Javascript: promise 等待条件或在尝试次数后中止

Question

如何在 JavaScript 中将此算法编码为 promise？

// condition: some situation that is not true
// interval: period to wait before trying again
// retriesAllowed: maximum number of retries to make before aborting
// retriesMade: number of retries made so far
while() {
    if (condition) return true; // indicates success
    if (retriesMade < retriesAllowed) return false; // or throw an exception
    ++retriesMade;
    wait(interval);
}

这是行不通的，因为它实际上并没有在继续之前等待检查结果为真或假:

var check = () => {
    if (condition) return true; // indicates success
    if (retriesMade < retriesAllowed) return false; // or throw an exception
    ++retriesMade;
    setTimeout(check,interval)
}
check();

Answer 1

This doesn't work, since it doesn't actually wait for the check to resolve to true or false before moving on.

其中一部分可能是失败/退出条件似乎是相反的，因此它可能总是在第一轮检查条件之后退出:

// retriesMade starts at 0, which is likely already less-than retriesAllowed
if (retriesMade < retriesAllowed) return false;

// probably should be
if (retriesMade >= retriesAllowed) return false;

---

除此之外，没有等待是通过异步操作设计的，例如 setTimeout() 等待给定的延迟间隔。周围的代码将始终继续执行而不等待它们的结果。

与其期望他们等待，您需要建立一种方法，以便在流程实际完成时得到通知，并根据该信息继续进行。 Promises 是一种选择。

Side note: A thorough explanation of the behavior and other options are provided in "How do I return the response from an asynchronous call?"

有了它们，它们提供了 resolve() 和 reject()，而不是使用 return 或 throw > 在调用时分别表示成功或失败的函数。它们的 .then() 和 .catch() 方法可用于设置延续。

var tries = new Promise((resolve, reject) => {
    var retriesMade = 0;
    var interval = 1000;

    var check = () => {
        if (condition) { resolve(); }
        if (retriesMade >= retriesAllowed) { reject(); }
        ++retriesMade;
        setTimeout(check, interval);
    });

    check();
});

tries.then(
    () => console.log('success'),
    () => console.log('failure')
);

---

还有一个更彻底地使用 Promises 的替代版本，包含函数中的迭代 (untilLimited) 以及允许条件 (exitUpon) 是异步的，如果需要的话。

function delay(milliseconds) {
  return new Promise(function (resolve) {
    setTimeout(resolve, milliseconds);
  });
}

function untilLimited(limit, waitTime, exitUpon) {
  return Promise.resolve()
    .then(exitUpon)
    .then(result => {
      if (result) return true;
      if (!(--limit > 0)) return Promise.reject('Limit exceeded.');
      return delay(waitTime)
        .then(() => untilLimited(limit, waitTime, exitUpon))
    });
}

untilLimited(5, 500, () => false /* condition */)
  .then(
    (result) => console.log('success', result),
    (result) => console.log('failure', result)
  );