Strive for loosely coupled designs between objects that interact

The power of loose coupling

1. When two objects are loosely coupled, they can interact, but have very little knowledge of each other.
2. The Observer Pattern provides an object design where subjects and observers are loosely coupled.
3. The only thing the subject knows about an observer is that it implements a certain Observer interface. It doesn’t need to know the concrete class of the observer, what it does, or anything else about it.
4. We can add new observers any time. Because the only thing the subject depends on is a list of objects that implement the Observer interface, we can add new observers whenever we want. In fact, we can replace any observer at runtime with another observer and the subject will keep purring along. Likewise, we can remove observers at any time.
5. We never need to modify the subject to add new types of observers. If a new concrete class comes along that needs to be an observer, we don’t need to make any changes to the subject to accommodate the new class type; all we have to do is implement the Observer interface in the new class and register as an observer. The subject doesn’t care; it will deliver notifications to any object that implements the Observer interface.
6. We can reuse subjects or observers independently of each other. If we have another use for a subject or an observer, we can easily reuse them because the two aren’t tightly coupled.
7. Changes to either the subject or the observer will not affect the other. Because the two are loosedly coupled, we are free to make changes to either, as long as the objects still meet their obligations to implement the subject or observer interfaces.

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Observer pattern