Chain of responsibility

Static diagram :

Description :

Type :

Be (Behavioral)

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.

Strong points :

1. Extensibility
1. 1.1 Objects chaining factorization
2. 1.2 Unique access point for the client class
3. 1.3 Addition or removal of a new element in the chain does not need code modification

Pattern's participants :

Handler
Defines an interface for handling requests.
(Optional) implements the successor link.
ConcreteHandler
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 the request to a ConcreteHandler object on the chain.

Problems :

1. Knowledge chaining