Static Class Structure

• Subject
  • Refers to the Observer interface to update dependent objects (observers) and is independent of how the objects are updated.
  • Maintains a list of dependent objects (observers).
  • Defines an interface for registering and unregistering observers (attach(o), detach(o)).
  • Defines an interface for notifying observers (notify()), i.e., calling update() on all registered observers: for each o in observers: o.update().
  • Usually, calls notify() on itself when its state changes.
• Subject1
  • Stores state observers depend on.
• Observer
  • Defines an interface for updating state (update()), i.e., synchronizing observer's state with subject's state.
• Observer1,Observer2,,…
  • Dependent objects that implement the Observer interface.
  • Store state that should stay consistent with subject's state.
  • Maintain a reference (subject) to the subject they observe to get the changed data (getState).

Dynamic Object Collaboration

• In this sample scenario, Observer1 and Observer2 objects register themselves on a Subject1 object and are subsequently notified and updated when Subject1 changes state.
• The interaction starts with the Observer1 and Observer2 objects that call attach(this) on Subject1 to register themselves.
• Thereafter, let's assume that the state of Subject1 changes, Subject1 calls notify() on itself.
• notify() calls update() on the registered Observer1 and Observer2 objects, which in turn get the changed data (getState()) from Subject1 to update (synchronize) their state.
• There are different ways to exchange (push/pull) data between Subject and Observer objects (see Implementation).
• See also Sample Code / Example 1.