Observer Pattern

The observer pattern is a Behavioural Design Pattern that allows certain classes (known as observers) to subscribe to changes that happen in another class (known as the subject). The subscribing feature can be changed at runtime.

Usecase §

• When you have classes that need to respond to a change in another class. More specifically, when a class has a one-waydependency on another class, and there are a lot of such dependencies.
• The dependencies could also be time-specific, such listening for a one-time event.
• This could be either a one-to-one dependency (A listens to B) to many-to-one dependency (A,B,C all listen to D).

Properties §

• Subject/Publisher: The class that is being ‘observed’. It must store all the ‘subscribers’ (instances of the subscriber Interface) in an array or list (or some other data structure). Furthermore it must have an opt-in based subscribing
  • This means it must have a public Method that opts-in a class to the array
  • And one that opts-out.
• Subscriber Interface: An interface that contains a public method which is called when an ‘update’ happens.
• Observer/Subscriber: Inherits from the subscriber interface. These classes ‘observe’ the subject for any changes, which is then notified via the interface method.

UML Class §

classDiagram
direction LR
class Publisher {
	ISubscriber[] subscribers
	+subscribe(ISubscriber s)
	+unsubscribe(ISubscriber s)
	+NotifyAllSubscribers()
}
class ISubscriber {
	+update()
}
class Subscriber
<<interface>> ISubscriber
ISubscriber <|-- Subscriber
Publisher --o ISubscriber

• subscribe() and unsubscribe() append or delete a concreteSubscriber from the list, subscribers
• When a change occurs, Publisher calls NotifyAllSubscribers(), which iterates through the subscribers list and calls update() on each item.
• Every concrete subscriber handles update() differently

Pros vs Cons §

Pros §

• Dynamic subscriber allocation
• Open-Closed Principle: You can introduce new subscriber/observer classes without having to change the publisher/subject’s code

Cons §

• ??? #TODO [Cons in the observer pattern]

Example §