对象之间常有依赖关系，观察者模式可以让我们方便地监听一个对象，当它发生改变时，其他对象会自动收到通知并作出相应的反应。
实例
public class Subject {
private int mState;
private List<Observer> mObservers = new ArrayList<>();
public void setState(int state) {
mState = state;
notifyObservers();
}
@Override
public String toString() {
return "State: " + mState;
}
public boolean addObserver(Observer o) {
return mObservers.add(o);
}
public boolean removeObserver(Observer o) {
return mObservers.remove(o);
}
private void notifyObservers() {
for (Observer o : mObservers) {
o.update(this);
}
}
public interface Observer {
void update(Subject subject);
}
}
测试
Subject subject = new Subject();
subject.setState(0);
Subject.Observer so = System.out::println;
subject.addObserver(so);
subject.setState(1);
subject.removeObserver(so);
subject.setState(2);
State: 1
