Behavioral Patterns - Observer Pattern Exercise

behavioral-patterns-observer-pattern-exercise

The Observer Pattern is widely used to solve one-to-many object state changes notification problems. The pub/sub model uses the Observer Pattern as the base pattern. It is also used in the event-driven systems. In this exercise, we will create a naive implementation of the pub/sub model to understand the components of the Observer Pattern.

Reading Materials

  1. https://en.wikipedia.org/wiki/Observer_pattern (10 minutes reading)

Practice Materials

  1. Use the Observer Pattern to create a naive implementation of the pub/sub model.

Components:

  1. Observer (The interface that defines the callback method signature or the update method when it is notified.) - Observer
  2. Subject (The interface that observers listen to) - Subject
  3. AbstractSubscriber (The abstract class that attach the observer to a subject) - AbstractSubscriber
  4. AbstractTopic (The abstract class that keep track of a list of observers and provides observer registration implementations) - AbstractTopic
  5. TextMessageSubscriber (The concrete class that represents a subscriber that will print out the message that is published) - TextMessageSubscriber
  6. TextMessageTopic (The concrete class that represents a topic that defines the message publishing implementation) - TextMessageTopic
  7. Client (The class that creates topics and subscribers, registers and unregisters them, and publishes messages to topics) - App

Tip

  • A Queue could be used to keep track of published message to the Topic. After notifying all subscribers about the head message of the queue, the message should be dequeued.

Solution

Pub/Sub Example

Questions to discuss

  1. What are the common use cases to apply an Observer Pattern?

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