# 共享附加作用

文档中一些特征序列，会有如下特性：

共享 [附加作用](https://3440217568.gitbook.io/rxswift/recipes/side_effects)：

* [Driver](https://3440217568.gitbook.io/rxswift/rxswift_core/observable/driver)
* [Signal](https://3440217568.gitbook.io/rxswift/rxswift_core/observable/signal)
* [ControlEvent](https://3440217568.gitbook.io/rxswift/rxswift_core/observable/control_event)
* ...

不共享 [附加作用](https://3440217568.gitbook.io/rxswift/recipes/side_effects)：

* [Single](https://3440217568.gitbook.io/rxswift/rxswift_core/observable/single)
* [Completable](https://3440217568.gitbook.io/rxswift/rxswift_core/observable/completable)
* [Maybe](https://3440217568.gitbook.io/rxswift/rxswift_core/observable/maybe)
* ...

那什么是**共享** [**附加作用**](https://3440217568.gitbook.io/rxswift/recipes/side_effects)，什么是**不共享** [**附加作用**](https://3440217568.gitbook.io/rxswift/recipes/side_effects)？

## 共享 [附加作用](https://3440217568.gitbook.io/rxswift/recipes/side_effects)：

```swift
...
let observable: Observable<Teacher> = API.teacher(teacherId: 1)
let shareSideEffects: Driver<Teacher> = observable.asDriver(onErrorDriveWith: .empty())

let observer0: (Teacher) -> () = ...
let observer1: (Teacher) -> () = ...

shareSideEffects.drive(onNext: observer0)
shareSideEffects.drive(onNext: observer1) // 第二次订阅
```

如果一个序列**共享** [**附加作用**](https://3440217568.gitbook.io/rxswift/recipes/side_effects)，那在第二次订阅时，不会重新发起网络请求，而是共享第一次网络请求（[附加作用](https://3440217568.gitbook.io/rxswift/recipes/side_effects)）。

## 不共享 [附加作用](https://3440217568.gitbook.io/rxswift/recipes/side_effects)：

```swift
...
let observable: Observable<Teacher> = API.teacher(teacherId: 1)
let notShareSideEffects: Single<Teacher> = observable.asSingle()

let observer0: (Teacher) -> () = ...
let observer1: (Teacher) -> () = ...

notShareSideEffects.subscribe(onSuccess: observer0)
notShareSideEffects.subscribe(onSuccess: observer1) // 第二次订阅
```

如果一个序列**不共享** [**附加作用**](https://3440217568.gitbook.io/rxswift/recipes/side_effects)，那在第二次订阅时，会重新发起网络请求，而不是共享第一次网络请求（[附加作用](https://3440217568.gitbook.io/rxswift/recipes/side_effects)）。

因此我们需要注意，如果一个网络请求序列，他**不共享** [**附加作用**](https://3440217568.gitbook.io/rxswift/recipes/side_effects)，那每一次订阅时就会单独发起网络请求。这时最好改用 **共享** [**附加作用**](https://3440217568.gitbook.io/rxswift/recipes/side_effects) 的序列，或者使用 [share](https://3440217568.gitbook.io/rxswift/decision_tree/sharereplay) 操作符。


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