考虑下面以串行/顺序方式读取文件数组的代码。readFiles返回一个承诺,只有在顺序读取所有文件后才会解析这个承诺。

var readFile = function(file) {
  ... // Returns a promise.
};

var readFiles = function(files) {
  return new Promise((resolve, reject) => {
    var readSequential = function(index) {
      if (index >= files.length) {
        resolve();
      } else {
        readFile(files[index]).then(function() {
          readSequential(index + 1);
        }).catch(reject);
      }
    };

    readSequential(0); // Start with the first file!
  });
};

上面的代码可以工作,但是我不喜欢为了使事情按顺序发生而进行递归。是否有一种更简单的方法可以重写这段代码,这样我就不必使用奇怪的readSequential函数了?

最初我尝试使用Promise。但是这会导致所有的readFile调用并发发生,这不是我想要的:

var readFiles = function(files) {
  return Promise.all(files.map(function(file) {
    return readFile(file);
  }));
};

当前回答

我不得不运行大量的顺序任务,并使用这些答案来伪造一个函数,将照顾处理任何顺序任务…

function one_by_one(objects_array, iterator, callback) {
    var start_promise = objects_array.reduce(function (prom, object) {
        return prom.then(function () {
            return iterator(object);
        });
    }, Promise.resolve()); // initial
    if(callback){
        start_promise.then(callback);
    }else{
        return start_promise;
    }
}

该函数接受2个参数+ 1个可选参数。第一个参数是我们将要处理的数组。第二个参数是任务本身,一个返回承诺的函数,只有当这个承诺解决时,下一个任务才会开始。第三个参数是在所有任务完成后运行的回调。如果没有传递回调,则函数返回它创建的promise,以便我们可以处理结束。

下面是一个用法示例:

var filenames = ['1.jpg','2.jpg','3.jpg'];
var resize_task = function(filename){
    //return promise of async resizing with filename
};
one_by_one(filenames,resize_task );

希望它能节省一些时间…

其他回答

只需使用.then(resPrevTask => nextTask())

(顺便说一下,下一个代码需要4秒。)

函数task1() { return new Promise((resolve) => { setTimeout(() => { 解决(console.log(任务1)) }, 3000) }) } 函数task2() { return new Promise((resolve) => { setTimeout(() => { 解决(console.log(任务2)) }, 1000) }) } 函数seqTasks () { task1 () .then(() => task2()) } seqTasks ();

使用Array.prototype。Reduce,并记住将承诺包装在函数中,否则它们将已经在运行!

// array of Promise providers

const providers = [
  function(){
     return Promise.resolve(1);
  },
  function(){
     return Promise.resolve(2);
  },
  function(){
     return Promise.resolve(3);
  }
]


const inSeries = function(providers){

  const seed = Promise.resolve(null); 

  return providers.reduce(function(a,b){
      return a.then(b);
  }, seed);
};

很好很简单…… 您应该能够为性能等重新使用相同的种子。

在使用reduce时,防止空数组或只有一个元素的数组是很重要的,所以这个技术是你最好的选择:

   const providers = [
      function(v){
         return Promise.resolve(v+1);
      },
      function(v){
         return Promise.resolve(v+2);
      },
      function(v){
         return Promise.resolve(v+3);
      }
    ]

    const inSeries = function(providers, initialVal){

        if(providers.length < 1){
            return Promise.resolve(null)
        }

        return providers.reduce((a,b) => a.then(b), providers.shift()(initialVal));
    };

然后像这样叫它:

inSeries(providers, 1).then(v => {
   console.log(v);  // 7
});

我发现自己多次回到这个问题上,答案并没有给我我想要的,所以把这个放在这里给任何需要这个问题的人。

下面的代码执行顺序承诺执行(一个接一个),每一轮由多个调用组成:

async function sequence(list, cb) {
  const result = [];
  await list.reduce(async (promise, item) => promise
    .then(() => cb(item))
    .then((res) => result.push(res)
  ), Promise.resolve());
  return result;
}

展示:

<script src="https://cdnjs.cloudflare.com/ajax/libs/axios/0.15.3/axios.min.js"></script> <script src="https://unpkg.com/@babel/standalone@7/babel.min.js"></script> <script type="text/babel"> function sleep(ms) { return new Promise(resolve => setTimeout(resolve, ms)); } async function readFile(url, index) { console.log('Running index: ', index); // First action const firstTime = await axios.get(url); console.log('First API response: ', firstTime.data.activity); // Second action await sleep(1000); // Third action const secondTime = await axios.get(url); console.log('Second API response: ', secondTime.data.activity); // Fourth action await sleep(1000); return secondTime.data; } async function sequence(urls, fn) { const result = []; await urls.reduce(async (promise, url, index) => promise.then(() => fn(url, index)).then((res) => result.push(res)), Promise.resolve()); return result; } const urls = [ 'https://www.boredapi.com/api/activity', 'https://www.boredapi.com/api/activity', 'https://www.boredapi.com/api/activity', ]; (async function init() { const result = await sequence(urls, readFile); console.log('result', result); })() </script>

我真的很喜欢@joelnet的回答,但对我来说,这种编码风格有点难以消化,所以我花了几天时间试图弄清楚如何以更可读的方式表达相同的解决方案,这就是我的想法,只是使用了不同的语法和一些注释。

// first take your work
const urls = ['/url1', '/url2', '/url3', '/url4']

// next convert each item to a function that returns a promise
const functions = urls.map((url) => {
  // For every url we return a new function
  return () => {
    return new Promise((resolve) => {
      // random wait in milliseconds
      const randomWait = parseInt((Math.random() * 1000),10)
      console.log('waiting to resolve in ms', randomWait)
      setTimeout(()=>resolve({randomWait, url}),randomWait)
    })
  }
})


const promiseReduce = (acc, next) => {
  // we wait for the accumulator to resolve it's promise
  return acc.then((accResult) => {
    // and then we return a new promise that will become
    // the new value for the accumulator
    return next().then((nextResult) => {
      // that eventually will resolve to a new array containing
      // the value of the two promises
      return accResult.concat(nextResult)
    })
  })
};
// the accumulator will always be a promise that resolves to an array
const accumulator = Promise.resolve([])

// we call reduce with the reduce function and the accumulator initial value
functions.reduce(promiseReduce, accumulator)
  .then((result) => {
    // let's display the final value here
    console.log('=== The final result ===')
    console.log(result)
  })

下面是我比较喜欢的连续运行任务的方式。

function runSerial() {
    var that = this;
    // task1 is a function that returns a promise (and immediately starts executing)
    // task2 is a function that returns a promise (and immediately starts executing)
    return Promise.resolve()
        .then(function() {
            return that.task1();
        })
        .then(function() {
            return that.task2();
        })
        .then(function() {
            console.log(" ---- done ----");
        });
}

有更多任务的情况会怎样呢?10 ?

function runSerial(tasks) {
  var result = Promise.resolve();
  tasks.forEach(task => {
    result = result.then(() => task());
  });
  return result;
}