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2: Structural Patterns: Decorator, Proxy, and Bridge
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// The TaggedPhoto decorator keeps track of the tag number which gives it // a specific place to be written class TaggedPhoto : Photo { Photo photo; string tag; int number; static int count; List <string> tags = new List <string> ( ); public TaggedPhoto(Photo p, string t) { photo = p; tag = t; tags.Add(t); number = ++count; } public override void Drawer(Object source, PaintEventArgs e) { photo.Drawer(source,e); e.Graphics.DrawString(tag, new Font("Arial", 16), new SolidBrush(Color.Black), new PointF(80,100+number*20)); } public string ListTaggedPhotos( ) { string s = "Tags are: "; foreach (string t in tags) s +=t+" "; return s; } } static void Main ( ) { // Application.Run acts as a simple client Photo photo; TaggedPhoto foodTaggedPhoto, colorTaggedPhoto, tag; BorderedPhoto composition; // Compose a photo with two TaggedPhotos and a blue BorderedPhoto photo = new Photo( ); Application.Run(photo); foodTaggedPhoto = new TaggedPhoto (photo,"Food"); colorTaggedPhoto = new TaggedPhoto (foodTaggedPhoto,"Yellow"); composition = new BorderedPhoto(colorTaggedPhoto, Color.Blue); Application.Run(composition); Console.WriteLine(colorTaggedPhoto.ListTaggedPhotos( )); // Compose a photo with one TaggedPhoto and a yellow BorderedPhoto photo = new Photo( ); tag = new TaggedPhoto (photo,"Jug"); composition = new BorderedPhoto(tag, Color.Yellow); Application.Run(composition);
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Console.WriteLine(tag.ListTaggedPhotos( )); } } /* Output TaggedPhotos are: Food Yellow TaggedPhotos are: Food Yellow Jug */
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An important point about the Decorator pattern is that it is based around new objects being created with their own sets of operations. Some of these might be inherited, but only down one level. For example, when implementing the Photo Decorator program, we could try to alter properties of the Windows Form class, such as Height and Width, from within a decorator object. For the component itself and for a first-level decorator, this would work. But as soon as there was a second level of decorators, the changes would not get through. The reason is evident from the object diagram in Figure 2-3: the first decorator holds a reference to the actual Windows Form object, but the second-level decorator does not. For this kind of manipulation a different pattern, such as Strategy (discussed in 7), would be more fitting. In the example of the Photo Decorator program, we did not actually add any behavior; we only overrode it. A Client implemented as the Application.Run harness uses the event model to invoke the PaintEventHandler implicitly. It does not give an opportunity for calling other methods explicitly. Of course, other events (such as MouseMove and OnClick) can be programmed, and then the other behaviors in the decorators will make sense (see the Exercises section later).
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Here are four ways the Decorator pattern is used in the real world: As our small example illustrated, the Decorator pattern fits well in the graphics world. It is equally at home with video and sound; for instance, video streaming can be compressed at different rates, and sound can be input to a simultaneous translation service. At a more mundane level, decorators abound in the I/O APIs of C#. Consider the following hierarchy:
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System.IO.Stream System.IO.BufferedStream System.IO.FileStream System.IO.MemoryStream System.Net.Sockets.NetworkStream System.Security.Cryptography.CryptoStream
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The subclasses decorate Stream because they inherit from it, and they also contain an instance of a Stream that is set up when an object is constructed. Many of their properties and methods relate to this instance.
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2: Structural Patterns: Decorator, Proxy, and Bridge
In today s world of mobile devices, web browsers and other mobile applications thrive on the Decorator pattern. They can create display objects suitable for smaller screens that include scroll bars and exclude banners that would be standard on desktop display browsers, for example. The Decorator pattern is so useful that there are now actual Decorator classes in .NET 3.0. The one in System.Windows.Controls provides a base class for elements that apply effects onto or around a single child element, such as Border or Viewbox. The following table summarizes when, in general, to use the Decorator pattern.
Use the Decorator pattern when You have: An existing component class that may be unavailable for subclassing. You want to: Attach additional state or behavior to an object dynamically. Make changes to some objects in a class without affecting others. Avoid subclassing because too many classes could result. But consider using instead: The Adapter pattern, which sets up an interface between different classes. The Composite pattern, which aggregates an object without also inheriting its interface. The Proxy pattern, which specifically controls access to objects. The Strategy pattern, which changes the original object rather than wrapping it.
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