Jason Gedge

Listening In

Posted on July 24, 2009

So it came to our attention recently that our application was making abundant use of the Observer/Listener pattern. For those not familiar with this pattern, you’d use this guy when you want the outside world to know about state changes in an object. This pattern is used often when developing with various architectural patterns, such as Model-View-Controller (MVC). Other examples, in Java, include many of the components in Swing, java.util.Observable, and java.beans.PropertyChangeListener.

I personally had issues with the extensive use of the listener pattern for us. Every time we added a new data class, we’d have to rewrite code for storing and notifying the listeners. Note that some of this could have been factored out into its own class. Nevertheless, if we ever changed the listener interface, change had to be made for everyone using that interface. Since we use Eclipse, this isn’t too big of an issue, but it still bugged me a little. The final issue was, sometimes things needed to listen to events on EVERY instance of a data class, not just a specific instance. To make our lives a little easier, this required us to create a new type of listener that would be statically available (read: like a singleton).

My alternative was to create a messaging system. Before I started this messaging system, I thought about design so that I could generalize it so that it may be of use to others. A couple of the main design decisions that came up were:

  • There should be a concept of a message sender, receiver, and a delivery system to coordinate message sending and receiving.
  • There should be registration facilities to allow the system to become as type-safe and interface-like as possible.
  • Java annotations (retention set to RUNTIME) will be used to define messages and receiver methods
  • When registering a specific receiver instance to receive messages, weak references should be used so that a) the outside world doesn’t have to concern themselves with unregistering the instance and b) considering (a), so that the garbage collector can destroy that instance (when necessary).

A test program I wrote up that uses this message system looks something like this. I personally find this a reasonably elegant system, for one that uses reflection. So what do we get out of this? I’ll start with the cons (that I can think of) followed by (what I consider to be) the pros:

  • Cons
    • We lose a lot of compile-time error checking
    • We introduce some overhead, mainly due to using the reflection API
  • Pros
    • Adding or removing messages (generally) will require less work elsewhere in the code
    • Receivers only need to implement the messages they want to receive
    • Receivers are not required to name their methods as per an interface
    • Receivers can define what I call “catchall” methods, methods that accept all messages from a specified sender (this could also be done using the observable/listener pattern too, but I believe it would be a little less elegant)
    • Receivers define an accept method which allows them to dynamically control which instances they receive messages from

Currently I’m holding on to this until I feel it satisfies the needs of our score editing project completely, but after that I think I’ll release it to the public so that someone else might find some use out of it.

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