.NET Rx优于经典事件的优势？

Question

.NET 4.0的β2已经介绍的的IObservable和IObserver接口.

与传统的.NET事件相比有哪些优势？这不是解决同样的问题吗？

Answer 1

您可以将IObservable用作事件,替换使用IObservable类型的属性公开事件的代码,但这不是重点.

关于IObservable有两个重要的事情要理解:

1. 它统一了我们之前不知道如何统一的两个概念:异步操作(通常返回单个值)和事件(通常永远持续).

2. 它是可组合的.与CLR事件,IAsyncResult或INotifyCollectionChanged不同,它允许我们从一般事件和异步操作中构建特定事件.

这是我今天下午在工作中遇到的一个例子.

在Silverlight中,有一些效果可以应用于无法应用于普通控件的图像控件.为了解决控件内容更改时的这些限制,我可以等待其视觉外观更新并截取它的屏幕截图.然后我想隐藏其可视化表示,用快照替换它,并将视觉效果应用于图像.现在我可以将图像效果应用于控件(假设它不是交互式的).

这个程序将是微不足道的,但它必须是异步的.在将效果应用于图像之前,我必须等待两个连续的异步操作才能完成:

1. 控件的内容已更改
2. 控件的视觉外观已更新

这是我如何使用Rx解决这个问题:

// A content control is a control that displays content.  That content can be
// anything at all like a string or another control.  Every content control contains
// another control: a ContentPresenter.  The ContentPresenter's job is to generate
// a visual representation of the Content property. For example, if the Content property
// of the ContentControl is a string, the ContentPresenter creates a TextBlock and inserts
// the string into it.  On the other hand if the Content property is another control the 
// ContentPresenter just inserts it into the visual tree directly.
public class MyContentControl : ContentControl
{
   // A subject implements both IObservable and IObserver.  When IObserver methods
   // are called, it forwards those calls to all of its listeners.
   // As a result it has roughly the same semantics as an event that we can "raise."
   private Subject<object> contentChanged = new Subject<object>();

   // This is a reference to the ContentPresenter in the ContentControl's template
   private ContentPresenter contentPresenter; 

   // This is a reference to the Image control within ContentControl's template.  It is displayed on top of the ContentPresenter and has a cool blur effect applied to it.
   private Image contentImageControl; 

   public MyContentControl()
   {
      // Using Rx we can create specific events from general events.
      // In this case I want to create a specific event ("contentImageChanged") which
      // gives me exactly the data I need to respond and update the UI.
      var contentImageChanged = 
         // get the content from the content changed event
         from content in contentChanged
         where content != null
         // Wait for the ContentPresenter's visual representation to update.
         // ContentPresenter is data bound to the Content property, so it will
         // update momentarily.
         from _ in contentPresenter.GetLayoutUpdated().Take(1)
         select new WritableBitmap(contentPresenter, new TranslateTransform());

      contentImageChanged.Subscribe(
         contentImage => 
         {
            // Hide the content presenter now that we've taken a screen shot              
            contentPresenter.Visibility = Visibility.Collapsed; 

            // Set the image source of the image control to the snapshot
            contentImageControl.ImageSource = contentImage;
         });
   }

   // This method is invoked when the Content property is changed.
   protected override OnContentChanged(object oldContent, object newContent)
   {
      // show the content presenter before taking screenshot
      contentPresenter.Visibility = Visibility.Visible;  

      // raise the content changed "event"
      contentChanged.OnNext(newContent);   

      base.OnContentChanged(oldContent, newContent);
   }
}

这个例子特别简单,因为只有两个连续的序列操作.即使在这个简单的例子中,我们也可以看到Rx增加了价值.没有它,我将不得不使用状态变量来确保事件按特定顺序触发.我也不得不写一些非常难看的代码来明确分离LayoutUpdated事件.

当您使用Rx进行编程时,诀窍是想"我希望我的框架提供什么样的事件？" 然后去创造它.我们训练将事件视为简单的输入驱动事物("鼠标悬停","鼠标点击","键盘"等).但是,没有理由事件不能非常复杂和特定于您的应用程序("GoogleMsdnMashupStockDataArrived","DragStarting"和"ImageContentChanged").当您以这种方式构建程序时(完全创建我需要的事件,然后通过更改状态来响应它),您会发现它们具有更少的状态错误,变得更有序,并且更加自我描述.

得到它了？:-)