如何 return 许多 Promise 并在做其他事情之前等待它们全部

Question

我有一个循环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，我还有另一个循环，只有当所有异步的东西都完成后才需要执行。

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 promise 不是很熟悉，谁能帮我实现这个？

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

您可以为此使用 Promise.all (spec, MDN)：它接受一堆单独的承诺，并在您给它的所有承诺都已解决时返回给您一个已解决的承诺, 或当其中任何一个被拒绝时被拒绝。

因此，如果您做出 doSomeAsyncStuff return 承诺，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 有一篇关于承诺的文章 here。我还在我的书 JavaScript：新玩具 的第 8 章中详细介绍了承诺，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）：

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

Answer 2

一个可重复使用的函数非常适合这种模式：

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));

一个相关的模式，正在遍历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

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 4

这是我为自己编写的代码，以便理解此处陈述的答案。我在 for 循环中有猫鼬查询，所以我把 asyncFunction 放在这里代替它。希望它能帮助任何人。您可以运行节点中的此脚本或任何 Javascript 运行次。

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 5

/*** 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 6

如果您想多次执行相同的操作，这里有一个适合您的优雅解决方案：

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承诺数组。

如何 return 许多 Promise 并在做其他事情之前等待它们全部

How to return many Promises and wait for them all before doing other stuff

javascript

asynchronous

promise

ecmascript-6

es6-promise