# BehaviorSubject

![](https://4217506537-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-MWj545abP2yK86-qksJ%2Fsync%2F7f0ac0a950c9daf1d0dd61c026221e2d7e6a6bc0.png?generation=1616819382073703\&alt=media)

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。

![](https://4217506537-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-MWj545abP2yK86-qksJ%2Fsync%2F2bc049a8f48c6369ea6ecbf77e83e9f9f608e397.png?generation=1616819381916641\&alt=media)

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

```swift
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("🍊")
```

**输出结果：**

```swift
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(🍊)
```