Message Handler Pattern

All message clients perform two functions: they retrieve and process a message. It's possible to weave these two activities tightly together, but in more complex systems it can result in some pretty complex code. We want to isolate the latter function in a way that lets us share code across different message types. The Message Handler pattern abstracts the code responsible for receiving the actual message away from the code responsible for processing the message content.

This pattern allows us to change the method by which we receive a message without changing the code for processing the message. Changing message handling itself becomes easier, since we can drop message handlers in and out without major changes to the rest of the application (we can even do it dynamically). There's also a testing benefit, since you can write standalone test cases for an isolated handler method more easily than for an integrated component.