Observer
A one-to-many dependency between objects so that when an object changes its state. It notifies its dependent objects.
update()
observers
ConcreteSubject
getState()
setState()
Subject
attach()
detach()
notify()
Observer
update()
subject
while (e.hasMoreElements()) {
Observer o = (Observer) e.nextElement();
o.update(this);
}
state = newState;
notify();
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