As a new Android developer, I discovered a tip that has helped me tremendously as I've worked on Saver, a savings planning application. To debug with Android source and Eclipse, install the following packages (usually via Help -> Install New Software...):
The Observer pattern allows a developer to signal one or more objects when an object changes state (1:N). The Design Patterns text describes the interacting classes in the Observer pattern as a subject and its observers. The subject is under observation and when its state changes the observers are notified. This is also known as publish-subscribe.
A few things to consider with the Observer pattern:
The Observer pattern should be used when you either don't know how many objects need to be changed when the subject changes or an object needs to notify other objects without assuming too much about the other objects
Subject and observer are not tightly coupled in this situation so layering can remain intact
The subject can broadcast information to observers but can still add and remove observers at any time
More information including a sample implementation is available here.