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What is the Chain?

What is the Chain?. It’s a behavioral design pattern. It deals with how objects make requests and how they are handled. Question. When an object makes a request, who handles it?. Conventional Approach. The object that makes the request sends it directly to the object what handles it.

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What is the Chain?

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

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

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

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

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

  6. 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.

  7. Example:Context Sensitive Help

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

  9. 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.

  10. Flow of Requests

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

  12. Interaction Diagram

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

  14. 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.

  15. Class Diagram

  16. Things to Consider

  17. 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.

  18. 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.

  19. 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.

  20. Pros and Cons

  21. 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.

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

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

  24. Questions?

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