如何将Decorator模式与C#MEF相结合？

2019年7月21日 185次阅读

我正在尝试增强我目前使用C#MEF建立的程序集.由于这些组件已经在生产中使用,因此直接修改单个类并不是一种可行的方法.我主要是在现有的行为中添加新的行为.例如,我有：

public IExtension
{
     Object Execute(); 
}


public BaseExtension : IExtension
{
     // other methods and members

     public virtual Object Execute()
     {
         // do operations here. 
     }
}

[Export(typeof(IExtension)]
public AppRecordExtension : BaseExtension
{
     // .. other methods and members
     public override Object Execute()
     {
         base.Execute(); // shown just for example..
         this.someOperation(); 
     }
}

// other extensions made.

现在,当MEF容器在驱动程序的方法中调用扩展时,上述工作原理：

[ImportMany(typeof(IExtension)]
private IEnumerable<Lazy<IExtension>> operations;

public void ExecuteExtensions() 
{
     var catalog = new AggregateCatalog( new AssemblyCatalog(Assembly.GetExecutingAssembly()), new DirectoryCatalog("extensions", ".dll")); 
     CompositionContainer container = new CompositionContainer(catalog); 
     container.ComposeParts(this); 

     Dictionary<IExtension, object> result = new Dictionary<IExtension, object>(); 

     foreach(Lazy(IExtension> extension in operations) 
     {
         result.Add((extension.Value, extension.Value.Execute()); 

     }
}

但是,如果我想为IExtension或BaseExtension实现特定的装饰器,我会把它们放在容器中,或者我应该如何将属性放在装饰器上,以便加载所有原始的IExtension具体类,执行其他行为. IExtension装饰器的一个示例：

// do I put an attribute here? 
// if an attribute is put here, how does the MEF container call it?
public BatchableExtension : BaseExtension 
{
     private IExtension extension = null; 


     public BatchableExtension( IExtension extension) 
     {
        this.extension = extension; 
     }

     public override Object Execute() 
     {
        this.extension.Execute(); 
        doSomeBatchSpecificOperation(); 
     }
}

// do I put an attribute here? 
// if an attribute is put here, how does the MEF container call it?
public  MonitoringExtension : BaseExtension 
{
     private IExtension extension = null; 


     public MonitoringExtension( IExtension extension) 
     {
        this.extension = extension; 
     }

     public override Object Execute() 
     {
        this.extension.Execute(); 
        doSomeMonitoringSpecificOperation(); 
        doSomeMoreBehaviors(); 
     }

有人可以帮忙吗？我想确保当容器选择扩展时,新的行为也会被拾取,具体取决于传递的参数(例如,如果isBatchable = true,则添加BatchableExtension等).如果它是非MEF,上面的内容将类似于：

 public void Main(String[] args) 
 {
     IExtension ext = new AppRecordExtension(); 
     // this is the part where I want to simulate when I use MEF. 
     IExtension ext2 = new MonitoringExtension(new BatchableExtension(ext)); 
     ext2.Execute(); 
 }

最佳答案 MEF不支持这种功能,所以你必须自己动手.您可以使用
Export Metadata公开构建装饰对象的数据 – 然后您将导出扩展名,如下所示：

[ExtensionExport(IsBatch = true, IsMonitoring = false)]
public AppRecordExtension : BaseExtension
{
     // ...
}

并在导入扩展的类中：

[ImportMany]
private IEnumerable<Lazy<IExtension, IExtensionMetadata>> operations;

public void ExecuteExtensions()
{
    // ...

    foreach(Lazy(IExtension, IExtensionMetadata> extension in operations) 
    {
        IExtension decoratedExtension = DecorateExtension(extension);
        result.Add(decoratedExtension, decoratedExtension.Execute()); 
    }
}

private IExtension DecorateExtension(Lazy<IExtension, IExtensionMetadata> exportedExtension)
{
    IExtension ext = exportedExtension.Value;
    if (exportedExtension.Metadata.IsBatch)
    {
        ext = new BatchableExtension(ext);
    }
    if (exportedExtension.Metadata.IsMonitoring)
    {
        ext = new MonitoringExtension(ext);
    }

    // Other decorating logic...

    return ext;
}
点赞