MEF 程式設計指南（六）：DeploymentCatalog 目錄服務與動態組合

MEF 透過屬性導向程式模型，依目錄動態發現組件內所有可組合部件。WPF 使用 DirectoryCatalog，Silverlight 則以 DeploymentCatalog 管理目錄、下載與組合。以下示範在 Silverlight 取得目前應用程式目錄資訊：

var catalog = new AssemblyCatalog(Assembly.GetExecutingAssembly());
var assembly = catalog.Assembly;
var parts = catalog.Parts;

封裝 IDeploymentService 介面

為提升彈性，以介面抽象目錄操作：

public interface IDeploymentService
{
    void AddXap(string relativeUri, Action<AsyncCompletedEventArgs> completedAction);
}

DeploymentCatalogService 完整實作

內部使用 AggregateCatalog 聚合多個 DeploymentCatalog，並透過 CompositionHost.Initialize 啟動 MEF 容器：

[Export(typeof(IDeploymentService))]
public class DeploymentCatalogService : IDeploymentService
{
    private static AggregateCatalog _aggregateCatalog;
    private readonly Dictionary<string, DeploymentCatalog> _catalogs = new();

    public static void Initialize()
    {
        _aggregateCatalog = new AggregateCatalog();
        _aggregateCatalog.Catalogs.Add(new DeploymentCatalog());
        CompositionHost.Initialize(_aggregateCatalog);
    }

    public void AddXap(string relativeUri, Action<AsyncCompletedEventArgs> completedAction)
    {
        if (!_catalogs.TryGetValue(relativeUri, out var catalog))
        {
            catalog = new DeploymentCatalog(relativeUri);
            catalog.DownloadCompleted += (s, e) => completedAction?.Invoke(e) ?? DownloadCompleted(e);
            catalog.DownloadAsync();
            _catalogs[relativeUri] = catalog;
            _aggregateCatalog.Catalogs.Add(catalog);
        }
    }

    private void DownloadCompleted(object sender, AsyncCompletedEventArgs e)
    {
        if (e.Error != null)
            throw new InvalidOperationException(e.Error.Message, e.Error);
    }
}

一鍵完成 XAP 下載與組合

呼叫端極簡單：

private void button1_Click(object sender, RoutedEventArgs e)
{
    this.Service.AddXap("MEFTraining.MefCatalogs.Parts.xap", null);
}

後續可結合契約介面與中繼資料策略實現集中控制，詳細將於後續文章說明。