c# barcode code 39 IComponent Component Operation Decorator Client IComponent in Visual C#.NET

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IComponent Component Operation Decorator Client IComponent
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Any photo A plain photo To display a photo A tagged photo Creator of photos and tagged photos Any photo
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From this list, we can see that the following would be valid statements in a Client wanting to put two tags on a photo:
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Photo photo = new Photo( ); Tag foodTag = new Tag (photo, "Food",1); Tag colorTag = new Tag (foodTag, "Yellow",2);
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By the is-a relationship, photo, foodTag, and colorTag are all IComponent objects. Each of the tags (the decorators) is created with a Component, which might be a photo or an already tagged photo. The resulting object diagram is shown in Figure 2-3. As you can see, there are actually three separate photo objects in the system. How they interact is discussed in the upcoming Implementation section.
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In most of the patterns we will encounter, the players can appear in multiple guises. To keep the UML diagrams clear and simple, not all of the options will be shown. However, we should consider the implications of these multiple players on the design of the pattern:
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2: Structural Patterns: Decorator, Proxy, and Bridge
Multiple components Different components that conform to the interface can also be decorated. For example, we could have a class that draws people, houses, ships, and so on from simple shapes and lines. They too could be tagged. It is for this reason that the IComponent interface is important, even if it does not contain any operations. In the case where we are sure there will only ever be one class of components, we can dispense with the IComponent interface and have the decorators directly inherit from Component. Multiple decorators We have seen that we can create different instances of a Tag decorator. We can also consider having other types of decorators, such as Border decorators or even decorators that make the photo invisible. No matter what the decorators are, each contains a component object, which might itself be a decorator, setting off a chain of changes (as suggested in Figure 2-3). Some Decorator pattern designs include an IDecorator interface, but it generally serves no purpose in C# implementations. Multiple operations Our illustration focuses on drawing as the chief operation for photos and decorations. Other examples will lend themselves to many more optional operations. Some of these will be part of the original component and its interface, whereas some will be added behaviors in certain decorators only. The client can call any of the operations individually on any of the components (decorated or otherwise) to which it has access.
Implementation
The Decorator pattern s key feature is that it does not rely on inheritance for extending behavior. If the Tag class had to inherit from the Photo class to add one or two methods, Tags would carry everything concerned with Photos around with them, making them very heavyweight objects. Instead, having the Tag class implement a Photo interface and then add behavior keeps the Tag objects lean. They can: Implement any methods in the interface, changing the initial behavior of the component Add any new state and behavior Access any public members via the object passed at construction Before we carry on with the photo example, consider the theoretical version of the Decorator pattern in Example 2-1. Short examples such as this one are useful for showing the interaction between classes and objects of a pattern in a direct mapping to the UML. Once we move on to a real-world example, optimizations and extensions might be employed that make it more difficult to detect and visualize the pattern. Moreover, in a real-world example, the names of the players can be completely different than those in the pattern description.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 using System; class DecoratorPattern { // Decorator Pattern Judith Bishop Dec 2006 // Shows two decorators and the output of various // combinations of the decorators on the basic component interface IComponent { string Operation( ); } class Component : IComponent { public string Operation ( ) { return "I am walking "; } } class DecoratorA : IComponent { IComponent component; public DecoratorA (IComponent c) { component = c; } public string Operation( ) { string s = component.Operation( ); s += "and listening to Classic FM "; return s; } } class DecoratorB : IComponent { IComponent component; public string addedState = "past the Coffee Shop "; public DecoratorB (IComponent c) { component = c; } public string Operation ( ) { string s = component.Operation ( ); s += "to school "; return s; } public string AddedBehavior( ) { return "and I bought a cappuccino "; } } class Client { static void Display(string s, IComponent c) {
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