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今天看David Wynne 写的一篇如果在MVVM中使用MEF进行扩展的文章，总的来说例子写的很好，可是有一个很命的地方，View是硬编码(hardcoded)到Main window中的，我就借花献佛，把它的例子稍稍改一下，以使它真正的好扩展。

基本上改动不大，直接贴代码吧

App.cs 

 public partial class App : Application
    {
        private static CompositionContainer _container;internal static CompositionContainer Container
        {
            get
            {
                return _container;
            }
        }
               [Import(
"MainWindow")]
        public new Window MainWindow
        {
            get { return base.MainWindow; }
            set { base.MainWindow = value; }
        }protected override void OnStartup(StartupEventArgs e)
        {
            base.OnStartup(e);
            var catalog = new AggregateCatalog();
            catalog.Catalogs.Add(new AssemblyCatalog(Assembly.GetExecutingAssembly()));
            //catalog.Catalogs.Add(new DirectoryCatalog("Extensions"));
            _container = new CompositionContainer(catalog);
            var batch = new CompositionBatch();
            batch.AddPart(this);try
            {
                _container.Compose(batch);
                MainWindow.Show();
            }
            catch
            {
                Shutdown();
            }
                    }
    }

下面是对应的MainWindow.Xaml

<Window x:Class="WpfApplication1.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"       
        Loaded="Window_Loaded"
        Title="MainWindow">
    <StackPanel x:Name="WidgetsPanel"></StackPanel>
</Window>

MainWindow.xaml.cs

代码

[Export("MainWindow", typeof(Window))]
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }
        [ImportMany(
"wpf1app", typeof(UIElement))]
        public Lazy<UIElement>[] Widgets { get; set; }private void Window_Loaded(object sender, RoutedEventArgs e)
        {
            foreach (var widget in Widgets)
            {
                this.WidgetsPanel.Children.Add(widget.Value);
            }
        }
    }

Widget1.Xaml

<UserControl x:Class="WpfApplication1.Component1"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008"             
             Loaded="UserControl_Loaded"
            >
    <Grid>
        <Button Content="{Binding Path=Content}" Background="{Binding Path=BackgroudColor}" />
    </Grid>
</UserControl>

Widget1.xaml.cs

代码

[Export("wpf1app", typeof(UIElement))]
    public partial class Component1 : UserControl
    {
        public Component1()
        {           
            InitializeComponent();
        }
              [Import(
typeof(Widget1ModelView))]
        public Widget1ModelView ViewModel
        {
            get;
            private set;
        }private void UserControl_Loaded(object sender, RoutedEventArgs e)
        {
            this.DataContext = ViewModel;
        }
    }

Widget1ModelView

代码

 [Export(typeof(Widget1ModelView))]
    public class Widget1ModelView
    {
        public string Content
        {
            get
            {
                return "Click Me";
            }
        }public string BackgroudColor
        {
            get
            {
                return "Red";
            }
        }
    }