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.
Drawbacks
• Objects must know who handles each type of request.
• Objects must have links to many other objects.
Chain Approach
• Objects are arranged in a “chain of responsibility.”
• Requests are passed along the chain until they are handled.
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.
Example:Context Sensitive Help
Suppose:
• User can click any part of the interface to get specific help.
• If no specific help is available, more general information is provided.
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.
Flow of Requests
Implementation
• Each object must have a HandleHelp() method.
• This method either handles the help request or passes it along the chain.
Interaction Diagram
Implementation (cont.)
• We define a HelpHandler class with a HandleHelp() method.
• All other classes are subclasses to HelpHandler.
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.
Class Diagram
Things to Consider
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.
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.
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.
Pros and Cons
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.
Cons
• Care must be taken to ensure all requests are handled properly.
References
• Gamma, Erich et al. Design Patterns: Elements of Reusable Object-Oriented Software. Addison-Wesley, 1995.
Questions?
