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4: Structural Patterns: Adapter and Fa ade
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types that it adapts. In each of them, it does the delegate assignments (one, or more than one if there are further methods for rerouting).
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C# Feature Delegates
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A method that is specified in an interface is implemented with the same name in the base class. However, such close coupling is not always appropriate. The delegate construct can be used to break the coupling for this purpose. A delegate is a type that defines a method signature. A delegate instance is then able to accept a method of that signature, regardless of its method name or the type that encapsulates it. The delegate syntax in C# evolved considerably from Versions 1.0 to 2.0 to 3.0. We shall concentrate on the 3.0 version, which is the simplest to code. The language has predefined standard generic delegate types, as follows:
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delegate R delegate R delegate R // ... and Func<R>( ); Func<A1, R>(A1 a1); Func<A1, A2, R>(A1 a1, A2 a2); up to many arguments
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where R is the return type and the As and as represent the argument types and names, respectively. Thus, declaring a delegate instance is now straightforward. For example, we can define a Request delegate that takes an integer parameter and returns a string:
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public Func <int,string> Request;
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Next, we can assign an actual method to Request, as in:
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Request = Target.Estimate;
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The delegate can then be invoked just as any other method would be:
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string s = Request(5);
This statement would invoke the Estimate method in the Target class, returning a string. cf. C# Language Specification Version 3.0, September 2007, Section 10.8
Example: CoolBook
Our last Adapter pattern example picks up on an earlier example that we explored with the Proxy and Bridge patterns: SpaceBook. Recall that Example 2-4 introduced the SpaceBook class and its authentication frontend, MySpaceBook. Then, Example 2-6 showed how we could create a Bridge to an alternative version of MySpaceBook called MyOpenBook, which did not have authentication. Now, we are going to consider going GUI. The input and output of SpaceBook (wall writing, pokes, etc.) will be done via Windows forms. There will be a separate form for each user, and users will be able to write on each other s pages as before. However, now the input will be interactive, as well as being simulated by method calls in the program. Thus, we will have a prototype of a much more realistic system.
Adapter Pattern
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C# Feature Anonymous Functions
Anonymous functions simplify the creation of one-time behavior for delegates. They are useful when additional behavior is to be added before or after a method is invoked. For example:
Request = delegate(int i) { return "Estimate based on precision is " + (int) Math.Round(Precise (i,3)); };
Here, the method to be invoked is Precise. The parameters are different from the ones in the delegate, as is the return value. The anonymous function can wrap up the changes and assign a complete solution to the delegate for later invocation. cf. C# Language Specification Version 3.0, September 2007, Section 6.5
Creating a GUI and handling its events is a specialized function, and it is best to isolate it as much as possible from the ordinary logic of the system. In setting up CoolBook, we wrote a minimal GUI system called Interact. All Interact does is set up a window with a TextBox and a Button called Poke, and pass any click events on the Button to a delegate (see sidebar). Separately from this, we wrote MyCoolBook, which mimics the functionality of MyOpenBook and, for reasons of simplicity at this stage, maintains its own community of users. Given the following client program, the output will be as shown in Figure 4-3.
static void Main( ) { MyCoolBook judith = new MyCoolBook("Judith"); judith.Add("Hello world"); MyCoolBook tom = new MyCoolBook("Tom"); tom.Poke("Judith"); tom.Add("Hey, We are on CoolBook"); judith.Poke("Tom"); Console.ReadLine( ); }
The second Tom : Poked you was created interactively by Tom typing in Judith on his wall, selecting it, and clicking the Poke button. Judith then wrote on her own wall, and was getting ready to poke Tom when the snapshot was taken.
MyCoolBook builds on top of Interact and acts as the adapter class. As can be seen in the Client, MyOpenBook and MySpaceBook have been completely plugged out and replaced by MyCoolBook. We can just change the instantiations back, and everything
will revert to the old system. This is what a pluggable adapter achieves. Consider the insides of the adapter in Example 4-4. It inherits from MyOpenBook and, through inheritance, it makes use of the MySpaceBook object stored there, as well as the Name
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