The document discusses the Observer design pattern and how to create observable classes in ExtJS that can notify observer objects when events occur. It provides an example of an ObservableStack class that inherits from Ext.util.Observable and defines push and pop events that are fired when those methods are called. An example shows setting up event handlers on an ObservableStack instance and calling its methods to demonstrate event handling.

1. ExtJS Ext.util.Observable By Aaron Conran