async简单使用

node的异步io虽然好用，但是控制异步流程确实一个比较麻烦的事情，比如在爬虫中控制并发数量，避免并发过大导致网站宕机或被加入黑名单。因此需要一个工具来控制并发，这个工具可以自己写或者使用async(官方文档点击这里)。代码基于node 8.x，如版本过低可能会出现错误。

说明

async本身有七十多个方法，这里只说明几个比较常用的简单函数用法，想进一步学习可参考文档。总的来说分为两大类。

一、第一个参数为函数集合，也就是遍历执行集合中的函数。

1.顺序执行 series（tasks , function(err,res){ }）

tasks为函数数组，数组中的每一项都为待执行函数。

a.下面是一个最简单示例，待执行函数为非异步

 const asyncx = require( 'async' );
 
 let tasks = [];
 for( let i = 0 ; i < 10 ; i++ )
 {
     tasks.push(
         function( callback ){
             //dosomething
             console.log( i );
             //本函数用来通知async本次任务完成情况，并把结果带出去。
             callback( null , i ); //第一个参数为异常参数，如果传入一个error( 比如new Error('error') ),并发结束，调用series里的回调。
         }
     )
 }
 asyncx.series( tasks , function(err , res ){
     if( err )
         console.log( err );
     console.log( res );
 } )

运行结果

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" 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b.待执行函数为异步

如果待处理的函数也是异步函数可将callback参数传入到异步函数中，在真正结束的时候调用callback。

 const asyncx = require( 'async' );  //模块命名为asyncx避免和es7中的async/await冲突
 
 let tasks = [];
 for( let i = 0 ; i < 10 ; i++ )
 {
     tasks.push(
         function( callback ){
             setTimeout( function(){
                 console.log(i);
                 callback( null , 1 ); //在这里整个处理才是真正完成，然后调用callback通知async本任务结束
             } , 2000 );
         }
     )
 }
 asyncx.series( tasks , function(err , res ){
     if( err )
         console.log( err );
     console.log( res );
 } )

如果想要在任务中使用es7的async/await 可将待处理代码放在一个闭包中（直接在function前加async会报错），下面示例。

 const asyncx = require( 'async' );  //模块命名为asyncx避免和es7中的async/await冲突
 
 let tasks = [];
 for( let i = 0 ; i < 10 ; i++ )
 {
     tasks.push(
         function( callback ){
             (async function(){
                 let sum = await doSomething( i );
                 callback( null , sum );
             })();
         }
     )
 }
 
 async function doSomething( i ){
     return new Promise( function( resolve , reject ){
         setTimeout(function(){
             console.log( i );
             resolve( i );
         } , 1000 );
     } );
 }
 
 asyncx.series( tasks , function(err , res ){
     if( err )
         console.log( err );
     console.log( res );
 } )

2.并发执行 parallel( tasks , function(err,res){} )

参数如上，不限制并发

3.并发限制执行parallelLimit( tasks , num , function(err,res){} )

参数同上，num为最大并发数量

二、第一个参数为非函数集合。

比如

async.map(['file1','file2','file3'], function(item,callback){
    //dosomething
    callback( null , 'done' );
    }, function(err, results) {
    // results is now an array of stats for each file
});

第二个参数为一个异步函数，要能接受两个参数item(前面集合中的一项),callback 通知async任务完成

还有其他的函数用法都是类似只是具体作用不一样。可参考官方文档说明。