# scan

**持续的将 `Observable` 的每一个元素应用一个函数，然后发出每一次函数返回的结果**

![](/files/-MWlwROiJZkrzIGtzJ_R)

**scan** 操作符将对第一个元素应用一个函数，将结果作为第一个元素发出。然后，将结果作为参数填入到第二个元素的应用函数中，创建第二个元素。以此类推，直到遍历完全部的元素。

这种操作符在其他地方有时候被称作是 **accumulator**。

## 演示

```swift
let disposeBag = DisposeBag()

Observable.of(10, 100, 1000)
    .scan(1) { aggregateValue, newValue in
        aggregateValue + newValue
    }
    .subscribe(onNext: { print($0) })
    .disposed(by: disposeBag)
```

**输出结果：**

```swift
11
111
1111
```


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