async修改一个方法,表示其为异步方法。而await表示等待一个异步任务的执行。js方面,在es7中开始得以支持;而.net在c#5.0开始支持。本文章将分别简单介绍他们在js和.net中的基本用法。
一、在js中的实现
js中的异步,还是基于Promise实现的。没有Promise就办法谈异步了。并且await只能出现async修改的方法中;以及reject会触发catch(异常)。
class AsyncTest{//simple example async run(){//按照顺序等待后输出let one = await this.output("one", 1000);console.log('output:' one);let two = await this.output("two", 3000);console.log(two);console.log('run.....');}//await and Promise.all difference async runDiff(){ let one = this.output('diff one', 2000);let two = this.output('diff two', 2000);console.log( await two await one ); //在2秒之后,两个都输出了,而不是各自都等待两秒console.log('runDiff.....');}//Promise.all realize runAll(){let nowTime = new Date();console.log('b:' nowTime.toTimeString());let array = ["a", "b", "c"];let that = this;array.forEach(async function(item){console.log( await that.output(item, 2000) );//2秒后同时输出 });let fn = async ()=>{for(let item of array){let v = await this.output(item, 2000);console.log(v ); //分步骤两秒执行 }}fn.call(this);}premosFn(){let nowTime = new Date();console.log('b:' nowTime.toTimeString());let array = ["a", "b", "c"];let that = this;//promise.alllet preFn = async function(){let promises = array.map(function(item){return that.output(item,2000); //同时开启多个定时器 });let r = await Promise.all(promises);console.log(r.join(','));}preFn();}reject(){let rejectFn = function(){return new Promise((resolve, reject)=>{setTimeout(()=>{reject();},2000);});}let asyncReject = async function(){try{await rejectFn();}catch( e) {console.log('reject.....');}}asyncReject();}output(log, time){return new Promise(resolve=>{setTimeout(()=>{var nowTime = new Date();resolve( nowTime.toTimeString() ":" log "\r\n");}, time);});}}
方法说明如下:
output:简单的输出方法,但返回了一Promise。run: 使用await来等待两次对output的执行runDiff:调用output时即创建promise。两个promise会同步执行runAll:多任务同步执行和按步骤执行的实现方法。也就是forEach和for方法体中使用await的区别premosFn: promise.all的使用。reject: promise的reject会触发await的异常。
二、在c#中的实现
C#中异常是通过Task来实现的,所以标记了async的方法,其方法体中都可以出现await,否则不可以。以及Task中抛出的异常,如果没有同步等待,则不能获取异常
public class AsyncDemo{private Task<string> Output(string val, int time){return System.Threading.Tasks.Task.Run(() =>{System.Threading.Thread.Sleep(time * 1000);return (DateTime.Now.ToLongTimeString()) ": " val "\r\n";});}public async System.Threading.Tasks.Task Run(){string oneVal = await Output("One", 2);string twoVal = await Output("Two", 2);System.Console.WriteLine("Run \r\n" oneVal " " twoVal);}public async System.Threading.Tasks.Task RunDiff(){Task<string> oneTask = Output("one", 2);Task<string> twoTask = Output("two", 2);string val = await oneTask await twoTask; System.Console.WriteLine("RunDiff \r\n" val);}public async System.Threading.Tasks.Task RunAll(){System.Console.WriteLine("b:" (DateTime.Now.ToLongTimeString()));string[] array = new string[3] { "a", "b", "c" };foreach(var item in array){string v = await Output(item, 2);System.Console.WriteLine(v);}}public async System.Threading.Tasks.Task PromiseFn(){System.Console.WriteLine("b:" (DateTime.Now.ToLongTimeString()));string[] array = new string[3] { "a", "b", "c" };List<System.Threading.Tasks.Task<string>> tasks = new List<System.Threading.Tasks.Task<string>>();foreach (var item in array){tasks.Add(Output(item, 2));}//waitAll返回值不能获取,他返回为void,而WhenAll则返回为一个Task(这个Task就有其列表值)string[] r = await System.Threading.Tasks.Task.WhenAll(tasks.ToArray());System.Console.WriteLine(string.Join(",",r));}public async System.Threading.Tasks.Task Reject(){Func<System.Threading.Tasks.Task> func = async () =>{throw new Exception("custom...");await Output("reject", 2);};await func();}}
调用代码如下:
AsyncDemo asyncDemo = new AsyncDemo();asyncDemo.Run().Wait();asyncDemo.RunDiff().Wait();asyncDemo.RunAll().Wait();asyncDemo.PromiseFn().Wait();try{asyncDemo.Reject().Wait();}catch(Exception e){System.Console.WriteLine("reject ex");}
上述代码就是Js的async和await在c#中的翻版实现。 其中每个异步方法的调用,都用到了Wait方法来进行同步等待。以获取到结果。而没有像Js中那么难以控制。尤其注意,async方法中异常的捕获。
三、两者的异同点
js中的async方法的调用,是没有wait方法来等待结果的执行的,只能通过promise来监听执行结果c#中的async方法,由于推荐返回Task或者Task<T>,所以可以用Wait来等待执行结果,如果async方法返回为void,则与js类似。 C#中的下面示例方法的调用者捕获不了异常:public async void Run(){string oneVal = await Output("One", 2);string twoVal = await Output("Two", 2);System.Console.WriteLine("Run" oneVal " " twoVal);}
c#中的Task可以异步方法的链式调用,即可将前一任务的执行结果作为第二任务的参数传入,当然js的Promise也完全是可以很轻松的实现:
System.Console.WriteLine("b:" (DateTime.Now.ToLongTimeString()));string[] array = new string[3] { "a", "b", "c" };var r = await this.Output(array.First(), 2).ContinueWith((tsc) =>{string v = tsc.GetAwaiter().GetResult();return this.Output(v "02", 2);});System.Console.WriteLine(r.Result);
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