一个接一个地(即按顺序)解决承诺? [英] Resolve promises one after another (i.e. in sequence)?

问题描述

考虑以下以串行/顺序方式读取文件数组的代码.readFiles 返回一个承诺，只有在按顺序读取所有文件后才会解决.

Consider the following code that reads an array of files in a serial/sequential manner. readFiles returns a promise, which is resolved only once all files have been read in sequence.

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!
  });
};

上面的代码有效，但我不喜欢为了顺序发生的事情而进行递归.有没有更简单的方法可以重写这段代码，这样我就不必使用我奇怪的 readSequential 函数?

The above code works, but I don't like having to do recursion for things to occur sequentially. Is there a simpler way that this code can be re-written so that I don't have to use my weird readSequential function?

最初我尝试使用 Promise.all，但这导致所有 readFile 调用同时发生，这不是我想要:

Originally I tried to use Promise.all, but that caused all of the readFile calls to happen concurrently, which is not what I want:

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

推荐答案

Update 2017:如果环境支持，我会使用异步函数:

Update 2017: I would use an async function if the environment supports it:

async function readFiles(files) {
  for(const file of files) {
    await readFile(file);
  }
};

如果您愿意，可以使用异步生成器(如果您的环境支持)推迟读取文件，直到您需要它们:

If you'd like, you can defer reading the files until you need them using an async generator (if your environment supports it):

async function* readFiles(files) {
  for(const file of files) {
    yield await readFile(file);
  }
};

<小时>

更新:再想一想 - 我可能会改用 for 循环:

---

Update: In second thought - I might use a for loop instead:

var readFiles = function(files) {
  var p = Promise.resolve(); // Q() in q

  files.forEach(file =>
      p = p.then(() => readFile(file)); 
  );
  return p;
};

或者更简洁，使用reduce:

Or more compactly, with reduce:

var readFiles = function(files) {
  return files.reduce((p, file) => {
     return p.then(() => readFile(file));
  }, Promise.resolve()); // initial
};

在其他 Promise 库(例如 when 和 Bluebird)中，您有用于此的实用方法.

In other promise libraries (like when and Bluebird) you have utility methods for this.

例如，Bluebird 将是:

For example, Bluebird would be:

var Promise = require("bluebird");
var fs = Promise.promisifyAll(require("fs"));

var readAll = Promise.resolve(files).map(fs.readFileAsync,{concurrency: 1 });
// if the order matters, you can use Promise.each instead and omit concurrency param

readAll.then(function(allFileContents){
    // do stuff to read files.
});

尽管今天确实没有理由不使用异步等待.

Although there is really no reason not to use async await today.

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