chainofresponsibility

Chain of Responsibility

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.

Applicability

Use Chain of Responsibility when

• more than one object may handle a request, and the handler isn't known a priori. The handler should be ascertained automatically
• you want to issue a request to one of several objects without specifying the receiver explicitly
• the set of objects that can handle a request should be specified dynamically

Participants

• Handler (HelpHandler)
  • defines an interface for handling requests
  • (optional) implements the successor link
• ConcreteHandler (PrintButton, PrintDialog)
  • handles requests it is responsible for
  • can access its successor
  • if the ConcreteHandler can handle the request, it does so; otherwise it forwards the request to its successor
• Client
  • initiates a request to a ConcreteHandler object on the chain

Related Patterns

Chain of Responsibility is often applied in conjunction with Composite. There, a component's parent can act as its successor.