Advantages (+)

• Decouples subject from observers.
  • Subject only refers to and knows about the simple Observer interface for updating (synchronizing) state (update()).
  • "Because Subject and Observer aren't tightly coupled, they can belong to different layers of abstraction in a system. [GoF, p296]
  • Loosely coupled objects are easier to implement, change, test, and reuse.
• Makes adding/withdrawing observers easy.
  • Observers can be added to (attach(o)) and withdrawn from a subject independently and dynamically.
  • Usually, observers are responsible for registering and unregistering themselves on a subject.
  • Subject's sole responsibility is to hold a list of observers and notify (call update() on) them when its state changes.

Disadvantages (–)

• Can make the update behavior complex.
  • A change on the subject may cause a cascade of updates to observers and their dependent objects.
  • The Mediator design pattern can be applied to implement a complex dependency relationship between subject(s) and observers.