> For the complete documentation index, see [llms.txt](https://3440217568.gitbook.io/rxswift/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://3440217568.gitbook.io/rxswift/rxswift_core/observable_and_observer/behavior_subject.md).

# BehaviorSubject

![](/files/-MWlwLhF8wz12_FbT0Ku)

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。

![](/files/-MWlwLhGiL7lTDXIojNl)

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(🍊)
```