Question

虽然关于Promise.all的所有问题都集中在如何等待所有承诺，但我想采取另一种方式 - 当任何承诺失败时，停止其他承诺，或者甚至停止整个剧本。

这是一个简短的例子来说明：

＆＃13;

const promise1 = new Promise((resolve, reject) => {
  setTimeout(resolve, 1000, 'resolve1');
}).then(a => { console.log('then1'); return a; });

const promise2 = new Promise((resolve, reject) => {
  setTimeout(reject, 2000, 'reject2');
}).then(a => { console.log('then2'); return a; });

const promise3 = new Promise((resolve, reject) => {
  setTimeout(resolve, 3000, 'resolve3');
}).then(a => { console.log('then3'); return a; });

Promise.all([promise1, promise2, promise3])
  .then(values => { console.log('then', values); })
  .catch(err => { console.log('catch', err); throw err; });

// results:
// > "then1"
// > "catch" "reject2"
// > "then3"    <------- WHY?

＆＃13;

脚本继续解析promise3，即使最终all(...).catch()抛出！有人可以解释原因吗？我能做些什么来阻止他们拒绝的其他承诺呢？

Answer 1

如评论中所述promises cannot be canceled.

您需要使用第三方承诺库或rxjs observables。

Answer 2

取消承诺不包含在Promises/A+ specification。

中

然而，一些Promise库有这样的取消扩展。以bluebird：

为例

＆＃13;

Promise.config({ cancellation: true }); // <-- enables this non-standard feature

const promise1 = new Promise((resolve, reject) => {
    setTimeout(resolve, 1000, 'resolve1');
}).then(a => { console.log('then1'); return a; });

const promise2 = new Promise((resolve, reject) => {
    setTimeout(reject, 2000, 'reject2');
}).then(a => { console.log('then2'); return a; });

const promise3 = new Promise((resolve, reject) => {
    setTimeout(resolve, 3000, 'resolve3');
}).then(a => { console.log('then3'); return a; });

const promises = [promise1, promise2, promise3];

Promise.all(promises)
    .then(values => { 
        console.log('then', values); 
    })
    .catch(err => { 
        console.log('catch', err); 
        promises.forEach(p => p.cancel()); // <--- Does not work with standard promises
    });

＆＃13;

<script src="https://cdn.jsdelivr.net/bluebird/latest/bluebird.core.min.js"></script>

＆＃13;

请注意，即使取消了promise3，仍会调用其setTimeout回调。但它不会触发then或catch回调。就好像这个承诺永远不会解决......永远。

如果您还希望停止触发计时器事件，则这与承诺无关，可以使用clearTimeout完成。 Bluebird在Promise构造函数中公开了一个onCancel回调函数，它将在取消promise时调用。因此，您可以使用它来删除计时器事件：

＆＃13;

Promise.config({ cancellation: true }); // <-- enables this non-standard feature

const promise1 = new Promise((resolve, reject) => {
    setTimeout(resolve, 1000, 'resolve1');
}).then(a => { console.log('then1'); return a; });

const promise2 = new Promise((resolve, reject) => {
    setTimeout(reject, 2000, 'reject2');
}).then(a => { console.log('then2'); return a; });

const promise3 = new Promise((resolve, reject, onCancel) => { // Third argument (non-standard)
    var timer = setTimeout(resolve, 3000, 'resolve3');
    onCancel(_ => {
        clearTimeout(timer);
        console.log('cancelled 3');
    });
}).then(a => { console.log('then3'); return a; });

const promises = [promise1, promise2, promise3];

Promise.all(promises)
    .then(values => { 
        console.log('then', values); 
    })
    .catch(err => { 
        console.log('catch', err); 
        promises.forEach(p => p.cancel()); // <--- Does not work with standard promises
    });

＆＃13;

<script src="https://cdn.jsdelivr.net/bluebird/latest/bluebird.core.min.js"></script>

＆＃13;

Answer 3

如果不再可以实现承诺，则该过程将退出，因此有可能创建一个实现此目标的小帮手


function timeoutWhen(promises, bail) {
  const pending = promises
    .map(promise => Promise.race([ bail, promise ]))
  return Promise.all(pending)
}


const never = new Promise(() => {})
const done = Promise.resolve()

const cancel = new Promise(ok => setTimeout(ok, 1000))

timeoutWhen([ never, done ], cancel)
  .then(() => {
    console.log('done')
  })

即使never承诺永远无法解决，日志也会完成，然后退出。

当Promise.all（）拒绝时，停止其他承诺

3 个答案: