# withLatestFrom

**将两个 `Observables` 最新的元素通过一个函数组合起来，当第一个 `Observable` 发出一个元素，就将组合后的元素发送出来**

![](/files/-MWlwRVZR3bzyZSOoPY5)

**withLatestFrom** 操作符将两个 `Observables` 中最新的元素通过一个函数组合起来，然后将这个组合的结果发出来。当第一个 `Observable` 发出一个元素时，就立即取出第二个 `Observable` 中最新的元素，通过一个组合函数将两个最新的元素合并后发送出去。

## 演示

当第一个 `Observable` 发出一个元素时，就立即取出第二个 `Observable` 中最新的元素，然后把第二个 `Observable` 中最新的元素发送出去。

```swift
let disposeBag = DisposeBag()
let firstSubject = PublishSubject<String>()
let secondSubject = PublishSubject<String>()

firstSubject
     .withLatestFrom(secondSubject)
     .subscribe(onNext: { print($0) })
     .disposed(by: disposeBag)

firstSubject.onNext("🅰️")
firstSubject.onNext("🅱️")
secondSubject.onNext("1")
secondSubject.onNext("2")
firstSubject.onNext("🆎")
```

**输出结果：**

```swift
2
```

当第一个 `Observable` 发出一个元素时，就立即取出第二个 `Observable` 中最新的元素，将第一个 `Observable` 中最新的元素 `first` 和第二个 `Observable` 中最新的元素`second`组合，然后把组合结果 `first+second`发送出去。

```swift
let disposeBag = DisposeBag()
let firstSubject = PublishSubject<String>()
let secondSubject = PublishSubject<String>()

firstSubject
     .withLatestFrom(secondSubject) {
          (first, second) in
          return first + second
     }
     .subscribe(onNext: { print($0) })
     .disposed(by: disposeBag)

firstSubject.onNext("🅰️")
firstSubject.onNext("🅱️")
secondSubject.onNext("1")
secondSubject.onNext("2")
firstSubject.onNext("🆎")
```

**输出结果：**

```swift
🆎2
```


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://3440217568.gitbook.io/rxswift/decision_tree/withlatestfrom.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.