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 。