let

函数签名: let(function): Observable

让我拥有完整的 observable 。

let is no longer available, or necessary, with pipeable operators! (RxJS 5.5+)

示例

示例 1: 使用 let 重用错误处理逻辑

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// 自定义错误处理逻辑
const retryThreeTimes = obs =>
  obs.retry(3).catch(_ => Rx.Observable.of('ERROR!'));
const examplePromise = val =>
  new Promise(resolve => resolve(`Complete: ${val}`));

// 伪造的请求
const subscribe = Rx.Observable.of('some_url')
  .mergeMap(url => examplePromise(url))
  // 能够在使用 let 的多个地方重用错误处理逻辑
  .let(retryThreeTimes)
  // 输出: Complete: some_url
  .subscribe(result => console.log(result));

const customizableRetry = retryTimes => obs =>
  obs.retry(retryTimes).catch(_ => Rx.Observable.of('ERROR!'));

// 伪造的请求
const secondSubscribe = Rx.Observable.of('some_url')
  .mergeMap(url => examplePromise(url))
  // 能够在使用 let 的多个地方重用错误处理逻辑
  .let(customizableRetry(3))
  // 输出: Complete: some_url
  .subscribe(result => console.log(result));

示例 2: 使用 let 应用于 map

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// 将数组作为序列发出
const source = Rx.Observable.from([1, 2, 3, 4, 5]);
// 演示 let 和其他操作符间的区别
const test = source
  .map(val => val + 1)
  /*
    取消下面一行的注释会导致失败，let 的行为不同于大多数操作符
    val 在这里是 observable
    */
  //.let(val => val + 2)
  .subscribe(val => console.log('VALUE FROM ARRAY: ', val));

const subscribe = source
  .map(val => val + 1)
  // 'let' 会让你拥有整个的 observable 
  .let(obs => obs.map(val => val + 2))
  // 输出: 4,5,6,7,8
  .subscribe(val => console.log('VALUE FROM ARRAY WITH let: ', val));

示例 3: 使用 let 应用于多个操作符

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// 将数组作为序列发出
const source = Rx.Observable.from([1, 2, 3, 4, 5]);

// let 提供了灵活性，可以将多个操作符添加到源 observale，然后返回
const subscribeTwo = source
  .map(val => val + 1)
  .let(obs =>
    obs
      .map(val => val + 2)
      // 还可以只返回偶数
      .filter(val => val % 2 === 0)
  )
  // 输出: 4,6,8
  .subscribe(val => console.log('let WITH MULTIPLE OPERATORS: ', val));

示例 4: 通过函数应用于操作符

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// 将数组作为序列发出
const source = Rx.Observable.from([1, 2, 3, 4, 5]);

// 传入你自己的函数来将操作符添加到 observable 
const obsArrayPlusYourOperators = yourAppliedOperators => {
  return source.map(val => val + 1).let(yourAppliedOperators);
};
const addTenThenTwenty = obs => obs.map(val => val + 10).map(val => val + 20);
const subscribe = obsArrayPlusYourOperators(addTenThenTwenty)
    // 输出: 32, 33, 34, 35, 36
    .subscribe(val => console.log('let FROM FUNCTION:', val));

其他资源

• let - 官方文档

源码: https://github.com/ReactiveX/rxjs/blob/master/src/internal/operators/let.ts