observer

Observer Pattern

Intent

Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.

Applicability

Use the Observer pattern in any of the following situations:

• When an abstraction has two aspects, one dependent on the other. Encapsulating these aspects in separate objects lets you vary and reuse them independently.
• When a change to one object requires changing others, and you don't know how many objects need to be changed.
• When an object should be able to notify other objects without making assumptions about who these objects are. In other words, you don't want these objects tightly coupled.

Structure

Participants

• Subject
  • knows its observers. Any number of Observer objects may observe a subject.
  • provides an interface for attaching and detaching Observer objects.
• Observer
  • defines an updating interface for objects that should be notified of changes in a subject.
• ConcreteSubject
  • stores state of interest to ConcreteObserver objects.
  • sends a notification to its observers when its state changes.
• ConcreteObserver
  • maintains a reference to a ConcreteSubject object.
  • stores state that should stay consistent with the subject's.
  • implements the Observer updating interface to keep its state consistent with the subject's.

Example

Imagine the weather is changing, what will the boys and girls do?

Participants in this example:

• Weather is the Subject / ConcreteSubject.
• WeatherObserver is the Observer.
• Boy/Girl is the ConcreteObserver.

Scala Tips

N/A

Reference

• Design Patterns: Elements of Reusable Object-Oriented Software