When an Android application become complex, the communication between the different parts of your code become difficult and in this complexity the risk of introducing bugs grows.
A tiny library called Otto and developped by Jack Warthon, helps to simplify your code and to update easily different parts of your application following an event produced by an action.
In fact you might need to update the status of several acitvities after a change in the settings. A deep explication of the communication problem and the example using Otto are published on the
Square blog while the library is downloadable from
GitHub and there you will find also a complete tutorial