Understanding the Chain Design Pattern for Request Handling

2. What is the Chain? • It’s a behavioral design pattern. • It deals with how objects make requests and how they are handled.

3. Question • When an object makes a request, who handles it?

4. Conventional Approach • The object that makes the request sends it directly to the object what handles it.

5. Drawbacks • Objects must know who handles each type of request. • Objects must have links to many other objects.

6. Chain Approach • Objects are arranged in a “chain of responsibility.” • Requests are passed along the chain until they are handled.

7. Advantages • Objects don’t care who handles their request. • Objects only need one link, to their successor in the chain. • Multiple objects have a chance to handle requests.

8. Example:Context Sensitive Help

9. Suppose: • User can click any part of the interface to get specific help. • If no specific help is available, more general information is provided.

10. Use the Chain Pattern! • When the user wants help, the object he clicks on sends a request down the chain. • If an object in the chain can provide the needed help, it handles the request. • Otherwise, it passes the request to its successor.

11. Flow of Requests

12. Implementation • Each object must have a HandleHelp() method. • This method either handles the help request or passes it along the chain.

13. Interaction Diagram

14. Implementation (cont.) • We define a HelpHandler class with a HandleHelp() method. • All other classes are subclasses to HelpHandler.

15. Implementation (cont.) • By default, the HandleHelp() method passes the request to the next object in the chain. • If an object is to handle a request, HandleHelp() must be overloaded.

16. Class Diagram

17. Things to Consider

18. When to Use the Chain • If a request may be handled my multiple objects. • If the handler isn’t know ahead of time. • If you don’t want to explicitly specify the handler.

19. Linking Objects • Existing links between objects may be used if they fit your desired chain. OR • Designated links can be included in the handler class.

20. Request Types • Request types can be hard coded in the handler class. OR • A parameter can be passed to indicate the type of request and how it should be handled.

21. Pros and Cons

22. Pros • Coupling between objects is reduced. • Sender and receiver of request need no explicit knowledge of one another. • Chain allows flexibility in assigning responsibilities to objects.

23. Cons • Care must be taken to ensure all requests are handled properly.

24. References • Gamma, Erich et al. Design Patterns: Elements of Reusable Object-Oriented Software. Addison-Wesley, 1995.

25. Questions?