# flatMapLatest

**将 `Observable` 的元素转换成其他的 `Observable`，然后取这些 `Observables` 中最新的一个**

![](/files/-MWlwNsn9Z_jx27-0Z8m)

**flatMapLatest** 操作符将源 `Observable` 的每一个元素应用一个转换方法，将他们转换成 `Observables`。一旦转换出一个新的 `Observable`，就只发出它的元素，旧的 `Observables` 的元素将被忽略掉。

## 演示

**tips：**与 [flatMap](/rxswift/decision_tree/flatmap.md) 比较更容易理解

```swift
let disposeBag = DisposeBag()
let first = BehaviorSubject(value: "👦🏻")
let second = BehaviorSubject(value: "🅰️")
let variable = Variable(first)

variable.asObservable()
        .flatMapLatest { $0 }
        .subscribe(onNext: { print($0) })
        .disposed(by: disposeBag)

first.onNext("🐱")
variable.value = second
second.onNext("🅱️")
first.onNext("🐶")
```

**输出结果：**

```swift
👦🏻
🐱
🅰️
🅱️
```


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