Node.js異步流控制詳解
- 小云云原創
- 2018-01-22 10:11:591182瀏覽
在沒有深度使用函數回呼的經驗的時候，去看這些內容還是有一點吃力的。由於Node.js獨特的非同步特性，才出現了「回調地獄」的問題，在這篇文章中，我比較詳細的記錄瞭如何解決異步流問題。本文主要介紹了淺談Node.js之非同步流控制，小編覺得挺不錯的，現在分享給大家，也給大家做個參考。一起跟著小編過來看看吧，希望能幫助大家。
文章會很長，而且這篇是對非同步流模式的解釋。文中會使用一個簡單的網路蜘蛛的例子，它的作用是抓取指定URL的網頁內容並保存在專案中，在文章的最後，可以找到整篇文章中的原始碼demo。
1.原生JavaScript模式
本篇不針對初學者，因此會省略掉大部分的基礎內容的講解：
(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`);
}
});上邊的程式碼的流程大概是這樣的：
把url轉換成filename
判斷該檔案名稱是否存在，若存在直接返回，否則進入下一步
#發送請求，取得body
/// before
if (err) {
callback(err);
} else {
callback(null, filename, true);
}
/// after
if (err) {
return callback(err);
}
callback(null, filename, true);程式碼這麼寫，嵌套就會少一層，但經驗豐富的程式設計師會認為，這樣寫過重強調了error，我們程式設計的重點應該放在處理正確的資料上，在可讀性上也存在這樣的要求。 另一個優化是函式拆分，上邊程式碼中的spider函數中，可以把下載檔和儲存檔拆分出去。
(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`);
}
});上邊的程式碼基本上是採用原生最佳化後的結果，但這個蜘蛛的功能太簡單，我們現在需要抓取某個網頁中的所有url，這樣才會引申出串列和並行的問題。
(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`);
}
});上邊的程式碼相比之前的程式碼多了兩個核心功能，首先是透過輔助類別取得到了某個body中的links：
const links = utilities.getPageLinks(currentUrl, body);內部實作就不解釋了，另一個核心程式碼就是：
/// 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);
}可以說上邊這一小段程式碼，就是採用原生實作非同步串列的pattern了。除了這些之外，還引入了nesting的概念，透過這是這個屬性，可以控制抓取層次。
到這裡我們就完整的實現了串行的功能，考慮到性能，我們要開發並行抓取的功能。
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`);
}
});/// 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);
});
}上邊的程式碼可以說是實作並發的一個pattern。利用迴圈遍歷來實現。另一個核心是，既然是並發的，那麼利用fs.exists 就會存在問題，可能會重複下載同一文件，這裡的解決方案是：
使用Map快取某一url，url應該作為key
現在我們又有了新的需求，要求限制同時並發的最大數，那麼在這裡就引進了一個我認為最重要的概念：隊列。
(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;上邊的程式碼就是佇列的實作程式碼，核心是next() 方法，可以看出，當task加入佇列後，會立刻執行，這不是說這個任務一定馬上執行，而是指的是next會立刻呼叫。
(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，為了限制並發的個數，只需在spiderLinks 方法中，把task遍歷放入佇列就可以了。這相對來說很簡單。 到這裡為止，我們使用原生JavaScript實作了一個有相對完整功能的網路蜘蛛，既能串行，也能並發，還可以控制並發個數。 把不同的功能放到不同的函式中，會為我們帶來巨大的好處，async函式庫十分流行，它的性能也不錯，它內部基於callback。
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`);
}
});在上邊的程式碼中，我們只使用了async的三個功能：
const series = require("async/series"); // 串行
const eachSeries = require("async/eachSeries"); // 并行
const queue = require("async/queue"); // 队列###由於比較簡單，就不會解釋了。 async中的佇列的程式碼在(spider_v7.js)中，和上邊我們自訂的佇列很相似，也不做更多解釋了。 #########3.Promise#########Promise是一個協議，有很多函式庫實作了這個協議，我們用的是ES6的實作。簡單來說promise就是一個約定，如果完成了，就呼叫它的resolve方法，失敗了就呼叫它的reject方法。它內有實作了then方法，then回傳promise本身，這樣就形成了呼叫鏈。 ###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 asyncpision(pidend, pisor, cb) {
return new Promise((resolve, reject) => {
"use strict";
process.nextTick(() => {
const result = pidend / pisor;
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);
});
});
}
asyncpision(10, 2, (err, result) => {
"use strict";
if (err) {
return console.log(err);
}
console.log(result);
});
asyncpision(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 。
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