如何多次解决承诺？ [英] How to resolve a promise multiple times?

问题描述

这可能听起来很奇怪，但我正在寻找一种方法来多次解决承诺。有没有办法让这成为可能？

It might sound weird, but I'm looking for a way to resolve a promise multiple times. Are there any approaches to make this possible?

想想以下例子：

getPromise() {
  const event = new Event('myEvent');

  setTimeout(() => {
    window.dispatchEvent(event);
  }, 5000);

  setTimeout(() => {
    window.dispatchEvent(event);
  }, 7000);

  return new Promise((resolve) => {
    window.addEventListener('myEvent', () => {
      resolve('some value'));
    });

    resolve('some value'));
  });
};

然后.then（）：

getPromise().then(data => {console.log(data)})

应该给出以下结果：

some value // initial
some value // after 5000ms
some value // after 7000ms

所以我知道有流媒体库数据，但我真的在寻找一种原生的非callbak方法来实现这一目标。

So I know there are libraries to stream data, but I'm really looking for a native non-callbak approach to achieve this.

推荐答案

如何多次解决承诺？

How to resolve a promise multiple times?

你不能。承诺只能解决一次。一旦他们得到解决，他们再也不会改变他们的状态了。它们本质上是单向状态机，具有待处理，已满足和拒绝的三种可能状态。一旦他们从待决到履行或从待决到被拒，他们就无法改变。

You can't. Promises can only be resolved once. Once they have been resolved, they never ever change their state again. They are essentially one-way state machines with three possible states pending, fulfilled and rejected. Once they've gone from pending to fulfilled or from pending to rejected, they cannot be changed.

所以，你几乎不能也不应该使用承诺你想多次出现。事件监听器或观察者比这样的事情的承诺要好得多。您的承诺只会通知您收到的第一个事件。

So, you pretty much cannot and should not be using promises for something that you want to occur multiple times. Event listeners or observers are a much better match than promises for something like that. Your promise will only ever notify you about the first event it receives.

我不知道为什么你在这种情况下试图避免回调。 Promise在 .then（）处理程序中也使用回调。您需要在某处进行回调才能使解决方案正常运行。你能解释为什么你不直接使用 window.addEventListener（'myEvent'，someCallback），因为那样你会做什么吗？

I don't know why you're trying to avoid callbacks in this case. Promises use callbacks too in their .then() handlers. You will need a callback somewhere to make your solution work. Can you explain why you don't just use window.addEventListener('myEvent', someCallback) directly since that will do what you want?

您可以返回一个类似承诺的界面（不遵循Promise标准），它会多次调用其通知回调。为了避免与promises混淆，我不会使用 .then（）作为方法名称：

You could return a promise-like interface (that does not follow Promise standards) that does call its notification callbacks more than once. To avoid confusion with promises, I would not use .then() as the method name:

function getNotifier() {
  const event = new Event('myEvent');

  setTimeout(() => {
    window.dispatchEvent(event);
  }, 500);

  setTimeout(() => {
    window.dispatchEvent(event);
  }, 700);

  let callbackList = [];
  const notifier = {
      notify: function(fn) {
          callbackList.push(fn);
      }
  };
  window.addEventListener('myEvent', (data) => {
      // call all registered callbacks
      for (let cb of callbackList) {
          cb(data);
      }
  });
  return notifier;
};

// Usage:
getNotifier().notify(data => {console.log(data.type)})

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