APM之基于事件的异步模式（EAP）-2

EAP是针对Windows窗体开发提供的方便使用的异步模式，可以在IDE中可视化的设计和使用

            // The System.Net.WebClient class supports the Event-based Asynchronous Pattern

            WebClient wc = new WebClient();

            // When a string completes downloading, the WebClient object raises the 

            // DownloadStringCompleted event which will invoke our ProcessString method         

            wc.DownloadStringCompleted += (s, e) =>

            {

                System.Windows.Forms.MessageBox.Show((e.Error != null) ? e.Error.Message : e.Result);

            };

            // Start the asynchronous operation (this is like calling a BeginXxx method)

            wc.DownloadStringAsync(new Uri("http://Wintellect.com"));

.net有很多的类是这个模式

System.ComponentModel.Component-derived types 

System.ComponentModel.BackgroundWorker

System.Media.SoundPlayer

System.Net.WebClient

System.Net.NetworkInformation.Ping

System.Windows.Forms.PictureBox (derived from Control)

System.Object-derived types

System.Net.Mail.SmtpClient

System.Deployment.Application.ApplicationDeployment

System.Deployment.Application.InPlaceHostingManager

System.Activities.WorkflowInvoker

System.ServiceModel.Activities.WorkflowControlClient

System.Net.PeerToPeer.PeerNameResolver

System.Net.PeerToPeer.Collaboration.ContactManager

System.Net.PeerToPeer.Collaboration.Peer

System.Net.PeerToPeer.Collaboration.PeerContact

System.Net.PeerToPeer.Collaboration.PeerNearMe

System.ServiceModel.Discovery.AnnouncementClient

System.ServiceModel.Discovery.DiscoveryClient

使用TaskCompletionSource将EAP转换为Task

            // The System.Net.WebClient class supports the Event-based Asynchronous Pattern
            WebClient wc = new WebClient();

            // Create the TaskCompletionSource and its underlying Task object
            var tcs = new TaskCompletionSource<String>();

            // When a string completes downloading, the WebClient object raises the 
            // DownloadStringCompleted event which will invoke our ProcessString method
            wc.DownloadStringCompleted += (sender, ea) =>
            {
                // This code always executes on the GUI thread; set the Task’s state
                if (ea.Cancelled) tcs.SetCanceled();
                else if (ea.Error != null) tcs.SetException(ea.Error);
                else tcs.SetResult(ea.Result);
            };

            // Have the Task continue with this Task that shows the result in a message box
            // NOTE: The TaskContinuationOptions.ExecuteSynchronously flag is required to have this code
            // run on the GUI thread; without the flag, the code runs on a thread pool thread 
            tcs.Task.ContinueWith(t =>
            {
                try
                {
                    System.Windows.Forms.MessageBox.Show(t.Result);
                }
                catch (AggregateException ae)
                {
                    System.Windows.Forms.MessageBox.Show(ae.GetBaseException().Message);
                }
            }, TaskContinuationOptions.ExecuteSynchronously);

            // Start the asynchronous operation (this is like calling a BeginXxx method)
            wc.DownloadStringAsync(new Uri("http://Wintellect.com"));

APM之EAP APM比较、

	优点	缺点
EAP	可以和Visual Stuido配合使用，提供设计时支持 EAP是通过SynchronizationContext处理线程模型的，因此GUI程序中使用方便	比APM消耗的更多地内存、更慢的速度 EAP的异常不会抛出
APM	APM更接近事物本质 EAP内部一般都是用APM事先	-

详细参考：

Clr Via C#

http://transbot.blog.163.com

http://ys-f.ys168.com/?CLR_via_CSharp_3rd_Edition_Code_by_Jeffrey_Richter.zip_55bism1e0e7bkisjthit2bso0cm5bs4bs1b5bktnql0c0bu22f05f12z