# takeUntil

**忽略掉在第二个 `Observable` 产生事件后发出的那部分元素**

![](https://4217506537-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-MWj545abP2yK86-qksJ%2Fsync%2Fc4773a61d5d9cc5dd41465e1e105127d36a0a3c5.png?generation=1616819381464083\&alt=media)

**takeUntil** 操作符将镜像源 `Observable`，它同时观测第二个 `Observable`。一旦第二个 `Observable` 发出一个元素或者产生一个终止事件，那个镜像的 `Observable` 将立即终止。

## 演示

```swift
let disposeBag = DisposeBag()

let sourceSequence = PublishSubject<String>()
let referenceSequence = PublishSubject<String>()

sourceSequence
    .takeUntil(referenceSequence)
    .subscribe { print($0) }
    .disposed(by: disposeBag)

sourceSequence.onNext("🐱")
sourceSequence.onNext("🐰")
sourceSequence.onNext("🐶")

referenceSequence.onNext("🔴")

sourceSequence.onNext("🐸")
sourceSequence.onNext("🐷")
sourceSequence.onNext("🐵")
```

**输出结果：**

```swift
next(🐱)
next(🐰)
next(🐶)
completed
```