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2: Structural Patterns: Decorator, Proxy, and Bridge
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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 55 56 57 58 59 60 61 62 private class Subject { public string Request( ) { return "Subject Request " + "Choose left door\n"; } } public class Proxy : ISubject { Subject subject; public string Request( ) { // A virtual proxy creates the object only on its first method call if (subject == null) { Console.WriteLine("Subject inactive"); subject = new Subject( ); } Console.WriteLine("Subject active"); return "Proxy: Call to " + subject.Request( ); } } public class ProtectionProxy : ISubject { // An authentication proxy first asks for a password Subject subject; string password = "Abracadabra"; public string Authenticate (string supplied) { if (supplied!=password) return "Protection Proxy: No access"; else subject = new Subject( ); return "Protection Proxy: Authenticated"; } public string Request( ) { if (subject==null) return "Protection Proxy: Authenticate first"; else return "Protection Proxy: Call to "+ subject.Request( ); } } } class Client : SubjectAccessor { static void Main( ) { Console.WriteLine("Proxy Pattern\n"); ISubject subject = new Proxy( ); Console.WriteLine(subject.Request( )); Console.WriteLine(subject.Request( )); ProtectionProxy subject = new ProtectionProxy( ); Console.WriteLine(subject.Request( ));
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63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 Console.WriteLine((subject as ProtectionProxy).Authenticate("Secret")); Console.WriteLine((subject as ProtectionProxy).Authenticate("Abracadabra")); Console.WriteLine(subject.Request( )); } } /* Output Proxy Pattern Subject inactive Subject active Proxy: Call to Subject Request Choose left door Subject active Proxy: Call to Subject Request Choose left door Protection Protection Protection Protection */ Proxy: Proxy: Proxy: Proxy: Authenticate first No access Authenticated Call to Subject Request Choose left door
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The program is structured with an enclosing class called SubjectAccessor that is there to group the types of the proxies and subjects. The interface and the proxies are all declared as public (lines 7, 17, and 31), so that they and their publicly modified members are fully accessible to clients. The intention of both the virtual and protection proxies is to provide a frontend to the Subject. For this reason, the Subject is declared as private (line 11). Its Request method is public, but it is only visible to classes that can see the class i.e., those within SubjectAccessor. We can perform two tests to check that the accessibility is set up just right: A more natural grouping for proxies and subjects would have been a namespace, but classes within namespaces are not permitted to be private. See what happens when you change line 6 to namespace SubjectAccessor and add a using SubjectAccessor statement after line 1. The same applies if you remove the SubjectAccessor class altogether, as then the types all belong to the default namespace. Add the following statement in the Main method (at line 56):
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ISubject test = new Subject( );
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The code for a client to declare a real subject will not compile because the program is organized so that it can only declare proxies. However, if you remove the private modifier from Subject (line 11), the instantiation will be accepted. The reason for this is that in order to gain access to the classes in SubjectAccessor, the client inherits from it, thus giving it access to internal members. An alternative way of composing the program is discussed in the exercises.
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2: Structural Patterns: Decorator, Proxy, and Bridge
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As this discussion has illustrated, you can make the proxies and the Subject available for use in just the ways you need. They are not intended to be independent of each other hence their grouping in a class. The proxies are free to aggregate objects of the Subject. They keep them in line with each other and with what they offer to the Client.
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