javascript - 如何仅使用 native Promise 编写异步计数器，即具有用于异步代码的同步接口(interface)的计数器？

Question

我想创建一个公开以下接口(interface)的对象:

// Create the object that counts to three
const c = counter(3);
// c.finished is a promise that resolves when c.count() is invoked three times
c.finished.then(() => console.log('counted three times!'));

// Somewhere else, c.count() is invoked in async body
setTimeout(() => c.count(), 500);
setTimeout(() => c.count(), 1000);
setTimeout(() => c.count(), 1500);

我预计 c.finished 在 1.5 秒后解析。

如何仅使用 native Promise API 编写 counter(countTimes)？

披露我已经有 a solution对于上面的问题，想知道最优雅的方法是什么。

编辑

我原来的解决方案是:

class AsyncCounter {
  constructor(countTimes) {
    let currentCount = 0;
    this.countTimes = countTimes;
    this.ready = new Promise(resolveReady => {
      this.finished = new Promise(resolveFinished => {
        const count = () => {
          currentCount++;
          if (currentCount >= this.countTimes) {
            resolveFinished();
          }
          return currentCount;
        };
        this.count = () => this.ready.then(() => count());
        resolveReady();
      });
    });
  }
}

const counter = countTimes => new AsyncCounter(countTimes);

按照@Bergi的建议并根据MDN docs for executor function :

the executor is called before the Promise constructor even returns the created object

因此，上述解决方案中的 ready promise 并不是必需的。

Answer 1

你会写

function counter(n) {
    let i=0, resolve;
    return {
        count() {
            if (++i == n) resolve();
        },
        finished: new Promise(res => {
            resolve = res;
        })
    };
}

或者，您也可以不将 resolve 放入外部变量中

function counter(n) {
    let i=0;
    const res = {};
    res.finished = new Promise(resolve => { 
        res.count = () => {
            if (++i == n) resolve();
        };
    });
    return res;
}