• subject maintains a list of dependent observers
• subject provides an interface (i.e., a set of methods) for attaching and detaching observer objects
• subject sends notifications via an update method on each observer
• observer provides an updating interface used by the subject for notification

Subject                   Observer
  attach(Observer)          update(infoObject)
  detach(Observer)
  notify(infoObject)

• issue: how to tell observers "what changed" in subject?
• push model vs pull model
• push model: subject sends "what changed" at update time (e.g., via infoObject).
  • more efficient, but forces subject to assume needs of observers
• pull model: subject sends nothing at update time. observers must determine "what changed" by requesting information from subject.
  • less efficient. more work for each observer.
• middle ground: category types. subject provides a separate interface for each category of change.
  • e.g., Java's java.awt.Component class provides separate addMouseListener() and addMouseMotionListener() methods