The Observer Pattern, in the Real World

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

Being Loosely Coupled

- Subjects and observers are "loosely coupled"

They interact,

but have little knowledge of each other.

- Subject only knows

The observer implements a specific interface.

Does not need to know concrete class or anything else.

The Observer

- Subject relies on a list of observers.

- Observers can be:

+ Added at any time.

+ Removed at any time.

+ Or even replaced.

- Subject does not care, it keeps doing its job.

If we want to add new type of observers, modification of the subject is never needed.

- Observers just need to implement the Observer interface.

In fact, no changes to the subject or observers will affect the other.

> That's loose coupling.

Because object interdependencies are minimized, loose coupling allows us to build more flexible OO architectures.

Design Principle #4

Strive for loosely coupled designs between objects that interact.