使用MEF构建可扩展的Silverlight应用

“托管扩展性框架（Managed Extensibility Framework，简称MEF），是微软.NET框架下为提高应用和组件复用程度而推出的，用于使组件能够最大化的重用。使用MEF能够使静态编译的.NET应用程序转换为动态组合，这将是创建可扩展应用、可扩展框架和应用扩展的好途径。它将做为.NET Framework 4.0的组成部分之一发布。现在，MEF也将被包含在Silverlight 4.0中。

那么MEF是怎样工作的呢？简单分为三个步骤：

• Export （输出）
• Import （输入）
• Compose （组合）

简短说一下MEF的工作原理，MEF的核心包括一个catalog和一个CompositionContainer。category用于发现扩展，而container用于协调创建和梳理依赖性。每个可组合的Part提供了一个或多个Export，并且通常依赖于一个或多个外部提供的服务或Import。每个Part管理一个实例为应用程序运行。

下面我们做一个小型可扩展计算器示例来解释这三个过程

1. 首先下载MEF框架包，Silverlight 4.0会自带，不过微软已经将其开源了。
   http://www.codeplex.com/MEF
2. 创建一个Silverlight Navigate Application ,并添加程序集引用(MEF_Beta_2\bin\SL目录下 System.ComponentModel.Composition.dll)
   在项目下添加两个类文件Package.cs和PackageCatalog.cs，这两个文件在最新的MEF版本中没有提供，主要用于加载silverlight的Xap包。
   这两个文件在MEF框架的Sample中提供了(MEF_Beta_2\Samples\PictureViewer\PictureViewer.Common)，将这两个类的访问修饰符改为public, 添加后注意修改命名空间。
3. 修改Home.cs 类
   

   using System;
   using System.Net;
   using System.Windows;
   using System.Windows.Controls;
   using System.Windows.Documents;
   using System.Windows.Ink;
   using System.Windows.Input;
   using System.Windows.Media;
   using System.Windows.Media.Animation;
   using System.Windows.Shapes;
   using System.ComponentModel.Composition;
   using System.ComponentModel;
   
   namespace MefDemo
   {
       //用于更新界面的委托
       public delegate void OperateHandler(IOperate Op);
   
       /// <summary>
       /// 运算器接口
       /// </summary>
       public interface IOperate
       {
           double Op(double left, double right);
           string Symbol { set; get; }
           string Label { get; set; }
       }
   
       /// <summary>
       /// 加法运算器
       /// </summary>
       [Export(typeof(IOperate))]
       public class AddButton : Button, IOperate
       {
           [Import("AddButtonContract",AllowRecomposition = true)]
           public string Label { get { return this.Content.ToString(); } set { this.Content = value; } }
   
           [Import("AddSybomContract", AllowRecomposition = true)]
           public string Symbol { set; get; }
   
           [Import("ClickHandler")]
           public OperateHandler ClickAction { get; set; }
   
           #region IOperate 成员
   
           public double Op(double left, double right)
           {
               return left + right;
           }
   
           #endregion
   
           public AddButton()
           {
               this.Click += (s, e) => ClickAction(this);
           }
       }
   
       /// <summary>
       /// 减法运算器
       /// </summary>
       [Export(typeof(IOperate))]
       public class SubButton : Button, IOperate
       {
           [Import("SubButtonContract",AllowRecomposition=true)]
           public string Label { get { return this.Content.ToString(); } set { this.Content = value; } }
   
           [Import("SubSybomContract", AllowRecomposition = true)]
           public string Symbol { set; get; }
   
           [Import("ClickHandler")]
           public OperateHandler ClickAction { get; set; }
   
           #region IOperate 成员
   
           public double Op(double left, double right)
           {
               return left - right;
           }
   
           #endregion
   
           public SubButton()
           {
               this.Click += (s, e) => ClickAction(this);
           }
       }
   
       /// <summary>
       /// 为每个运算器的属性提供值
       /// </summary>
       public class ComponentAttributeProvider
       {
           [Export("AddButtonContract")]
           public string AddLabel  { get { return "Add"; } }
           [Export("AddSybomContract")]
           public string AddSymbol { get { return "+"; } }
           [Export("SubButtonContract")]
           public string SubLabel  { get { return "Sub"; } }
           [Export("SubSybomContract")]
           public string SubSymbol { get { return "-"; } }
       }
   }

4. 修改 Home.xaml
   

   <navigation:Page x:Class="MefDemo.Home" 
       xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" 
       xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" 
       xmlns:d="http://schemas.microsoft.com/expression/blend/2008" xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
       xmlns:navigation="clr-namespace:System.Windows.Controls;assembly=System.Windows.Controls.Navigation"
       mc:Ignorable="d" d:DesignWidth="640" d:DesignHeight="480"
       Title="Home"
       Style="{StaticResource PageStyle}">
   
       <navigation:Page.Resources>
           <ItemsPanelTemplate x:Key="ItemsPanelTemplate1">
               <StackPanel Orientation="Horizontal" HorizontalAlignment="Center" VerticalAlignment="Center"/>
           </ItemsPanelTemplate>
       </navigation:Page.Resources>
   
       <Grid x:Name="LayoutRoot">
           <ScrollViewer x:Name="PageScrollViewer" Style="{StaticResource PageScrollViewerStyle}">
   
               <StackPanel x:Name="ContentStackPanel" Background="Black">
                   <StackPanel   Orientation="Horizontal" Width="455" Height="89" Margin="91,0,91,-30">
                       <TextBox x:Name="LeftNum" HorizontalAlignment="Left"   VerticalAlignment="Center" Width="83" TextWrapping="Wrap"/>
                       <TextBlock x:Name="Symbol" Width="62" Text="+" TextWrapping="Wrap" FontSize="24" Foreground="#FFF80606" TextAlignment="Center" VerticalAlignment="Center" HorizontalAlignment="Left"/>
                       <TextBox x:Name="RightNum" HorizontalAlignment="Left"   VerticalAlignment="Center" Width="78" TextWrapping="Wrap"/>
                       <TextBlock Width="64" Text="=" TextWrapping="Wrap" Foreground="#FFF20808" FontSize="21.333" TextAlignment="Center" VerticalAlignment="Center" HorizontalAlignment="Left"/>
                       <TextBox x:Name="Result" HorizontalAlignment="Left"   VerticalAlignment="Center" Width="146" TextWrapping="Wrap"/>
                   </StackPanel>
                   <ListBox x:Name="operateList"  ItemsSource="{Binding}" ItemsPanel="{StaticResource ItemsPanelTemplate1}" Height="99" Background="{x:Null}" BorderBrush="{x:Null}"/>
           <Button x:Name="DynamicLoadButton" Height="40" Width="196" Content="DynamicLoadOperate"/>
               </StackPanel>
           </ScrollViewer>
       </Grid>
   
   </navigation:Page>

5. 新建类 OperatorComponent.cs
   

   using System;
   using System.Net;
   using System.Windows;
   using System.Windows.Controls;
   using System.Windows.Documents;
   using System.Windows.Ink;
   using System.Windows.Input;
   using System.Windows.Media;
   using System.Windows.Media.Animation;
   using System.Windows.Shapes;
   using System.ComponentModel.Composition;
   using System.ComponentModel;
   
   namespace MefDemo
   {
       //用于更新界面的委托
       public delegate void OperateHandler(IOperate Op);
   
       /// <summary>
       /// 运算器接口
       /// </summary>
       public interface IOperate
       {
           double Op(double left, double right);
           string Symbol { set; get; }
           string Label { get; set; }
       }
   
       /// <summary>
       /// 加法运算器
       /// </summary>
       [Export(typeof(IOperate))]
       public class AddButton : Button, IOperate
       {
           [Import("AddButtonContract",AllowRecomposition = true)]
           public string Label { get { return this.Content.ToString(); } set { this.Content = value; } }
   
           [Import("AddSybomContract", AllowRecomposition = true)]
           public string Symbol { set; get; }
   
           [Import("ClickHandler")]
           public OperateHandler ClickAction { get; set; }
   
           #region IOperate 成员
   
           public double Op(double left, double right)
           {
               return left + right;
           }
   
           #endregion
   
           public AddButton()
           {
               this.Click += (s, e) => ClickAction(this);
           }
       }
   
       /// <summary>
       /// 减法运算器
       /// </summary>
       [Export(typeof(IOperate))]
       public class SubButton : Button, IOperate
       {
           [Import("SubButtonContract",AllowRecomposition=true)]
           public string Label { get { return this.Content.ToString(); } set { this.Content = value; } }
   
           [Import("SubSybomContract", AllowRecomposition = true)]
           public string Symbol { set; get; }
   
           [Import("ClickHandler")]
           public OperateHandler ClickAction { get; set; }
   
           #region IOperate 成员
   
           public double Op(double left, double right)
           {
               return left - right;
           }
   
           #endregion
   
           public SubButton()
           {
               this.Click += (s, e) => ClickAction(this);
           }
       }
   
       /// <summary>
       /// 为每个运算器的属性提供值
       /// </summary>
       public class ComponentAttributeProvider
       {
           [Export("AddButtonContract")]
           public string AddLabel  { get { return "Add"; } }
           [Export("AddSybomContract")]
           public string AddSymbol { get { return "+"; } }
           [Export("SubButtonContract")]
           public string SubLabel  { get { return "Sub"; } }
           [Export("SubSybomContract")]
           public string SubSymbol { get { return "-"; } }
       }
   }

6. 运行。 这样就构建了一个简单的运算器，其中的Export、Import就像一个个管道一样相互连接。
   
   
7. 按照这样的设计，我们想要对其进行扩展，就必须把接口分离。新建一个Silverlight ClassLibrary Project(Named ContractLibrary),这个Library用来封装所有的扩展接口，定义Import/Export契约。
   现在把原项目中的OperatorComponent.cs 类中的接口迁移到Library项目中，新建类文件OperateContract.cs 。
   

   using System;
   using System.Net;
   using System.Windows;
   using System.Windows.Controls;
   using System.Windows.Documents;
   using System.Windows.Ink;
   using System.Windows.Input;
   using System.Windows.Media;
   using System.Windows.Media.Animation;
   using System.Windows.Shapes;
   
   namespace ContractLibrary
   {
       public delegate void OperateHandler(IOperate Op);
   
       /// <summary>
       /// 运算器接口
       /// </summary>
       public interface IOperate
       {
           double Op(double left, double right);
           string Symbol { set; get; }
           string Label { get; set; }
       }
   }

   编译通过后在Silverlight主工程(MefDemo)中添加对ContractLibrary项目的引用
8. 再新建一个Silverlight ClassLibrary Project (Named StaticExtension),，这个工程就是我们用来静态扩展的DLL。
   a). 在工程下新建我们的运算器类，AddButton.cs , SubButton.cs.(代码不变).
   b). 但注意要添加对ContractLibrary项目的引用和MEF的框架集引用) 。
   c). 添加全局属性配置类(ComponentConfiguration.cs)
   d). 删除主工程中的ComponetOperater.cs.
   e). 添加对StaticExtension的引用.
    
   

9. OK,这样我们就可以任意扩展运算器，添加更多的扩展运算了。

10. 那么下面是添加一个新的乘法运算所要做的工作。
    在StaticExtension中添加新类 Multiply.cs
    

    using System;
    using System.Net;
    using System.Windows;
    using System.Windows.Controls;
    using System.Windows.Documents;
    using System.Windows.Ink;
    using System.Windows.Input;
    using System.Windows.Media;
    using System.Windows.Media.Animation;
    using System.Windows.Shapes;
    using System.ComponentModel.Composition;
    using ContractLibrary;
    
    namespace StaticExtension
    {
        /// <summary>
        /// 乘法运算器
        /// </summary>
        [Export(typeof(IOperate))]
        public class MultiplyButton : Button, IOperate
        {
            [Import("MultiplyButtonContract", AllowRecomposition = true)]
            public string Label { get { return this.Content.ToString(); } set { this.Content = value; } }
    
            [Import("MultiplySybomContract", AllowRecomposition = true)]
            public string Symbol { set; get; }
    
            [Import("ClickHandler")]
            public OperateHandler ClickAction { get; set; }
    
            #region IOperate 成员
    
            public double Op(double left, double right)
            {
                return left * right;
            }
    
            #endregion
    
            public MultiplyButton()
            {
                this.Click += (s, e) => ClickAction(this);
            }
        }
    }

11. 上面的是静态加载，那么现在我们使用MEF实现动态扩展运算器。桌面程序的动态扩展是动态加载DLL，而对于Silverlight的Web程序则是动态加载Xap包了。
    新建普通Silverlight Application（Named DynamicExtension）.
    
    
    去掉勾选Add a test page that references the application.
    1). 删掉App和Main等不必要的文件，只留一个空的Silverlight项目，以减少Xap包的大小。
    2). 添加ContractLibrary和MEF框架集的引用(可以将引用程序集属性CopyLocal设置为false，因为我们在主工程中已经添加了，可以重用)
    3). 添加类Division.cs.
    
    

    using System;
    using System.Net;
    using System.Windows;
    using System.Windows.Controls;
    using System.Windows.Documents;
    using System.Windows.Ink;
    using System.Windows.Input;
    using System.Windows.Media;
    using System.Windows.Media.Animation;
    using System.Windows.Shapes;
    using System.ComponentModel.Composition;
    using ContractLibrary;
    
    namespace DynamicExtension
    {
        /// <summary>
        /// 乘法运算器
        /// </summary>
        [Export(typeof(IOperate))]
        public class DivisionButton : Button, IOperate
        {
            [Import("DivisionButtonContract", AllowRecomposition = true)]
            public string Label { get { return this.Content.ToString(); } set { this.Content = value; } }
    
            [Import("DivisionSybomContract", AllowRecomposition = true)]
            public string Symbol { set; get; }
    
            [Import("ClickHandler")]
            public OperateHandler ClickAction { get; set; }
    
            #region IOperate 成员
    
            public double Op(double left, double right)
            {
                return left * right;
            }
    
            #endregion
    
            public DivisionButton()
            {
                this.Click += (s, e) => ClickAction(this);
            }
        }
    }

    4).  添加配置类ComponentConfiguration.cs
    

    using System;
    using System.Net;
    using System.Windows;
    using System.Windows.Controls;
    using System.Windows.Documents;
    using System.Windows.Ink;
    using System.Windows.Input;
    using System.Windows.Media;
    using System.Windows.Media.Animation;
    using System.Windows.Shapes;
    using System.ComponentModel.Composition;
    
    namespace DynamicExtension
    {
        /// <summary>
        /// 为每个运算器的属性配置值
        /// </summary>
        public class ComponentConfiguration
        {
            [Export("DivisionButtonContract")]
            public string AddLabel { get { return "Div"; } }
            [Export("DivisionSybomContract")]
            public string AddSymbol { get { return "/"; } }
        }
    }

     
    5). 修改Home.cs ，为其注册下载包的相关事件和回调

1. using System;
   using System.Windows;
   using System.Windows.Controls;
   using System.Windows.Navigation;
   using System.Collections.ObjectModel;
   using System.ComponentModel.Composition.Hosting;
   using System.Reflection;
   using System.ComponentModel.Composition;
   using ContractLibrary;
   
   namespace MefDemo
   {
       public partial class Home : Page
       {
           [ImportMany(typeof(IOperate),AllowRecomposition = true)]
           public ObservableCollection<IOperate> Operates = new ObservableCollection<IOperate>();
   
           [Export("ClickHandler")]
           public OperateHandler ClickHandler { get { return OperateButton_Click; } }
   
           private PackageCatalog Catalog;
   
           /// <summary>
           /// 用于界面控件响应运算后的一些更新工作
           /// </summary>
           /// <param name="Operate">运算器</param>
           public void OperateButton_Click(IOperate Operate)
           {
               try
               {
                   Symbol.Text = Operate.Symbol;
                   double left = double.Parse(LeftNum.Text);
                   double right = double.Parse(RightNum.Text);
                   this.Result.Text = Operate.Op(left, right).ToString();
               }
               catch (Exception e)
               {
                   ChildWindow errorWin = new ErrorWindow(e);
                   errorWin.Show();
               }
           }
   
           public Home()
           {
               InitializeComponent();
   
               this.Loaded += new RoutedEventHandler(Home_Loaded);
               //注册按钮事件
               this.DynamicLoadButton.Click += (s, e) =>
               {
                   //下载包
                   Package.DownloadPackageAsync(
                       new Uri("DynamicExtension.xap", UriKind.Relative),
                       (args, package) => Catalog.AddPackage(package)
                   );
                   //包被添加到PackageCatalog后会自动重新组合
                   //并对添加了AllowRecomposition = true属性的Import导入器重新输入数据
               };
           }
   
           void Home_Loaded(object sender, RoutedEventArgs e)
           {
               //组合当前XAP包中所有部件(Parts)
               Catalog = new PackageCatalog();
               Catalog.AddPackage(Package.Current);
               CompositionContainer container = new CompositionContainer(Catalog);
               container.ComposeParts(this);
   
               //组合后所有实现运算接口(IOperate)的运算器都将被自动填充到 Operates 集合。
               //将运算器绑定到 ListBox 控件，用于呈现。
               this.operateList.DataContext = Operates;
           }
   
           // Executes when the user navigates to this page.
           protected override void OnNavigatedTo(NavigationEventArgs e)
           {
           }
   
       }
   }

   Ok，最终界面。
    
   点击DynamicLoadOperate按钮后
   
   

　　　　程序中还有很多细节没有展开说明，理论性的介绍可以参考MSDN和CodePlex上的文档。
　　　　

　　　　源码下载
　　　　
　　　　值此新春佳节，祝大家春节快乐！:)