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CHAPTER 6 WRITING ALGORITHMS
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However, unlike the Decorator pattern, individual implementations of the Proxy pattern don t work together. The best way to understand this concept is to consider a collection. Imagine for the moment that collections were not thread safe. And imagine writing an application that didn t use threads that happened to use an unsafe collection. Now imagine taking that application and adding threads. The collection won t work with multiple threads because it was never meant to be used in a multithreaded context. In this case, you face the challenge of either not using multiple threads or rewriting the collection class. Another solution that illustrates the Proxy pattern is to create an implementation with the same interface as the collection has. In the new implementation, the existing collection is encapsulated, and the methods of the interface are implemented. The client gets a reference to the new implementation, not the original collection. Then whenever a client calls a collection method, the new implementation synchronizes access and delegates the call to the encapsulated method. Now that you know a little about the Proxy pattern, let s see how you d go about using it.
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Implementing the Proxy Pattern
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The essence of the Proxy pattern1 is to provide a surrogate or a placeholder for another class that manages the access and control of just that class. Figure 6-1 illustrates the Proxy pattern in UML terms.
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Figure 6-1. UML diagram for the Proxy pattern As you can see in this diagram, Client interacts with the interface ICollection, which is implemented by both the LinkedList and SynchronizedList types. Because LinkedList is from the .NET library API, it can t be changed in functionality. Attempting to do so isn t appropriate, as LinkedList is managed by Microsoft. Therefore, the only possible option is to use the Proxy
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1. Erich Gamma et al., Design Patterns: Elements of Reusable Object-Oriented Software (Boston: AddisonWesley, 1995), p. 207.
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CHAPTER 6 WRITING ALGORITHMS
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pattern. The class SynchronizedList implements the ICollection interface and references a parent ICollection implementation. The parent reference is responsible for doing all the heavy lifting of managing the collection items. In code terms, you implement the SynchronizedList as follows (note that for clarity not all of the code is shown): class SynchronizedList<type>: ICollection<type> { private ICollection<type> _parent; public SynchronizedList(ICollection<type> parent) { _parent = parent; } public void Add(type item) { lock(this) { _parent.Add(item); } } } The class SynchronizedList<> uses Generics like the interface ICollection that it implements. What is distinctive about the SynchronizedList class is the constructor and the data member _parent. The constructor SynchronizedList<> requires as a parameter the parent ICollection implementation that will receive all delegated method calls. The implementation of the method Add does two things: perform the added value of the proxy and call the parent implementation. All methods that implement the Proxy pattern will do the same as the Add method. One caveat: the logic of the implementation might do different things depending on the method call, but if accepted the method call will always be delegated. It isn t acceptable for the Proxy pattern to act like the parent that the pattern implements. Doing that would be redundant and create consistency problems. Also extremely important is that the Proxy pattern encapsulates the parent implementation and doesn t expose the parent functionality directly. For example, in the implementation of the SynchronizedList<> class, the data member _parent can only be assigned via the constructor, ensuring a consistent Proxy pattern implementation. From the client perspective, the way to use the synchronized collection is as follows: ICollection<string> collection = new SynchronizedList<string>(new LinkedList<string>()); The variable collection is of the type ICollection<>, which refers to the SynchronizedList<> type. And the SynchronizedList<> type refers to the LinkedList<> type. The variable collection must be an interface reference, rather than a direct reference to the class like the one you see in this example: LinkedList<string> collection = new LinkedList<string>(); I include this direct reference example because very often developers will use the types directly when using collections, even though this is incorrect. Using the type makes it impossible to use the Proxy pattern. And frankly, it violates the original intention of the Bridge pattern.
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