# concat

**让两个或多个 `Observables` 按顺序串连起来**

![](/files/-MWlwP7ZOpWgZo1qW5zw)

**concat** 操作符将多个 `Observables` 按顺序串联起来，当前一个 `Observable` 元素发送完毕后，后一个 `Observable` 才可以开始发出元素。

**concat** 将等待前一个 `Observable` 产生完成事件后，才对后一个 `Observable` 进行订阅。如果后一个是“热” `Observable` ，在它前一个 `Observable` 产生完成事件前，所产生的元素将不会被发送出来。

[startWith](/rxswift/decision_tree/startwith.md) 和它十分相似。但是 [startWith](/rxswift/decision_tree/startwith.md) 不是在后面添加元素，而是在前面插入元素。

[merge](/rxswift/decision_tree/merge.md) 和它也是十分相似。[merge](/rxswift/decision_tree/merge.md) 并不是将多个 `Observables` 按顺序串联起来，而是将他们合并到一起，不需要 `Observables` 按先后顺序发出元素。

## 演示

```swift
let disposeBag = DisposeBag()

let subject1 = BehaviorSubject(value: "🍎")
let subject2 = BehaviorSubject(value: "🐶")

let subject = BehaviorSubject(value: subject1)

subject
    .asObservable()
    .concat()
    .subscribe { print($0) }
    .disposed(by: disposeBag)

subject1.onNext("🍐")
subject1.onNext("🍊")

subject.onNext(subject2)

subject2.onNext("I would be ignored")
subject2.onNext("🐱")

subject1.onCompleted()

subject2.onNext("🐭")
```

**输出结果：**

```swift
next(🍎)
next(🍐)
next(🍊)
next(🐱)
next(🐭)
```


---

# 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/concat.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.