C# & WPF 随手小计之一 ——初探async await 实现多线程处理

本是古典何须时尚 2022-07-15 01:54 32阅读 0赞

# C\# & WPF 随手小计之一 ——初探async await 实现多线程处理 #

时间 2014-11-25 22:37:00  [博客园精华区 ][Link 1]

*原文*  [ http://www.cnblogs.com/provissy/p/4122112.html][http_www.cnblogs.com_provissy_p_4122112.html]

主题 [ C\# ][C_] [WPF ][WPF]

嗯。。。我也是在园子待了不短时间的人了，一直以来汲取着园友的知识，感觉需要回馈什么。

于是以后有空我都会把一些小技巧小知识写下来，有时候可能会很短甚至很简单，但希望能帮到大家咯。

第一篇文章来说说async 和 await吧。

这是微软关于Async的介绍：http://msdn.microsoft.com/en-us/library/hh156513.aspx

这是await ：http://msdn.microsoft.com/en-us/library/hh156528.aspx

这是综合起来讲：http://msdn.microsoft.com/en-us/library/hh191443.aspx

async和await用起来并不是很难，大部分人是这么用的，微软的sample是这么写的：

private async void StartButton_Click(object sender, RoutedEventArgs e) {
    	// ExampleMethodAsync returns a Task<int>, which means that the method
    	// eventually produces an int result. However, ExampleMethodAsync returns
    	// the Task<int> value as soon as it reaches an await.
    	ResultsTextBox.Text += "\n";
    	try
    	{
    		int length = await ExampleMethodAsync();
    		// Note that you could put "await ExampleMethodAsync()" in the next line where
    		// "length" is, but due to when '+=' fetches the value of ResultsTextBox, you
    		// would not see the global side effect of ExampleMethodAsync setting the text.
    		ResultsTextBox.Text += String.Format("Length: {0}\n", length);
    	}
    	catch (Exception)
    	{
    		// Process the exception if one occurs.
    	}
    }
    
    public async Task<int> ExampleMethodAsync() {
    	var httpClient = new HttpClient();
    	int exampleInt = (await httpClient.GetStringAsync("http://msdn.microsoft.com")).Length;
    	ResultsTextBox.Text += "Preparing to finish ExampleMethodAsync.\n";
    	// After the following return statement, any method that's awaiting
    	// ExampleMethodAsync (in this case, StartButton_Click) can get the 
    	// integer result.
    	return exampleInt;
    }
    // Output:
    // Preparing to finish ExampleMethodAsync.
    // Length: 53292

上面是微软给的写法，给按钮的响应方法加上async，然后await ExampleMethodAsync，让一个Length变量来接收返回值。

但是其实async和await还有另一种用法：

private async void button_Click(object sender, RoutedEventArgs e) {
                TB_Result.Text = await exampleAsync();
            }

private async Task<string> exampleAsync() {
    			 return await Task.Run(() =>
    			 {
    				 WebClient w = new WebClient();
    				 w.DownloadFile("http://provissy.com", "webPage1");
    				 w.DownloadFile("http://microsoft.com", "webPage2");
    				 string a = w.DownloadString("http://microsoft.com");
    				 return a;
    			 });
    		}

Task.Run 和 Lambda表达式，用起来其实更加简单。

在 => \{ \}范围内写任何代码都是异步执行的，因此不用担心某几个语句是否会长时间堵住UI线程。

当然，你不能访问UI线程，否则会抛出异常。

如果不返回值，那就更简单了

private async Task exampleAsync() { // 用 await exampleAsync(); 来调用即可，会立即返回。 await Task.Run(() => { WebClient w = new WebClient(); w.DownloadFile("http://provissy.com", "webPage1"); w.DownloadFile("http://microsoft.com", "webPage2"); }); }

好了就这么多，以后会尽量写详细点。。。

[Link 1]: http://www.tuicool.com/sites/bAzeYv
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[C_]: http://www.tuicool.com/topics/11050001
[WPF]: http://www.tuicool.com/topics/11250011