chain-of-responsibility

Chain of Responsibility

The Chain of Responsibility Pattern helps 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.

Structure

The general structure for the pattern includes a Client that initiates the request, a Handler interface, which is responsible for defining the methods that each handler in the chain will be implementing, and the ConcreteHandlers that will be implementing the Handler interface to be able to answer to the Client. The class diagram below shows this structure in more detail:

classDiagram

class Client
class Handler {
    HandleRequest()
}

class ConcreteHandler1 {
    HandleRequest()
}

class ConcreteHandler2 {
    HandleRequest()
}

Client --> Handler : uses-a
Handler <|-- ConcreteHandler1 : implements
Handler <|-- ConcreteHandler2 : implements

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Working example

The working example for this project is a mailing classification system, responsible for handling different types of email: Spam, Fan and Complaint. Each kind of e-mail needs to be handled differently and the system uses the Chain of Responsibility Pattern to chain each handler, defaulting to a DefaultMailHandler if none of the previous handlers were able to deal with the incoming e-mail. Check out the MightyGumballMailing project for more details on the project and on how the pattern was applied.