在 RxJS 5 中，有没有办法在订阅 Observable 之前触发它？

Question

我知道 RxJS 5（和其他地方）中的 observable 是延迟执行的。换句话说，它们在有订阅者之前不会被执行。但是，我正在尝试预取一些数据。有没有办法在订阅之前触发 observable？

let obs = Rx.Observable.create(observer => {
  console.log('Observer executed');
  // This would actually be fetching data from a server:
  observer.next(42);
});

// Something like obs.warmup() happens here
console.log('Observer is ideally called before this point.');

// Some time later this is called, and hopefully the data is already retrieved.
obs.subscribe(value => {
  console.log('Got ' + value);
});

Answer 1

你想让一个冷的 observable 变热。 (what are hot and cold observables)

因此，如果您已经有一个冷 observable，您可以将 publish 运算符与 connect 一起使用。

let obs = Rx.Observable.create(observer => {
  console.log('Observer executed');
  // This would actually be fetching data from a server:
  observer.next(42);
}).publish(); // create a ConnectableObservable

obs.connect(); // Run the observer

// Something like obs.warmup() happens here
console.log('Observer is ideally called before this point.');

// Some time later this is called, and hopefully the data is already retrieved.
obs.subscribe(value => {
  console.log('Got ' + value);
});

<script src="https://cdnjs.cloudflare.com/ajax/libs/rxjs/5.0.0-rc.1/Rx.js"></script>

但通常有一个更简单的方法。我假设您有一个外部事件源，您想将其转换为可观察的。正确的方法是使用Subject。

let obs = new Rx.Subject();

console.log('Observer executed');
obs.next(42); // subscribers would receive this... 
// it could be something like `service.on("event", e => obs.next(e));`

// Something like obs.warmup() happens here
console.log('Observer is ideally called before this point.');

// Some time later this is called, and hopefully the data is already retrieved.
obs.subscribe(value => {
  console.log('Got ' + value);
});

<script src="https://cdnjs.cloudflare.com/ajax/libs/rxjs/5.0.0-rc.1/Rx.js"></script>