The observer pattern defines one to many dependency between objects, so that when one object changes state all of it's dependencies are notified and updated automatically.
Pattern type - Behavioral
This pattern focuses on push architecture instead of pooling. We distinguish observable
(aka subject) and observer object(s).
Imagine a process which needs to perform some calculations. Due to heaviness of a task, results cannot be provided instantly and in general it's hard to estimate expected time of arrival. In such case process performing computations will be regarded as an observable and it's responsibility is to notify when computing finishes.
Process object - ComputingProcess, implements an interface - IComputingProcess which provides methods:
- registerObservable(Observer obj)
- removeObservable(Observer obj)
- notify()
ComputingProcess takes computes_count as an argument in it's constructor
(there's some loop to model computational time).
ProcessClient (observer) class implements IProcessClient interface which provides method
update(). When ComputingProcess object finishes computations it'll call update()
method (push architecture) on client.