Node.js中怎么实现异步流控制,很多新手对此不是很清楚,为了帮助大家解决这个难题,下面小编将为大家详细讲解,有这方面需求的人可以来学习下,希望你能有所收获。
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:
把url转换成filename
判断该文件名是否存在,若存在直接返回,否则进入下一步
发请求,获取body
把body写入到文件中
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:
/// before
if (err) {
callback(err);
} else {
callback(null, filename, true);
}
/// after
if (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:
使用Map缓存某一url,url应该作为key
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);
}
});
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