BehaviorSubject
当观察者对 BehaviorSubject 进行订阅时，它会将源 Observable 中最新的元素发送出来（如果不存在最新的元素，就发出默认元素）。然后将随后产生的元素发送出来。
如果源 Observable 因为产生了一个 error 事件而中止， BehaviorSubject 就不会发出任何元素，而是将这个 error 事件发送出来。
演示
输出结果：
Last updated
Was this helpful?
当观察者对 BehaviorSubject 进行订阅时，它会将源 Observable 中最新的元素发送出来（如果不存在最新的元素，就发出默认元素）。然后将随后产生的元素发送出来。
如果源 Observable 因为产生了一个 error 事件而中止， BehaviorSubject 就不会发出任何元素，而是将这个 error 事件发送出来。
输出结果：
Last updated
Was this helpful?
Was this helpful?
let disposeBag = DisposeBag()
let subject = BehaviorSubject(value: "🔴")
subject
.subscribe { print("Subscription: 1 Event:", $0) }
.disposed(by: disposeBag)
subject.onNext("🐶")
subject.onNext("🐱")
subject
.subscribe { print("Subscription: 2 Event:", $0) }
.disposed(by: disposeBag)
subject.onNext("🅰️")
subject.onNext("🅱️")
subject
.subscribe { print("Subscription: 3 Event:", $0) }
.disposed(by: disposeBag)
subject.onNext("🍐")
subject.onNext("🍊")Subscription: 1 Event: next(🔴)
Subscription: 1 Event: next(🐶)
Subscription: 1 Event: next(🐱)
Subscription: 2 Event: next(🐱)
Subscription: 1 Event: next(🅰️)
Subscription: 2 Event: next(🅰️)
Subscription: 1 Event: next(🅱️)
Subscription: 2 Event: next(🅱️)
Subscription: 3 Event: next(🅱️)
Subscription: 1 Event: next(🍐)
Subscription: 2 Event: next(🍐)
Subscription: 3 Event: next(🍐)
Subscription: 1 Event: next(🍊)
Subscription: 2 Event: next(🍊)
Subscription: 3 Event: next(🍊)