前言
在沒有深度使用函數(shù)回調(diào)的經(jīng)驗(yàn)的時(shí)候，去看這些內(nèi)容還是有一點(diǎn)吃力的。由于Node.js獨(dú)特的異步特性，才出現(xiàn)了“回調(diào)地獄”的問題，這篇文章中，我比較詳細(xì)的記錄了如何解決異步流問題。
文章會(huì)很長(zhǎng)，而且這篇是對(duì)異步流模式的解釋。文中會(huì)使用一個(gè)簡(jiǎn)單的網(wǎng)絡(luò)蜘蛛的例子，它的作用是抓取指定URL的網(wǎng)頁內(nèi)容并保存在項(xiàng)目中，在文章的最后，可以找到整篇文章中的源碼demo。
1.原生JavaScript模式
本篇不針對(duì)初學(xué)者，因此會(huì)省略掉大部分的基礎(chǔ)內(nèi)容的講解：
(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`); }});上邊的代碼的流程大概是這樣的：
這是一個(gè)非常簡(jiǎn)單版本的蜘蛛，他只能抓取一個(gè)url的內(nèi)容，看到上邊的回調(diào)多么令人頭疼。那么我們開始進(jìn)行優(yōu)化。
首先，if else 這種方式可以進(jìn)行優(yōu)化，這個(gè)很簡(jiǎn)單，不用多說，放一個(gè)對(duì)比效果：
/// beforeif (err) { callback(err);} else { callback(null, filename, true);}/// afterif (err) { return callback(err);}callback(null, filename, true);代碼這么寫，嵌套就會(huì)少一層，但經(jīng)驗(yàn)豐富的程序員會(huì)認(rèn)為，這樣寫過重強(qiáng)調(diào)了error，我們編程的重點(diǎn)應(yīng)該放在處理正確的數(shù)據(jù)上，在可讀性上也存在這樣的要求。
另一個(gè)優(yōu)化是函數(shù)拆分，上邊代碼中的spider函數(shù)中，可以把下載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`); }});上邊的代碼基本上是采用原生優(yōu)化后的結(jié)果，但這個(gè)蜘蛛的功能太過簡(jiǎn)單，我們現(xiàn)在需要抓取某個(gè)網(wǎng)頁中的所有url，這樣才會(huì)引申出串行和并行的問題。
(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); }); })}/// 最大的啟發(fā)是實(shí)現(xiàn)了如何異步循環(huán)遍歷數(shù)組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`); }});上邊的代碼相比之前的代碼多了兩個(gè)核心功能，首先是通過輔助類獲取到了某個(gè)body中的links：
const links = utilities.getPageLinks(currentUrl, body);
內(nèi)部實(shí)現(xiàn)就不解釋了，另一個(gè)核心代碼就是：
/// 最大的啟發(fā)是實(shí)現(xiàn)了如何異步循環(huán)遍歷數(shù)組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);}可以說上邊這一小段代碼，就是采用原生實(shí)現(xiàn)異步串行的pattern了。除了這些之外，還引入了nesting的概念，通過這是這個(gè)屬性，可以控制抓取層次。
到這里我們就完整的實(shí)現(xiàn)了串行的功能，考慮到性能，我們要開發(fā)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); }); })}/// 最大的啟發(fā)是實(shí)現(xiàn)了如何異步循環(huán)遍歷數(shù)組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`); }});這段代碼同樣很簡(jiǎn)單，也有兩個(gè)核心內(nèi)容。一個(gè)是如何實(shí)現(xiàn)并發(fā)：
/// 最大的啟發(fā)是實(shí)現(xiàn)了如何異步循環(huán)遍歷數(shù)組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); });}上邊的代碼可以說是實(shí)現(xiàn)并發(fā)的一個(gè)pattern。利用循環(huán)遍歷來實(shí)現(xiàn)。另一個(gè)核心是，既然是并發(fā)的，那么利用 fs.exists 就會(huì)存在問題，可能會(huì)重復(fù)下載同一文件，這里的解決方案是：
現(xiàn)在我們又有了新的需求，要求限制同時(shí)并發(fā)的最大數(shù)，那么在這里就引進(jìn)了一個(gè)我認(rèn)為最重要的概念：隊(duì)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;上邊的代碼就是隊(duì)列的實(shí)現(xiàn)代碼，核心是 next() 方法，可以看出，當(dāng)task加入隊(duì)列中后，會(huì)立刻執(zhí)行，這不是說這個(gè)任務(wù)一定馬上執(zhí)行，而是指的是next會(huì)立刻調(diào)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); }); })}/// 最大的啟發(fā)是實(shí)現(xiàn)了如何異步循環(huán)遍歷數(shù)組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 => { /// 給隊(duì)列出傳遞一個(gè)任務(wù)，這個(gè)任務(wù)首先是一個(gè)函數(shù)，其次該函數(shù)接受一個(gè)參數(shù) /// 當(dāng)調(diào)用任務(wù)時(shí)，觸發(fā)該函數(shù)，然后給函數(shù)傳遞一個(gè)參數(shù)，告訴該函數(shù)在任務(wù)結(jié)束時(shí)干什么 downloadQueue.pushTask(done => { spider(link, nesting - 1, err => { /// 這里表示，只要發(fā)生錯(cuò)誤，隊(duì)列就會(huì)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，為了限制并發(fā)的個(gè)數(shù)，只需在 spiderLinks 方法中，把task遍歷放入隊(duì)列就可以了。這相對(duì)來說很簡(jiǎn)單。
到這里為止，我們使用原生JavaScript實(shí)現(xiàn)了一個(gè)有相對(duì)完整功能的網(wǎng)絡(luò)蜘蛛，既能串行，也能并發(fā)，還可以控制并發(fā)個(gè)數(shù)。
2.使用async庫
把不同的功能放到不同的函數(shù)中，會(huì)給我們帶來巨大的好處，async庫十分流行，它的性能也不錯(cuò)，它內(nèi)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); });}/// 最大的啟發(fā)是實(shí)現(xiàn)了如何異步循環(huán)遍歷數(shù)組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`); }});在上邊的代碼中，我們只使用了async的三個(gè)功能：
const series = require("async/series"); // 串行const eachSeries = require("async/eachSeries"); // 并行const queue = require("async/queue"); // 隊(duì)列由于比較簡(jiǎn)單，就不做解釋了。async中的隊(duì)列的代碼在(spider_v7.js)中，和上邊我們自定義的隊(duì)列很相似，也不做更多解釋了。
3.Promise
Promise是一個(gè)協(xié)議，有很多庫實(shí)現(xiàn)了這個(gè)協(xié)議，我們用的是ES6的實(shí)現(xiàn)。簡(jiǎn)單來說promise就是一個(gè)約定，如果完成了，就調(diào)用它的resolve方法，失敗了就調(diào)用它的reject方法。它內(nèi)有實(shí)現(xiàn)了then方法，then返回promise本身，這樣就形成了調(diào)用鏈。
其實(shí)Promise的內(nèi)容有很多，在實(shí)際應(yīng)用中是如何把普通的函數(shù)promise化。這方面的內(nèi)容在這里也不講了，我自己也不夠格
(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編程的本質(zhì)就是為了解決在函數(shù)中設(shè)置回調(diào)函數(shù)的問題/// 通過中間層promise來實(shí)現(xiàn)異步函數(shù)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') { /// 拋出錯(cuò)誤，這個(gè)方便與在整個(gè)異步鏈的最后通過呢catch來捕獲這個(gè)鏈中的錯(cuò)誤 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); });可以看到上邊的代碼中的函數(shù)都是沒有callback的，只需要在最后catch就可以了。
在設(shè)計(jì)api的時(shí)候，應(yīng)該支持兩種方式，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，他可以讓暫停函數(shù)和恢復(fù)函數(shù)，利用thunkify和co這兩個(gè)庫，我們下邊的代碼實(shí)現(xiàn)起來非常酷。
(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); }}/// 通過co就自動(dòng)處理了回調(diào)函數(shù)，直接返回了回調(diào)函數(shù)中的參數(shù)，把這些參數(shù)放到一個(gè)數(shù)組中，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); }});總結(jié)
我并沒有寫promise和generator并發(fā)的代碼。以上這些內(nèi)容來自于這本書nodejs-design-patterns 。
以上就是本文的全部?jī)?nèi)容，希望對(duì)大家的學(xué)習(xí)有所幫助，也希望大家多多支持武林網(wǎng)。
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