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Implementing the Chain of Responsibility Pattern
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The purpose of the Chain of Responsibility7 pattern is to allow a client to make a request, but have it processed by the appropriate handlers. In UML terms, the Chain of Responsibility pattern is defined as shown in Figure 5-5.
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Figure 5-5. UML definition of the Chain of Responsibility pattern The Chain of Responsibility pattern is implemented using an interface that a class implements. The various implementations are linked together in some type of list structure. When the client wants some information processed, a call is made to the front of the list or the first
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7. Design Patterns: Elements of Reusable Object-Oriented Software, p. 223.
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CHAPTER 5 IMPLEMENTING COMPONENT GROUPINGS
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interface instance. The first interface instance can decide to process the request or hand it off to another interface instance that may or may not process the request. This course of decision, processing, or delegation continues until somebody has processed the request or no more interface implementation instances are left. The client will know whether the request has been handled or not by the Boolean return value that the client receives. Based on the diagram in Figure 5-5, a classical implementation of the IHandler interface would be as follows: interface IContext { } interface IHandler { bool HandleRequest( IContext context); } The IHandler interface has a single method, HandleRequest, which has a single parameter, IContext. The use of a parameter is purely optional, and there could be multiple parameters. If parameters are specified, they need not be the type IContext, they could be Generic types or some other type that refers to the context of the request. If no parameters are specified, the contextual information could be provided using a singleton (an object-oriented global variable), but it s better to use a parameter, as a parameter occurs per request, involving fewer concurrency issues. Assuming that the IContext interface is used, then the following source code represents an implementation of the Chain of Responsibility pattern: class ContextForHandler1: IContext { } class ContextForHandler2: IContext { } class UnknownContext: IContext { } class ConcreteHandler1: IHandler { IHandler _next; public ConcreteHandler1(IHandler next) { _next = next; } public bool HandleRequest(IContext context) { if(context is ContextForHandler1) { return true; } else { return _next.HandleRequest(context); } } }
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CHAPTER 5 IMPLEMENTING COMPONENT GROUPINGS
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class ConcreteHandler2: IHandler { public bool HandleRequest(IContext context) { if(context is ContextForHandler2) { return true; } else { return false; } } } The classes ConcreteHandler1 and ConcreteHandler2 are implementations that will process a request. Look at how the constructor of ConcreteHandler1 references another IHandler implementation, whereas ConcreteHandler2 doesn t. This means that when the concrete handlers are linked together, ConcreteHandler2 will reference another concrete handler using the IContext interface. In the example code, this would mean ConcreteHandler1, but doesn t have to. In fact, the different concrete handlers don t need to be related in functionality. If the concrete handlers process e-mail requests, there is no reason why a concrete handler for HTTP requests could not be part of the list. Looking at the implementations of each HandleRequest method, the logic is to process it, hand the request off to another implementation, or return a false answer indicating that it wasn t processed. Notice how ConcreteHandler1 and ConcreteHandler2 decide to process the request based on the type of the IContext implementation. This is very common for a Chain of Responsibility implementation. But the processing request logic can be triggered in other ways, such as a property on the context or a condition on a singleton. The trigger can be whatever the developer needs, but it should be request related and not a configuration item. Following is the test code that illustrates how the Chain of Responsibility pattern is used: [TestFixture] public class TestChainOfResponsibility { [Test] public void Test() { ConcreteHandler1 handler = newConcreteHandler1( new ConcreteHandler2()); ContextForHandler1 context1 = new ContextForHandler1(); ContextForHandler2 context2 = new ContextForHandler2(); UnknownContext context3 = new UnknownContext(); Assert.IsTrue(handler.HandleRequest(context1)); Assert.IsTrue(handler.HandleRequest(context2)); Assert.IsFalse(handler.HandleRequest(context3)); } } In the source code, the structure of the concrete handlers is predefined before the requests are made. The order of the instances in the list is important, as otherwise certain steps might be missed or processed out of order.
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