chain-of-responsibility

Chain Of Responsibility Pattern

Intent

The chain of responsibility pattern decouples the sender of a request from its receiver, by giving more than one object an opportunity to handle the request. The request is processed by the chain until some 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

Structure

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. it does not know anything about the handlers are in the chain, it just send the requests to the chain.

Example

In this example, it demonstrates a software company handles customer's requests.

The SoftwareCommpany, as the interface who responses to customers' requests, constructs the chain of responsibility -- different roles handling different requests: developer can handle bugs, architect can handle features and CTO can handle products -- customers don't need to know whom should the request send to, they just send requests to the SoftwareCompany, in this way, it decouples the sender of a request from its receiver.

Participants in this example:

• App, as a customer, is the Client.
• RequestHandler is the Handler.
• Architect/CTO/Developer is the ConcreteHandler.

Scala Tips

• The Handler can be treated as a function in type Request => Response. In scala, we can implement Handler by partial function and chain them (partial functions) together by orElse method. Refer to the 'fp' package in this example for detail.

Reference

• Design Patterns: Elements of Reusable Object-Oriented Software