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 asertained 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.

Structure

Participants

• Handler: defines an interface for handling requests, implements the successor link.
• ConcreteHandler: handles requests it is responsible for, can access it's successor.
• Client: initiates the request to a ConcreteHandler object on the chain.

Collaborations

When a client issues a request, the request propogates along the chain until a ConcreteHandler object takes responsibility for handling it.

Consequences

COR has the following benefits:

• Reduced coupling: the pattern frees an object from knowing which other object handles a request.
• Added flexibility in assigning responsibilities to object.
• Receipt isn't guaranteed.

Sample Code

ChainOfResponsibility

Related Patterns

COR is often applied in conjunction with Composite. There, a component parent can act as it's successor.