Java设计模式之观察者模式

OBSERVER?（Object Behavioral）?
Purpose?
Lets one or more objects be notified of state changes in other?
objects within the system.?
Use When?
1 State changes in one or more objects should trigger behavior?
? in other objects?
2 Broadcasting capabilities are required.?
3 An understanding exists that objects will be blind to the?
? expense of notification.?
Example?
This pattern can be found in almost every GUI environment.?
When buttons, text, and other fields are placed in applications?
the application typically registers as a listener for those controls.?
When a user triggers an event, such as clicking a button, the?
control iterates through its registered observers and sends a?
notification to each.?
new?ConcreteObserver();??