浏览器自动化的一些体会11 webclient的异步操作

原来的代码大致如下：

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private void foo(string url) {        using (WebClient client = new WebClient())        {            client.DownloadProgressChanged += (obj, ev) =>            {                client.CancelAsync();            };                         client.DownloadStringCompleted += (obj, ev) =>            {                 ................                 Thread.Sleep(30000);                 bar(client, newUrl);            }        } }   private void bar(WebClient client, string url) {         client.DownloadStringAsync(url); }

　　

本来以为这样可以自动访问一串网址(这里省略了取下一个url的代码)，但运行后发现，经常会运行到一半停下来，程序并没有死掉，只是停止了，不继续访问下一个网址。

开始以为是Thread.Sleep()在做怪，把它移到bar方法里：

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private void bar(WebClient client, string url) {       Thread.Sleep(30000);       client.DownloadStringAsync(url); }

结果没用。想了半天，最后估计是访问某个网址时，一直在请求，但一直没收到数据，所以没有触发DownloadProgressChanged 事件。又因为是异步执行，所以UI没有阻塞。

最后用了个Timer控件，把代码改成：

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private void foo(string url) {        using (WebClient client = new WebClient())        {            client.DownloadProgressChanged += (obj, ev) =>            {                client.CancelAsync();            };                         client.DownloadStringCompleted += (obj, ev) =>            {                 ................                 if (timer1.Enabled)                 {                     timer1.Stop();                 }                 bar(client, newUrl);            }            timer1.Tick += (obj, ev) => {                 timer1.Stop();                 if (client.IsBusy)                 {                     client.CancelAsync();                 }                 else                 {                     bar(client, url);                 }            }            bar(client, url);        } }   private void bar(WebClient client, string url) {       Thread.Sleep(30000);       client.DownloadStringAsync(url);       timer1.Stop();       timer1.Start(); }

　　

Timer的间隔也设成30秒。测试了几百个url没有问题。