publishSubject = PublishSubject<String>()
        
        publishSubject.subscribe { (event) in
            print(event)
        }.disposed(by: disposeBag)
        
        publishSubject.onNext("1")
        publishSubject.onNext("2")
        publishSubject.onNext("3")
        publishSubject.onCompleted()

next(1)
next(2)
next(3)
completed

ReplaySubject

當你訂閱ReplaySubject的時候，你可以接收到訂閱他之后的事件，但也可以接受訂閱他之前發出的事件，接受幾個事件取決與bufferSize的大小

let disposeBag = DisposeBag()
let replaySubject = ReplaySubject<String>.create(bufferSize: 2)
        replaySubject.onNext("a")
        replaySubject.onNext("b")
       replaySubject.onNext("c")
        replaySubject subscribe { (event) in
            print(event)
        }.disposed(by: disposeBag)
        
        replaySubject.onNext("1")
        replaySubject.onNext("2")
        replaySubject.onNext("3")
        replaySubject.onCompleted()

next(b)
next(c)
next(1)
next(2)
next(3)
completed

BehaviorSubject

當你訂閱了BehaviorSubject，你會接受到訂閱之前的最后一個事件。

let disposeBag = DisposeBag()
let behaviorSubject = BehaviorSubject(value: "abc")
        behaviorSubject.onNext("a")
        behaviorSubject.onNext("b")
        behaviorSubject.onNext("c")
        behaviorSubject.subscribe { (event) in
            print(event)
        }.disposed(by: disposeBag)
        
        behaviorSubject.onNext("1")
        behaviorSubject.onNext("2")
        behaviorSubject.onNext("3")
        behaviorSubject.onCompleted()

next(c)
next(1)
next(2)
next(3)
completed

Variable

Variable是BehaviorSubject一個包裝箱，就像是一個箱子一樣，使用的時候需要調用asObservable()拆箱，里面的value是一個BehaviorSubject，他不會發出error事件，但是會自動發出completed事件。

let disposeBag = DisposeBag()
let variable = Variable("abc")
        variable.value = "1"
        variable.value = "2"
        variable.asObservable().subscribe { (evevt) in
            print(evevt)
        }.disposed(by: disposeBag)
        variable.value = "a"
        variable.value = "b"
        variable.value = "c"

next(2)
next(a)
next(b)
next(c)
completed