dp observer pattern introduction as per wikipedia"the observer pattern ( subset of the publish/subscribe patternis software design pattern in which an objectcalled the subjectmaintains list of its dependantscalled observersand notifies them automatically of any state changesusually by calling one of their methods it is mainly used to implement distributed event handling systems typically in the observer patternwe would have publisher class that would contain methods forregistering other objects which would like to receive notifications notifying any changes that occur in the main object to the registered objects (via registered object' methodunregistering objects that do not want to receive any further notifications subscriber class that would containa method that is used by the publisher classto notify the objects registered with itof any change that occurs an event that triggers state change that leads the publisher to call its notification method www testingperspective com