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

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。。。