如何返回许多 Promise 并在执行其他操作之前等待所有 Promise

Question

我有一个循环，它调用一个异步执行操作的方法。这个循环可以多次调用该方法。在这个循环之后，我有另一个循环，仅当所有异步工作完成时才需要执行。

所以这说明了我想要的：

for (i = 0; i < 5; i++) {
    doSomeAsyncStuff();    
}

for (i = 0; i < 5; i++) {
    doSomeStuffOnlyWhenTheAsyncStuffIsFinish();    
}

我对承诺不太熟悉，所以有人可以帮助我实现这一目标吗？

这就是我的 doSomeAsyncStuff() 的行为方式：

function doSomeAsyncStuff() {
    var editor = generateCKEditor();
    editor.on('instanceReady', function(evt) {
        doSomeStuff();
        // There should be the resolve() of the promises I think.
    })
}

也许我必须做这样的事情：

function doSomeAsyncStuff() {
    var editor = generateCKEditor();
    return new Promise(function(resolve,refuse) {
        editor.on('instanceReady', function(evt) {
            doSomeStuff();
            resolve(true);
        });
    });
}

但我不确定语法。

Answer 1

您可以使用 Promise.all （规范，< a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise/all" rel="nofollow noreferrer">MDN）：它接受一堆单独的承诺，并返回一个承诺，当您提供的所有承诺都得到履行时，该承诺得到履行，或者当其中任何一个承诺被拒绝时，该承诺被拒绝。履行数组中的结果与输入数组中的 Promise 的顺序相同。

因此，如果您让 doSomeAsyncStuff 返回一个 Promise，那么：

    const promises = [];
//  ^^^^^−−−−−−−−−−−−−−−−−−−−−−−−−−− use `const` or `let`, not `var`
    
    for (let i = 0; i < 5; i++) {
//       ^^^−−−−−−−−−−−−−−−−−−−−−−−− added missing declaration
        promises.push(doSomeAsyncStuff());
    }
    
    Promise.all(promises)
        .then(() => {
            for (let i = 0; i < 5; i++) {
//               ^^^−−−−−−−−−−−−−−−− added missing declaration
                doSomeStuffOnlyWhenTheAsyncStuffIsFinish();    
            }
        })
        .catch((e) => {
            // handle errors here
        });

MDN 有一篇关于 Promise 的文章 此处。我还在我的书 JavaScript：新玩具 的第 8 章中详细介绍了 promsies，如果您有兴趣，可以在我的个人资料中找到链接。

这是一个例子：

function doSomethingAsync(value) {
    return new Promise((resolve) => {
        setTimeout(() => {
            console.log("Resolving " + value);
            resolve(value);
        }, Math.floor(Math.random() * 1000));
    });
}
   
function test() {
    const promises = [];
       
    for (let i = 0; i < 5; ++i) {
        promises.push(doSomethingAsync(i));
    }
       
    Promise.all(promises)
        .then((results) => {
            console.log("All done", results);
        })
        .catch((e) => {
            // Handle errors here
        });
}
   
test();

示例输出（由于 Math.random，首先完成的内容可能会有所不同，但“全部完成”行将位于最后）：

Resolving 3
Resolving 2
Resolving 1
Resolving 4
Resolving 0
All done [0,1,2,3,4]

再次注意，fulfillment 数组的内容位于输入数组中承诺的顺序（不是承诺的履行顺序）。

Answer 2

可重用函数非常适合此模式：

function awaitAll(count, asyncFn) {
  const promises = [];

  for (i = 0; i < count; ++i) {
    promises.push(asyncFn());
  }

  return Promise.all(promises);
}

OP 示例：

awaitAll(5, doSomeAsyncStuff)
  .then(results => console.log('doSomeStuffOnlyWhenTheAsyncStuffIsFinished', results))
  .catch(e => console.error(e));

---

相关模式是迭代数组并对每个项目执行异步操作：

function awaitAll(list, asyncFn) {
  const promises = [];

  list.forEach(x => {
    promises.push(asyncFn(x));
  });

  return Promise.all(promises);
}

示例：

const books = [{ id: 1, name: 'foo' }, { id: 2, name: 'bar' }];

function doSomeAsyncStuffWith(book) {
  return Promise.resolve(book.name);
}

awaitAll(books, doSomeAsyncStuffWith)
  .then(results => console.log('doSomeStuffOnlyWhenTheAsyncStuffIsFinished', results))
  .catch(e => console.error(e));

Answer 3

/*** Worst way ***/
for(i=0;i<10000;i++){
  let data = await axios.get(
    "https://yourwebsite.com/get_my_data/"
  )
  //do the statements and operations
  //that are dependant on data
}

//Your final statements and operations
//That will be performed when the loop ends

//=> this approach will perform very slow as all the api call
// will happen in series


/*** One of the Best way ***/

const yourAsyncFunction = async (anyParams) => {
  let data = await axios.get(
    "https://yourwebsite.com/get_my_data/"
  )
  //all you statements and operations here
  //that are dependant on data
}
var promises = []
for(i=0;i<10000;i++){
  promises.push(yourAsyncFunction(i))
}
await Promise.all(promises)
//Your final statement / operations
//that will run once the loop ends

//=> this approach will perform very fast as all the api call
// will happen in parallal

Answer 4

const doSomeAsyncStuff = async (funcs) => {
  const allPromises = funcs.map(func => func());
  return await Promise.all(allPromises);
}

doSomeAsyncStuff([
  () => new Promise(resolve => setTimeout(() => resolve(), 100)),
  () => new Promise(resolve => setTimeout(() => resolve(), 100)),
  () => new Promise(resolve => setTimeout(() => resolve(), 100)),
  () => new Promise(resolve => setTimeout(() => resolve(), 100)),
  () => new Promise(resolve => setTimeout(() => resolve(), 100)),
]);

Answer 5

这是我为自己编写的代码，以便理解此处所述的答案。我在 for 循环中有猫鼬查询，因此我将 asyncFunction 放在此处来代替它。希望它对任何人都有帮助。您可以在节点或许多 Javascript 运行时中的任何一个中运行此脚本。

let asyncFunction = function(value, callback)
{
        setTimeout(function(){console.log(value); callback();}, 1000);
}



// a sample function run without promises

asyncFunction(10,
    function()
    {
        console.log("I'm back 10");
    }
);


//here we use promises

let promisesArray = [];

let p = new Promise(function(resolve)
{
    asyncFunction(20,
        function()
        {
            console.log("I'm back 20");
            resolve(20);
        }
    );
});

promisesArray.push(p);


for(let i = 30; i < 80; i += 10)
{
    let p = new Promise(function(resolve)
    {
        asyncFunction(i,
            function()
            {
                console.log("I'm back " + i);
                resolve(i);
            }
        );
    });
    promisesArray.push(p);
}


// We use Promise.all to execute code after all promises are done.

Promise.all(promisesArray).then(
    function()
    {
        console.log("all promises resolved!");
    }
)

Answer 6

如果您想多次执行相同的操作，这里有一个优雅的解决方案：

await Promise.all(new Array(10).fill(0).map(() => asyncFn()));

这将创建一个包含 10 个项目的数组，用零填充它，然后将其映射到一个 Promise 数组。