Design Patterns

Question 1

Observer (Publish-Subscribe)

Answer 1

• Different kinds of subscriber objects are interested in the state changes or events of a publisher object, and want to react in their own unique way when the publisher generates an event. Publisher wants to maintain low coupling to the subscribers.
• Solution: Define a “subscriber” or “listener” interface which Subscribers implement. The publisher can dynamically register subscribers who are interested in an event and notify them when an event occurs.

Question 2

Observer

Answer 2

• For example, when an item in a list changes, the on-screen list should
change.
• You could just send a message from the domain object to the UI object
• This would violate Model-View separation
• Thus various controls register for events, either external (button presses, keystrokes) or internal (new item added to a list) and act accordingly.

Question 3

GRASP Patterns

Answer 3

• Creator
• Information Expert
• Low Coupling
• Controller
• High Cohesion