15-Chain Of Responsibility.pdf

Design Patterns 宋 杰 Song Jie 东北大学软件学院 Software College, Northeastern University 15. Chain Of Responsibility Pattern Intent Avoid coupling the sender of a request to its receiver by giving more than one object a chance to handle the request. Chain the receiving objects and pass the request along the chain until an object handles it. The chain of responsibility could be a line, a ring, a hierarchy or a graph. 在责任链模式里，很多对象由每一个对象对其下家的引 用而连接起来形成一条链。请求在这个链上传递，直到 链上的某一个对象决定处理此请求。发出这个请求的客 户端并不知道链上的哪一个对象最终处理这个请求，这 使得系统可以在不影响客户端的情况下动态地重新组织 链和分配责任。 Example: Cash Machine +storeCoins() interface CashMachine Client Example: Rank in military Structure 继任者继承人 Participants Handler Defines an interface for handling requests. (Optional) Implements the successor link. ConcreteHandler Handles requests it is responsible for. If the ConcreteHandler can handle the request, it does so; otherwise it forwards the request to its successor. Client Initiates the request to a ConcreteHandler object on the chain. Collaborations When a client issues a request, the request propagates along the chain until a ConcreteHandler object takes responsibility for handling it. Consequences -benefits Reduced coupling, (nothing need to know). The client is free from knowing which object handles a request. The client only has to know that a request will be handled appropriately. Both the receiver and the sender have no explicit knowledge of each other. An handler in the chain doesnt have to know about the chains structure. Added flexibility in assigning responsibilities to objects. You can combine this with subclassing to specialize handlers statically. You can add or change responsibilities for handling a request by adding to or otherwise changing the chain at run-time. Consequences-drawbacks Receipt isnt guaranteed. Since a request has no explicit receiver, theres no guarantee itll be handled The request can fall off the end of the chain without ever being handled. A request ca