design_model（18）observe

1.观察者模式

 观察者模式定义了一种一对多的依赖关系，让多个观察者对象同时监听某一个主题对象，这个主题对象在状态发生变化的时，会通知所有观察者对象，使他们能够自动更新自己。

2.实例1

public class ConcreteSubject implements Subject {
	private int i;

	public void setI(int i) {
		this.i = i;
	}

	private List<Observer> list = new ArrayList<>();

	public void add(Observer observer) {
		list.add(observer);
	}

	public void update() {
		for (Observer observer : list) {
			observer.update(i);
		}
	}
}

public class Observer {
	private int i;

	public void update(int i) {
		this.i = i;
	}

	public int getI() {
		return i;
	}
}

public interface Subject {
	
}

public class Client {
	public static void main(String[] args) {
		ConcreteSubject cs = new ConcreteSubject();
		cs.setI(3);
		Observer observer1 = new Observer();
		Observer observer2 = new Observer();
		cs.add(observer1);
		cs.add(observer2);
		cs.update();
		System.out.println(observer1.getI());
		System.out.println(observer1.getI());
	}
}

3.实例2

public class ConcreteSubject extends Observable {
	private int i;

	public void set(int s) {
		this.i = s;
		setChanged();
		notifyObservers(i);
	}

	public int getI() {
		return i;
	}

	public void setI(int i) {
		this.i = i;
	}

}

public class Observe implements Observer {

	private int i;

	@Override
	public void update(Observable o, Object arg) {
		this.i = ((ConcreteSubject) o).getI();
	}

	public int getI() {
		return i;
	}

	public void setI(int i) {
		this.i = i;
	}

}

public class Client {
	public static void main(String[] args) {
		ConcreteSubject cs = new ConcreteSubject();
		Observe observer1 = new Observe();
		Observe observer2 = new Observe();
		cs.addObserver(observer1);
		cs.addObserver(observer2);
		cs.set(3);
		System.out.println(observer1.getI());
		System.out.println(observer1.getI());
	}
}

//源码

public void notifyObservers(Object arg) {
        Object[] arrLocal;

        synchronized (this) {
            if (!changed)
                return;
            arrLocal = obs.toArray();
            clearChanged();
        }

        for (int i = arrLocal.length-1; i>=0; i--)
            ((Observer)arrLocal[i]).update(this, arg);
    }