Consequences

• abstract coupling between subject and observer
• support for broadcast
• deals with unexpected updates

Implementation

• mapping subjects to observers
• observing more than one subject?
• who triggers the update - subject or client of subject
• dangling references to subject
• consistency of subject (before notification)
• adding observer-specific update protocols (push / pull)
• specifying modifications of interest (aspects)
• how to encapsulate update semantics