使用 MEF 对 ImportMany 进行重组时出现问题

Question

我在一个类上定义了一个属性，该类定义了 importManyAttribute ，声明如下：

public const string FontStyleProvidersPropertyName = "FontStyleProviders";
[ImportMany(typeof(IFontStyleProvider), RequiredCreationPolicy = CreationPolicy.Shared, AllowRecomposition=true)]
public List<IFontStyleProvider> FontStyleProviders { get; set; }

第一次运行时，我构建了我的组合容器，如下所示

private CompositionContainer BuildCompositionContainer()
{
    //build our composable parts catalog
    Assembly executingAssembly = Assembly.GetExecutingAssembly();
    CompositionContainer applicationContainer;
    string localPath = Path.GetDirectoryName(executingAssembly.Location);

    try
    {
        aggregateCatalog = new AggregateCatalog();
        aggregateCatalog.Catalogs.Add(new AssemblyCatalog(executingAssembly));

        if (!Directory.Exists(Path.Combine(localPath, ApplicationExtensionsPath)))
        {
            Directory.CreateDirectory(Path.Combine(localPath, ApplicationExtensionsPath));
        }

        exportsCatalog = new DirectoryCatalog(Path.Combine(localPath, ApplicationExtensionsPath));

        aggregateCatalog.Catalogs.Add(new DirectoryCatalog(Path.Combine(localPath, ApplicationExtensionsPath)));

        //create a composition container
        return applicationContainer = new CompositionContainer(aggregateCatalog);
    }
    catch (Exception e)
    {
        Debug.Fail("Catalog Construction Failed", e.StackTrace);
        throw;
    }
}

此时，一切都按预期工作，但我似乎无法触发“this”类实例的重新组合。我有一个导入方法如下：

private void Import()
{
    exportsCatalog.Refresh();
    CompositionBatch batch = new CompositionBatch();
    batch.AddPart(this);
    applicationContainer.Compose(batch);

    var copy = PropertyChanged;
    if (copy != null)
    {
        copy(this, new PropertyChangedEventArgs(FontStyleProvidersPropertyName));
        copy(this, new PropertyChangedEventArgs(MessageContainerViewModelsPropertyName));
    }
}

它在exportsCatalog使用的ApplicationExtensionPath文件夹中找到新类型就好了，但它实际上从未重新构建FontStyleProviders（或MessageContainerViewModels）

我已经浏览了几次文档，但我不能似乎明白为什么。

Answer 1

问题是您实际上并未将调用 Refresh() 的目录添加到 AggregateCatalog 中。更改为：

exportsCatalog = new DirectoryCatalog(Path.Combine(localPath, ApplicationExtensionsPath));
aggregateCatalog.Catalogs.Add(new DirectoryCatalog(Path.Combine(localPath, ApplicationExtensionsPath)));

改为：

exportsCatalog = new DirectoryCatalog(Path.Combine(localPath, ApplicationExtensionsPath));
aggregateCatalog.Catalogs.Add(exportsCatalog);

另外，一旦您的课程已撰写完毕，您就不应该再次撰写。只需调用 exportsCatalog.Refresh() 就足以导致重组。