1.原始的异步方法的调用

 

我们来看个简单的例子，在这里演示调用 WebClient.DownloadStringAsync 方法(这个方法不是 TAP)，然后由 WebClient.DownloadStringCompleted 事件通知 UI 更新，这是大多数人都会用的方法。

 

1. private
   void DownloadString(string address)
2. {
3.     WebClient wc = new WebClient();
4.     wc.DownloadStringCompleted += (sender, e) =>
5.     {
6.         if (e.Cancelled)
7.             this.textBox1.Text = "Cancel";
8.         else
   if (e.Error != null)
9.             this.textBox1.Text = "Error";
10.         else
11.             this.textBox1.Text = e.Result;
12.     };
13.  
14.     wc.DownloadStringAsync(new Uri(address));
15. }

客户端调用

 

1. private
   void button_DownloadString_Click(object sender, EventArgs e)
2. {
3.     DownloadString("https://www.google.com.tw/");
4. }

这是一个很简单的例子，一旦若项目里有成千上万的通知事件跟 UI 绑在一起，维护起来会相当的痛苦。

 

2.将 EAP 转换成 TAP步骤

 

• 命名规则以 Async 为后缀
• 返回 Task 或是 Task<TResult>
• 调用 TaskCompletionSource 方法

 

改变 Task 状态可调用以下三个方法：SetCanceled、SetException、SetResult

1. private Task<string> DownloadStringAsync(string address)
2. {
3.     var tcs = new TaskCompletionSource<string>();
4.     WebClient wc = new WebClient();
5.     wc.DownloadStringCompleted += (sender, e) =>
6.     {
7.         if (e.Cancelled)
8.             tcs.SetCanceled();
9.         else
   if (e.Error != null)
10.             tcs.SetException(e.Error);
11.         else
12.             tcs.SetResult(e.Result);
13.     };
14.  
15.     wc.DownloadStringAsync(new Uri(address));
16.     return tcs.Task;
17. }

 

客户端调用

1. private async void button_DownloadStringAsync_Click(object sender, EventArgs e)
2. {
3.     var task = DownloadStringAsync("https://www.google.com.tw/");
4.     await task;
5.     if (task.IsCanceled)
6.     {
7.         this.textBox1.Text = "Cancel";
8.     }
9.     else
   if (task.IsFaulted)
10.     {
11.         this.textBox1.Text = "Error";
12.     }
13.     else
    if (task.IsCompleted)
14.     {
15.         this.textBox1.Text = task.Result;
16.     }
17. }

 

转自：http://www.it165.net/pro/html/201308/6710.html

 

3.微软的封装

 

1. public
   static
   Task<byte[]> DownloadDataTask(this WebClient webClient, Uri address)
2. {
3.     // Create the task to be returned
4.     var tcs = new TaskCompletionSource<byte[]>(address);
5.  
6.     // Setup the callback event handler
7.     DownloadDataCompletedEventHandler handler = null;
8.     handler = (sender, e) => EAPCommon.HandleCompletion(tcs, e, () => e.Result, () => webClient.DownloadDataCompleted -= handler);
   
9.     webClient.DownloadDataCompleted += handler;
10.  
11.     // Start the async work
12.     try
13.     {
14.         webClient.DownloadDataAsync(address, tcs);
15.     }
16.     catch(Exception exc)
17.     {
18.         // If something goes wrong kicking off the async work,
19.         // unregister the callback and cancel the created task
20.         webClient.DownloadDataCompleted -= handler;
21.         tcs.TrySetException(exc);
22.     }
23.  
24.     // Return the task that represents the async operation
25.     return tcs.Task;
26. }

 

1. internal
   class EAPCommon
2. {
3.     internal
   static
   void HandleCompletion<T>(
4.         TaskCompletionSource<T> tcs, AsyncCompletedEventArgs e, Func<T> getResult, Action unregisterHandler)
5.     {
6.         // Transfers the results from the AsyncCompletedEventArgs and getResult() to the
7.         // TaskCompletionSource, but only AsyncCompletedEventArg's UserState matches the TCS
8.         // (this check is important if the same WebClient is used for multiple, asynchronous
9.         // operations concurrently). Also unregisters the handler to avoid a leak.
10.         if (e.UserState == tcs)
11.         {
12.             if (e.Cancelled) tcs.TrySetCanceled();
13.             else
    if (e.Error != null) tcs.TrySetException(e.Error);
14.             else tcs.TrySetResult(getResult());
15.             unregisterHandler();
16.         }
17.     }
18. }

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