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CHAPTER 5 IMPLEMENTING COMPONENT GROUPINGS
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An IHandler implementation doesn t need to be related to another IHandler implementation. The structuring of the IHandler implementations must not be a single linear order, but could be hierarchical where only certain portions of the tree will process the request. Not every IHandler implementation should process a request, and most likely only one implementation will process the request before exiting the iteration. Which IHandler implementation should perform the processing is based on the contextual information, and not a state that is defined externally. The only exception is if the contextual information is stored externally. The main idea isn t to use a configuration item to define which handler processes a request.
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Implementing the Command Pattern
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The purpose of the Command pattern8 is to make it possible to encapsulate a request as a command that can be executed at some later point in time. Multiple commands are linked together in a list to replicate a state. For example, to implement undo or redo operations in an application, you need the Command pattern. The UML diagram of the Command pattern is illustrated in Figure 5-6.
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Figure 5-6. UML implementation of the Command pattern The Command pattern has four main pieces: Invoker, ICommand, ConcreteCommand, and Receiver. The Invoker is typically a class that manages a collection of ICommand implementations, and when asked will execute them in a serial fashion. The ICommand interface will typically have a single method, Execute, to execute its own state. The class ConcreteCommand implements the ICommand interface and references a class called Receiver. Receiver could also be called a context, but that might be confused with the Chain of Responsibility pattern. The idea of a Receiver class is to provide an entry point to the infrastructure that ConcreteCommand can manipulate. Unlike the Chain of Responsibility pattern, the context used by the ConcreteCommand
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8. Design Patterns: Elements of Reusable Object-Oriented Software, p. 233.
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CHAPTER 5 IMPLEMENTING COMPONENT GROUPINGS
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doesn t contain state used by the ConcreteCommand. All of the state that ConcreteCommand needs is contained within ConcreteCommand. When ConcreteCommand is persisted and re-created, all of the state needed is persisted and re-created. Following is an example interface definition for ICommand: public interface ICommand<Receiver> { void Execute( Receiver receiver); } The interface ICommand uses Generics to define the Receiver type. In this example of defining the ICommand interface, there is the single method, Execute. The Invoker will call Execute, which then causes ICommand to execute its contained state. An example implementation of ICommand is as follows: public class CommandReceiver { public void Output(String buffer) { Console.WriteLine(buffer); } } [Serializable] public class Operation : ICommand< CommandReceiver> { private string _data; public Operation(string data) { _data = data; } public void Execute(CommandReceiver output) { output.Output( "My Data (" + _data + ")"); } } The class Operation implements the ICommand interface and uses the CommandReceiver class as a Receiver type. The constructor to Operation has a single string parameter that represents the state of the class. The state doesn t have to be assigned using the constructor, but could be assigned using methods and properties, or through the Memento pattern discussed in 8. The methods used to assign the state shouldn t be part of the ICommand interface. The method Execute executes the logic of Operation class, which is to output the state using the Receiversupplied method CommandReceiver.Output. If additional methods are assigned to the ICommand interface, then they must relate to the process of executing the command state, such as Initialize and Destroy. The method Initialize would be used to prepare the state of the command. The method Destroy would be used to tear down the state of the command. An example of setting up the state is the preparation of a database connection. Remember that commands can be used as macros in an application to carry out nonprogrammatically specified logic. Notice in the declaration of the Operation class the Serializable attribute is used. This is important because most likely the ICommand implementations will be serialized to a hard disk or network for later referral. To create a list of commands, it s necessary to execute an Invoker, which is defined as follows:
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