Silverlight的RIA应用中访问远端的WebService或WCF服务，都是通过异步线程模式调用的。在某些情况下我们的调用是需要同步进行，虽然Silverlight没有内置同步线程模式调用远端服务接口，但是我们可以通过多线程的处理来伪装出同步调用的实现。在.NET Framework的多线程编程中提供了丰富的线程接口，其中AutoResetEvent和ManualResetEvent在多线程编码中最为常用，本文将介绍如何通过AutoResetEvent的线程等待特性实现Silverlight同步调用远端WCF服务。
一、定义WCF服务
为了演示同步调用WCF服务的实现，提供一个简单的WCF服务接口，完成返回一本图书基本信息，WCF服务接口定义如下：
[ServiceContract] public interface IDataService{ [OperationContract] Book GetBook();} public class Book{ public int ID { get ; set ; } public string Name { get ; set ; } public string Author { get ; set ; } public double Price { get ; set ; }}
接口提供一个返回图书基本信息的方法，包括图书编好，图书名，图书作者以及图书价格。接口具体的实现如下代码：
public class DataService : IDataService{ public Book GetBook() { return new Book { ID = 1001 , Name = " 《三国演义》 " , Author = " 罗贯中 " , Price = 89.50 }; }}
如上提供可正常运行的WCF服务接口，在需要调用接口的地方通过WEB引用既可生成该服务的客户端代理对象。
二、基于MVVM模式的视图模型
MVVM模式的核心为INotifyPropertyChanged接口，对于实体模型对象和UI控件元素间提供了完善的同步更新特性。为了方便界面元素同步更新，这里引入了MVVP模式的简单应用。
public class ViewModelBase : INotifyPropertyChanged{ public event PropertyChangedEventHandler PropertyChanged; protected void RaisePropertyChangedEvent( string propertyName) { var handler = PropertyChanged; if (handler != null ) handler( this , new PropertyChangedEventArgs(propertyName)); }}
还需要对应于服务接口中的Book对象定义一个ViewModel对象，详细如下代码所示：
public class BookViewModel : ViewModelBase{ private int iD; /// <summary> /// 图书ID /// </summary> public int ID { get { return iD; } set { iD = value; RaisePropertyChangedEvent( " ID " ); } } private string name; /// <summary> /// 图书名称 /// </summary> public string Name { get { return name; } set { name = value; RaisePropertyChangedEvent( " Name " ); } } private string author; /// <summary> /// 图书作者 /// </summary> public string Author { get { return author; } set { author = value; RaisePropertyChangedEvent( " Author " ); } } private double price; /// <summary> /// 图书价格 /// </summary> public double Price { get { return price; } set { price = value; RaisePropertyChangedEvent( " Price " ); } }}
三、基于AutoResetEvent的同步实现
利用AutoResetEvent的线程等待特性，可以折中实现Silverlight同步调用远端WCF服务。其原理就是在Silverlight发起异步调用远端WCF的时候进行线程阻塞，比记录异步调用远端WCF服务接口的完成事件，当异步调用完成后就终止线程阻塞，从而获取状态事件对象中或得调用远程接口所返回的结果。由于视图模型对象实现了INotifyPropertyChanged接口能够及时的更新界面元素，以此间接的就实现了同步方式调用。
public class AsyncCallStatus < T > { public AsyncCallStatus() { } public T CompletedEventArgs { get ; set ; }}
public class BookFacade{ private AutoResetEvent autoResetEvent = new AutoResetEvent( false ); public void GetBook(BookViewModel viewModel) { if (viewModel == null ) { throw new ArgumentNullException( " viewModel " , " 参数不能为空。 " ); } DataService.DataServiceClient client = new DataService.DataServiceClient(); client.GetBookCompleted += client_GetBookCompleted; var status = new AsyncCallStatus < GetBookCompletedEventArgs > (); client.GetBookAsync(status); // 阻塞线程 autoResetEvent.WaitOne(); if (status.CompletedEventArgs.Error != null ) { throw status.CompletedEventArgs.Error; } var book = status.CompletedEventArgs.Result; viewModel.ID = book.ID; viewModel.Name = book.Name; viewModel.Author = book.Author; viewModel.Price = book.Price; } private void client_GetBookCompleted( object sender, GetBookCompletedEventArgs e) { var status = e.UserState as AsyncCallStatus < GetBookCompletedEventArgs > ; status.CompletedEventArgs = e; // 终止线程阻塞 autoResetEvent.Set(); }}
四、Silverlight前端调用
Siverlight前端就简单布局一个表单作为数据呈现界面，其代码如下：
< Grid x:Name ="LayoutRoot" Background ="White" > < Grid HorizontalAlignment ="Left" Name ="grid1" VerticalAlignment ="Top" Width ="300" Margin ="20" > < Grid.RowDefinitions > < RowDefinition Height ="30" ></ RowDefinition > < RowDefinition Height ="30" ></ RowDefinition > < RowDefinition Height ="30" ></ RowDefinition > < RowDefinition Height ="30" ></ RowDefinition > < RowDefinition Height ="30" ></ RowDefinition > </ Grid.RowDefinitions > < Grid.ColumnDefinitions > < ColumnDefinition Width ="60" ></ ColumnDefinition > < ColumnDefinition Width ="*" ></ ColumnDefinition > </ Grid.ColumnDefinitions > < sdk:Label HorizontalAlignment ="Left" Content ="图书编号：" VerticalAlignment ="Center" Grid.Column ="0" Grid.Row ="0" /> < TextBox Text =" {Binding ID} " Grid.Column ="1" Grid.Row ="0" ></ TextBox > < sdk:Label HorizontalAlignment ="Left" Content ="图书名称：" VerticalAlignment ="Center" Grid.Column ="0" Grid.Row ="1" /> < TextBox Text =" {Binding Name} " Grid.Column ="1" Grid.Row ="1" ></ TextBox > < sdk:Label HorizontalAlignment ="Left" Content ="图书作者：" VerticalAlignment ="Center" Grid.Column ="0" Grid.Row ="2" /> < TextBox Text =" {Binding Author} " Grid.Column ="1" Grid.Row ="2" ></ TextBox > < sdk:Label HorizontalAlignment ="Left" Content ="图书价格：" VerticalAlignment ="Center" Grid.Column ="0" Grid.Row ="3" /> < TextBox Text =" {Binding Price} " Grid.Column ="1" Grid.Row ="3" ></ TextBox > < Button Content ="查询" Grid.Column ="1" Grid.Row ="4" Width ="60" Height ="23" Click ="Button_Click" ></ Button > </ Grid > </ Grid >
通过按钮执行调用WCF服务接口查询图书信息，按钮事件直接使用上面所写的图书门面类（BookFacade）的调用服务方法即可。
private void Button_Click( object sender, RoutedEventArgs e){ try { ThreadPool.QueueUserWorkItem( delegate ( object o) { BookViewModel viewModel = new BookViewModel(); new BookFacade().GetBook(viewModel); Deployment.Current.Dispatcher.BeginInvoke(() => this .DataContext = viewModel); }); } catch (Exception ex) { MessageBox.Show(ex.ToString()); }}
最终的运行如下图所示效果：
