Observer defines a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically. The pattern has behavioral purpose and applies to the objects.
- when an abstraction has two aspects, one dependent on the other
- 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