# takeUntil

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

![](/files/-MWlwKxUQOgt1yQOV8V2)

**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
```


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