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nodejs毁掉地狱是一直被人诟病的,以下总结一下解决毁掉地狱的一些方法。(暂时研究的比较浅)
1.promise
promise模式在任何时刻都处于以下三种状态之一:未完成(unfulfilled)、已完成(resolved)和拒绝(rejected)。以CommonJS Promise/A 标准为例,promise对象上的then方法负责添加针对已完成和拒绝状态下的处理函数。then方法会返回另一个promise对象,以便于形成promise管道,这种返回promise对象的方式能够支持开发人员把异步操作串联起来,如then(resolvedHandler, rejectedHandler); 。resolvedHandler 回调函数在promise对象进入完成状态时会触发,并传递结果;rejectedHandler函数会在拒绝状态下调用。(其中rejectedHandler可选)。
以下为一个有几级嵌套的函数,看起来比较令人恶心。(如果换成缩进四个字符可想而知)
'use strict';
const md = require('markdown-it')();
const fs = require('fs');
fs.watchFile('nodejs.md', (curr, prev) => {
let mdStr = fs.readFile('./nodejs.md', 'utf-8', (err, data) => {
let mdData = md.render(data);
let htmlTemplate = fs.readFile('./index.html', 'utf-8', (err, data) => {
let html = data.replace('{{content}}', mdData);
console.log(mdData);
fs.writeFile('./nodejs.html', html, 'utf-8', (err, data) => {
if (err) {
throw err;
} else {
console.log('OK');
}
});
});
});
});
一下用promise的方式实现同样的效果,首先把异步函数封装一下,然后下面可以指教调用。可能看起来代码比之前的版本更多,但是封装的异步函数是可以复用的。等任务多了就不显得代码多了。(但看最后调用函数的部分是不是优雅了不少)
'use strict';
const fs = require('fs');
const md = require('markdown-it')();
var Q = require('q');
function fs_readFile(file, encoding) {
var deferred = Q.defer();
fs.readFile(file, encoding, function(err, data) {
if (err) deferred.reject(err); // rejects the promise with `er` as the reason
else
deferred.resolve(data) // fulfills the promise with `data` as the value
});
return deferred.promise; // the promise is returned
}
function fs_writeFile(file, data, encoding) {
var deferred = Q.defer();
fs.writeFile(file, data, encoding, function(err, data) {
if (err) deferred.reject(err); // rejects the promise with `er` as the reason
else deferred.resolve(data); // fulfills the promise with `data` as the value
});
return deferred.promise ;// the promise is returned
//return 1; // the promise is returned
}
function fs_watchFile(file, curr, prev) {
var deferred = Q.defer();
fs.watchFile(file, function(curr, prev) {
if (!prev) deferred.reject(err); // rejects the promise with `er` as the reason
else deferred.resolve(curr); // fulfills the promise with `data` as the value
});
return deferred.promise // the promise is returned
}
function markdowm_convert(file, encoding, mdData) {
var convertData = md.render(mdData);
console.log(convertData);
var deferred = Q.defer();
fs.readFile(file, encoding, function(err, data) {
if (err) deferred.reject(err); // rejects the promise with `er` as the reason
else {
data = data.replace('{{content}}', convertData);
deferred.resolve(data); // fulfills the promise with `data` as the value
}
})
return deferred.promise; // the promise is returned
}
// ===============promise实现 =====================
fs_watchFile('nodejs.md')
.then(function() {
return fs_readFile('./nodejs.md', 'utf-8');
})
.then(function(mdData) {
return markdowm_convert('./index.html', 'utf-8', mdData);
})
.then(function(data) {
fs_writeFile('./nodejs.html', data, 'utf-8');
});
2.async
node的async包有多的数不清的方法我暂时只实验了一个waterfall
waterfall瀑布流的意思和async中另一个函数series差不多都是按照顺序执行,不同之处是waterfall每执行完一个函数都会产生一个值,然后把这个值给下一个函数用。
以下是嵌套了两级的读写文件程序
fs.readFile('01.txt','utf-8',function(err,date){
fs.writeFile('02.txt',date,'utf-8',function(err,date){
console.log('复制完了');
});
})
用async.waterfall 后代码如下
async.waterfall([
function(cb){
fs.readFile('01.txt','utf-8',function(err,result){
cb(err,result);
});
},function(result,cb){
fs.writeFile('02.txt',result,'utf-8',function(err,result){
cb(err,result);
});
}
],function(err,result){
console.log('复制完了');
})
另外最近学习了generator及node co库的源码分析。详见这里 |
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