withLatestFrom
- 签名: withLatestFrom(other: Observable, project: Function): Observable
还提供另一个 observable 的最新值。
- 示例
  - 示例 1: 发出频率更快的第二个 source 的最新值
  - 示例 2: 第二个 source 发出频率更慢一些
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withLatestFrom

签名: `withLatestFrom(other: Observable, project: Function): Observable`

还提供另一个 observable 的最新值。

如果你希望每当任意 observable 发出值时各个 observable 的最新值，请尝试 combinelatest！

示例

示例 1: 发出频率更快的第二个 source 的最新值

import { withLatestFrom, map } from 'rxjs/operators';
import { interval } from 'rxjs/observable/interval';
// 每5秒发出值
const source = interval(5000);
// 每1秒发出值
const secondSource = interval(1000);
const example = source.pipe(
  withLatestFrom(secondSource),
  map(([first, second]) => {
    return `First Source (5s): ${first} Second Source (1s): ${second}`;
  })
);
/*
  输出:
  "First Source (5s): 0 Second Source (1s): 4"
  "First Source (5s): 1 Second Source (1s): 9"
  "First Source (5s): 2 Second Source (1s): 14"
  ...
*/
const subscribe = example.subscribe(val => console.log(val));

示例 2: 第二个 source 发出频率更慢一些

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import { withLatestFrom, map } from 'rxjs/operators';
import { interval } from 'rxjs/observable/interval';
// 每5秒发出值
const source = interval(5000);
// 每1秒发出值
const secondSource = interval(1000);
// withLatestFrom 的 observable 比源 observable 慢
const example = secondSource.pipe(
  // 两个 observable 在发出值前要确保至少都有1个值 (需要等待5秒)
  withLatestFrom(source),
  map(([first, second]) => {
    return `Source (1s): ${first} Latest From (5s): ${second}`;
  })
);
/*
  "Source (1s): 4 Latest From (5s): 0"
  "Source (1s): 5 Latest From (5s): 0"
  "Source (1s): 6 Latest From (5s): 0"
  ...
*/
const subscribe = example.subscribe(val => console.log(val));