前言
在没有深度使用函数回调的经验的时候，去看这些内容还是有一点吃力的。由于Node.js独特的异步特性，才出现了“回调地狱”的问题，这篇文章中，我比较详细的记录了如何解决异步流问题。
文章会很长，而且这篇是对异步流模式的解释。文中会使用一个简单的网络蜘蛛的例子，它的作用是抓取指定URL的网页内容并保存在项目中，在文章的最后，可以找到整篇文章中的源码demo。
1.原生JavaScript模式
本篇不针对初学者，因此会省略掉大部分的基础内容的讲解：
(spider_v1.js)
const request = require("request");const fs = require("fs");const mkdirp = require("mkdirp");const path = require("path");const utilities = require("./utilities");function spider(url, callback) { const filename = utilities.urlToFilename(url); console.log(`filename: ${filename}`); fs.exists(filename, exists => { if (!exists) { console.log(`Downloading ${url}`); request(url, (err, response, body) => { if (err) { callback(err); } else { mkdirp(path.dirname(filename), err => { if (err) { callback(err); } else { fs.writeFile(filename, body, err => { if (err) { callback(err); } else { callback(null, filename, true); } }); } }); } }); } else { callback(null, filename, false); } });}spider(process.argv[2], (err, filename, downloaded) => { if (err) { console.log(err); } else if (downloaded) { console.log(`Completed the download of ${filename}`); } else { console.log(`${filename} was already downloaded`); }});上边的代码的流程大概是这样的：
这是一个非常简单版本的蜘蛛，他只能抓取一个url的内容，看到上边的回调多么令人头疼。那么我们开始进行优化。
首先，if else 这种方式可以进行优化，这个很简单，不用多说，放一个对比效果：
/// beforeif (err) { callback(err);} else { callback(null, filename, true);}/// afterif (err) { return callback(err);}callback(null, filename, true);代码这么写，嵌套就会少一层，但经验丰富的程序员会认为，这样写过重强调了error，我们编程的重点应该放在处理正确的数据上，在可读性上也存在这样的要求。
另一个优化是函数拆分，上边代码中的spider函数中，可以把下载文件和保存文件拆分出去。
(spider_v2.js)
const request = require("request");const fs = require("fs");const mkdirp = require("mkdirp");const path = require("path");const utilities = require("./utilities");function saveFile(filename, contents, callback) { mkdirp(path.dirname(filename), err => { if (err) { return callback(err); } fs.writeFile(filename, contents, callback); });}function download(url, filename, callback) { console.log(`Downloading ${url}`); request(url, (err, response, body) => { if (err) { return callback(err); } saveFile(filename, body, err => { if (err) { return callback(err); } console.log(`Downloaded and saved: ${url}`); callback(null, body); }); })}function spider(url, callback) { const filename = utilities.urlToFilename(url); console.log(`filename: ${filename}`); fs.exists(filename, exists => { if (exists) { return callback(null, filename, false); } download(url, filename, err => { if (err) { return callback(err); } callback(null, filename, true); }) });}spider(process.argv[2], (err, filename, downloaded) => { if (err) { console.log(err); } else if (downloaded) { console.log(`Completed the download of ${filename}`); } else { console.log(`${filename} was already downloaded`); }});上边的代码基本上是采用原生优化后的结果，但这个蜘蛛的功能太过简单，我们现在需要抓取某个网页中的所有url，这样才会引申出串行和并行的问题。
(spider_v3.js)
const request = require("request");const fs = require("fs");const mkdirp = require("mkdirp");const path = require("path");const utilities = require("./utilities");function saveFile(filename, contents, callback) { mkdirp(path.dirname(filename), err => { if (err) { return callback(err); } fs.writeFile(filename, contents, callback); });}function download(url, filename, callback) { console.log(`Downloading ${url}`); request(url, (err, response, body) => { if (err) { return callback(err); } saveFile(filename, body, err => { if (err) { return callback(err); } console.log(`Downloaded and saved: ${url}`); callback(null, body); }); })}/// 最大的启发是实现了如何异步循环遍历数组function spiderLinks(currentUrl, body, nesting, callback) { if (nesting === 0) { return process.nextTick(callback); } const links = utilities.getPageLinks(currentUrl, body); function iterate(index) { if (index === links.length) { return callback(); } spider(links[index], nesting - 1, err => { if (err) { return callback(err); } iterate((index + 1)); }) } iterate(0);}function spider(url, nesting, callback) { const filename = utilities.urlToFilename(url); fs.readFile(filename, "utf8", (err, body) => { if (err) { if (err.code !== 'ENOENT') { return callback(err); } return download(url, filename, (err, body) => { if (err) { return callback(err); } spiderLinks(url, body, nesting, callback); }); } spiderLinks(url, body, nesting, callback); });}spider(process.argv[2], 2, (err, filename, downloaded) => { if (err) { console.log(err); } else if (downloaded) { console.log(`Completed the download of ${filename}`); } else { console.log(`${filename} was already downloaded`); }});上边的代码相比之前的代码多了两个核心功能，首先是通过辅助类获取到了某个body中的links：
const links = utilities.getPageLinks(currentUrl, body);
内部实现就不解释了，另一个核心代码就是：
/// 最大的启发是实现了如何异步循环遍历数组function spiderLinks(currentUrl, body, nesting, callback) { if (nesting === 0) { return process.nextTick(callback); } const links = utilities.getPageLinks(currentUrl, body); function iterate(index) { if (index === links.length) { return callback(); } spider(links[index], nesting - 1, err => { if (err) { return callback(err); } iterate((index + 1)); }) } iterate(0);}可以说上边这一小段代码，就是采用原生实现异步串行的pattern了。除了这些之外，还引入了nesting的概念，通过这是这个属性，可以控制抓取层次。
到这里我们就完整的实现了串行的功能，考虑到性能，我们要开发并行抓取的功能。
(spider_v4.js)
const request = require("request");const fs = require("fs");const mkdirp = require("mkdirp");const path = require("path");const utilities = require("./utilities");function saveFile(filename, contents, callback) { mkdirp(path.dirname(filename), err => { if (err) { return callback(err); } fs.writeFile(filename, contents, callback); });}function download(url, filename, callback) { console.log(`Downloading ${url}`); request(url, (err, response, body) => { if (err) { return callback(err); } saveFile(filename, body, err => { if (err) { return callback(err); } console.log(`Downloaded and saved: ${url}`); callback(null, body); }); })}/// 最大的启发是实现了如何异步循环遍历数组function spiderLinks(currentUrl, body, nesting, callback) { if (nesting === 0) { return process.nextTick(callback); } const links = utilities.getPageLinks(currentUrl, body); if (links.length === 0) { return process.nextTick(callback); } let completed = 0, hasErrors = false; function done(err) { if (err) { hasErrors = true; return callback(err); } if (++completed === links.length && !hasErrors) { return callback(); } } links.forEach(link => { spider(link, nesting - 1, done); });}const spidering = new Map();function spider(url, nesting, callback) { if (spidering.has(url)) { return process.nextTick(callback); } spidering.set(url, true); const filename = utilities.urlToFilename(url); /// In this pattern, there will be some issues. /// Possible problems to download the same url again and again。 fs.readFile(filename, "utf8", (err, body) => { if (err) { if (err.code !== 'ENOENT') { return callback(err); } return download(url, filename, (err, body) => { if (err) { return callback(err); } spiderLinks(url, body, nesting, callback); }); } spiderLinks(url, body, nesting, callback); });}spider(process.argv[2], 2, (err, filename, downloaded) => { if (err) { console.log(err); } else if (downloaded) { console.log(`Completed the download of ${filename}`); } else { console.log(`${filename} was already downloaded`); }});这段代码同样很简单，也有两个核心内容。一个是如何实现并发：
/// 最大的启发是实现了如何异步循环遍历数组function spiderLinks(currentUrl, body, nesting, callback) { if (nesting === 0) { return process.nextTick(callback); } const links = utilities.getPageLinks(currentUrl, body); if (links.length === 0) { return process.nextTick(callback); } let completed = 0, hasErrors = false; function done(err) { if (err) { hasErrors = true; return callback(err); } if (++completed === links.length && !hasErrors) { return callback(); } } links.forEach(link => { spider(link, nesting - 1, done); });}上边的代码可以说是实现并发的一个pattern。利用循环遍历来实现。另一个核心是，既然是并发的，那么利用 fs.exists 就会存在问题，可能会重复下载同一文件，这里的解决方案是：
现在我们又有了新的需求，要求限制同时并发的最大数，那么在这里就引进了一个我认为最重要的概念：队列。
(task-Queue.js)
class TaskQueue { constructor(concurrency) { this.concurrency = concurrency; this.running = 0; this.queue = []; } pushTask(task) { this.queue.push(task); this.next(); } next() { while (this.running < this.concurrency && this.queue.length) { const task = this.queue.shift(); task(() => { this.running--; this.next(); }); this.running++; } }}module.exports = TaskQueue;上边的代码就是队列的实现代码，核心是 next() 方法，可以看出，当task加入队列中后，会立刻执行，这不是说这个任务一定马上执行，而是指的是next会立刻调用。
(spider_v5.js)
const request = require("request");const fs = require("fs");const mkdirp = require("mkdirp");const path = require("path");const utilities = require("./utilities");const TaskQueue = require("./task-Queue");const downloadQueue = new TaskQueue(2);function saveFile(filename, contents, callback) { mkdirp(path.dirname(filename), err => { if (err) { return callback(err); } fs.writeFile(filename, contents, callback); });}function download(url, filename, callback) { console.log(`Downloading ${url}`); request(url, (err, response, body) => { if (err) { return callback(err); } saveFile(filename, body, err => { if (err) { return callback(err); } console.log(`Downloaded and saved: ${url}`); callback(null, body); }); })}/// 最大的启发是实现了如何异步循环遍历数组function spiderLinks(currentUrl, body, nesting, callback) { if (nesting === 0) { return process.nextTick(callback); } const links = utilities.getPageLinks(currentUrl, body); if (links.length === 0) { return process.nextTick(callback); } let completed = 0, hasErrors = false; links.forEach(link => { /// 给队列出传递一个任务，这个任务首先是一个函数，其次该函数接受一个参数 /// 当调用任务时，触发该函数，然后给函数传递一个参数，告诉该函数在任务结束时干什么 downloadQueue.pushTask(done => { spider(link, nesting - 1, err => { /// 这里表示，只要发生错误，队列就会退出 if (err) { hasErrors = true; return callback(err); } if (++completed === links.length && !hasErrors) { callback(); } done(); }); }); });}const spidering = new Map();function spider(url, nesting, callback) { if (spidering.has(url)) { return process.nextTick(callback); } spidering.set(url, true); const filename = utilities.urlToFilename(url); /// In this pattern, there will be some issues. /// Possible problems to download the same url again and again。 fs.readFile(filename, "utf8", (err, body) => { if (err) { if (err.code !== 'ENOENT') { return callback(err); } return download(url, filename, (err, body) => { if (err) { return callback(err); } spiderLinks(url, body, nesting, callback); }); } spiderLinks(url, body, nesting, callback); });}spider(process.argv[2], 2, (err, filename, downloaded) => { if (err) { console.log(`error: ${err}`); } else if (downloaded) { console.log(`Completed the download of ${filename}`); } else { console.log(`${filename} was already downloaded`); }});因此，为了限制并发的个数，只需在 spiderLinks 方法中，把task遍历放入队列就可以了。这相对来说很简单。
到这里为止，我们使用原生JavaScript实现了一个有相对完整功能的网络蜘蛛，既能串行，也能并发，还可以控制并发个数。
2.使用async库
把不同的功能放到不同的函数中，会给我们带来巨大的好处，async库十分流行，它的性能也不错，它内部基于callback。
(spider_v6.js)
const request = require("request");const fs = require("fs");const mkdirp = require("mkdirp");const path = require("path");const utilities = require("./utilities");const series = require("async/series");const eachSeries = require("async/eachSeries");function download(url, filename, callback) { console.log(`Downloading ${url}`); let body; series([ callback => { request(url, (err, response, resBody) => { if (err) { return callback(err); } body = resBody; callback(); }); }, mkdirp.bind(null, path.dirname(filename)), callback => { fs.writeFile(filename, body, callback); } ], err => { if (err) { return callback(err); } console.log(`Downloaded and saved: ${url}`); callback(null, body); });}/// 最大的启发是实现了如何异步循环遍历数组function spiderLinks(currentUrl, body, nesting, callback) { if (nesting === 0) { return process.nextTick(callback); } const links = utilities.getPageLinks(currentUrl, body); if (links.length === 0) { return process.nextTick(callback); } eachSeries(links, (link, cb) => { "use strict"; spider(link, nesting - 1, cb); }, callback);}const spidering = new Map();function spider(url, nesting, callback) { if (spidering.has(url)) { return process.nextTick(callback); } spidering.set(url, true); const filename = utilities.urlToFilename(url); fs.readFile(filename, "utf8", (err, body) => { if (err) { if (err.code !== 'ENOENT') { return callback(err); } return download(url, filename, (err, body) => { if (err) { return callback(err); } spiderLinks(url, body, nesting, callback); }); } spiderLinks(url, body, nesting, callback); });}spider(process.argv[2], 1, (err, filename, downloaded) => { if (err) { console.log(err); } else if (downloaded) { console.log(`Completed the download of ${filename}`); } else { console.log(`${filename} was already downloaded`); }});在上边的代码中，我们只使用了async的三个功能：
const series = require("async/series"); // 串行const eachSeries = require("async/eachSeries"); // 并行const queue = require("async/queue"); // 队列由于比较简单，就不做解释了。async中的队列的代码在(spider_v7.js)中，和上边我们自定义的队列很相似，也不做更多解释了。
3.Promise
Promise是一个协议，有很多库实现了这个协议，我们用的是ES6的实现。简单来说promise就是一个约定，如果完成了，就调用它的resolve方法，失败了就调用它的reject方法。它内有实现了then方法，then返回promise本身，这样就形成了调用链。
其实Promise的内容有很多，在实际应用中是如何把普通的函数promise化。这方面的内容在这里也不讲了，我自己也不够格
(spider_v8.js)
const utilities = require("./utilities");const request = utilities.promisify(require("request"));const fs = require("fs");const readFile = utilities.promisify(fs.readFile);const writeFile = utilities.promisify(fs.writeFile);const mkdirp = utilities.promisify(require("mkdirp"));const path = require("path");function saveFile(filename, contents, callback) { mkdirp(path.dirname(filename), err => { if (err) { return callback(err); } fs.writeFile(filename, contents, callback); });}function download(url, filename) { console.log(`Downloading ${url}`); let body; return request(url) .then(response => { "use strict"; body = response.body; return mkdirp(path.dirname(filename)); }) .then(() => writeFile(filename, body)) .then(() => { "use strict"; console.log(`Downloaded adn saved: ${url}`); return body; });}/// promise编程的本质就是为了解决在函数中设置回调函数的问题/// 通过中间层promise来实现异步函数同步化function spiderLinks(currentUrl, body, nesting) { let promise = Promise.resolve(); if (nesting === 0) { return promise; } const links = utilities.getPageLinks(currentUrl, body); links.forEach(link => { "use strict"; promise = promise.then(() => spider(link, nesting - 1)); }); return promise;}function spider(url, nesting) { const filename = utilities.urlToFilename(url); return readFile(filename, "utf8") .then( body => spiderLinks(url, body, nesting), err => { "use strict"; if (err.code !== 'ENOENT') { /// 抛出错误，这个方便与在整个异步链的最后通过呢catch来捕获这个链中的错误 throw err; } return download(url, filename) .then(body => spiderLinks(url, body, nesting)); } );}spider(process.argv[2], 1) .then(() => { "use strict"; console.log('Download complete'); }) .catch(err => { "use strict"; console.log(err); });可以看到上边的代码中的函数都是没有callback的，只需要在最后catch就可以了。
在设计api的时候，应该支持两种方式，及支持callback，又支持promise
function asyncDivision(dividend, divisor, cb) { return new Promise((resolve, reject) => { "use strict"; process.nextTick(() => { const result = dividend / divisor; if (isNaN(result) || !Number.isFinite(result)) { const error = new Error("Invalid operands"); if (cb) { cb(error); } return reject(error); } if (cb) { cb(null, result); } resolve(result); }); });}asyncDivision(10, 2, (err, result) => { "use strict"; if (err) { return console.log(err); } console.log(result);});asyncDivision(22, 11) .then((result) => console.log(result)) .catch((err) => console.log(err));4.Generator
Generator很有意思，他可以让暂停函数和恢复函数，利用thunkify和co这两个库，我们下边的代码实现起来非常酷。
(spider_v9.js)
const thunkify = require("thunkify");const co = require("co");const path = require("path");const utilities = require("./utilities");const request = thunkify(require("request"));const fs = require("fs");const mkdirp = thunkify(require("mkdirp"));const readFile = thunkify(fs.readFile);const writeFile = thunkify(fs.writeFile);const nextTick = thunkify(process.nextTick);function* download(url, filename) { console.log(`Downloading ${url}`); const response = yield request(url); console.log(response); const body = response[1]; yield mkdirp(path.dirname(filename)); yield writeFile(filename, body); console.log(`Downloaded and saved ${url}`); return body;}function* spider(url, nesting) { const filename = utilities.urlToFilename(url); let body; try { body = yield readFile(filename, "utf8"); } catch (err) { if (err.code !== 'ENOENT') { throw err; } body = yield download(url, filename); } yield spiderLinks(url, body, nesting);}function* spiderLinks(currentUrl, body, nesting) { if (nesting === 0) { return nextTick(); } const links = utilities.getPageLinks(currentUrl, body); for (let i = 0; i < links.length; i++) { yield spider(links[i], nesting - 1); }}/// 通过co就自动处理了回调函数，直接返回了回调函数中的参数，把这些参数放到一个数组中，但是去掉了err信息co(function* () { try { yield spider(process.argv[2], 1); console.log('Download complete'); } catch (err) { console.log(err); }});总结
我并没有写promise和generator并发的代码。以上这些内容来自于这本书nodejs-design-patterns 。
以上就是本文的全部内容，希望对大家的学习有所帮助，也希望大家多多支持武林网。
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