Mediator Pattern vs Observer Pattern

Mediator Pattern and Observer Pattern (Publisher-Subscriber Pattern) differences are often elusive. In most cases, you can implement either of these patterns; but sometimes you can apply both simultaneously. Let’s see how we can do that.

• The primary goal of Mediator is to eliminate mutual dependencies among a set of system components. Instead, these components become dependent on a single mediator object. The goal of Observer is to establish dynamic one-way connections between objects, where some objects act as subordinates of others

• There’s a popular implementation of the Mediator pattern that relies on Observer. The mediator object plays the role of publisher, and the components act as subscribers which subscribe to and unsubscribe from the mediator’s events. When Mediator is implemented this way, it may look very similar to Observer.

• When you’re confused, remember that you can implement the Mediator pattern in other ways. For example, you can permanently link all the components to the same mediator object. This implementation won’t resemble Observer but will still be an instance of the Mediator pattern.

• Now imagine a program where all components have become publishers, allowing dynamic connections between each other. There won’t be a centralized mediator object, only a distributed set of observers.