设计模式－观察者模式

观察者模式又称发布－订阅模式，用于定义对象间的一种一对多的关系，当一个对象的状态发生改变时，所有依赖它的对象都得到通知，并且自动更新。这一模式中关键对象是目标和观察者，一个目标可以有任意多个依赖它的观察者，一旦目标状态发生变化，所有观察者都得到通知，与目标保持状态同步。

Observer模式允许独立的改变目标和观察者，目标和观察者之间是抽象耦合的，但是它也有自己的缺点，因为一个目标对依赖于它的观察者一无所知，一个看似无害的操作可能会导致观察者出现问题。

一个观察者模式的简单实现：这里用老师和学生之间来描述，当老师的手机号改变的时候，需要通知他的学生们，以便学生随时都能联系到老师。

public interface MyObserver {

	public void update();
}

public interface Target {

	/*
	 * 一个目标有三个方法，跟Observer交互
	 */
	public void attach(MyObserver observer);
	
	public void detach(MyObserver observer);
	
	public void notifyChange();
}

public class Teacher implements Target{

	private List<MyObserver> observers;
	private String phoneNumber;
	
	public Teacher(String phoneNumber){
		observers = new ArrayList<MyObserver>();
		this.phoneNumber = phoneNumber;
	}
	public void attach(MyObserver observer) {
		observers.add(observer);
	}

	public void detach(MyObserver observer) {
		observers.remove(observer);
	}

	public void notifyChange() {
		for(MyObserver observer:observers) {
			observer.update();
		}
	}
	
	public void setPhoneNumber(String phoneNumber) {
		this.phoneNumber = phoneNumber;
		notifyChange();
	}

	public String getPhoneNumber() {
		return phoneNumber;
	}
}

public class Student implements MyObserver{

	private Teacher teacher;
	private String name; 
	public Student(String name,Teacher teacher) {
		this.name = name;
		this.teacher = teacher;
		System.out.println("student name:"+name+",teacher phone number:"+teacher.getPhoneNumber());
	}
	public void update() {
		System.out.println("teacher phone number:"+teacher.getPhoneNumber());
	}

}

public class Client {

	public static void main(String[] args) {
		Teacher teacher = new Teacher("098765");
		MyObserver sdutent1 = new Student("zhang san", teacher);
		MyObserver sdutent2 = new Student("li si", teacher);
		MyObserver sdutent3 = new Student("wang wu", teacher);
		MyObserver sdutent4 = new Student("zhao si", teacher);
		
		teacher.attach(sdutent1);
		teacher.attach(sdutent2);
		teacher.attach(sdutent3);
		teacher.attach(sdutent4);
		
		teacher.setPhoneNumber("112356");
	}
}

student name:zhang san,teacher phone number:098765
student name:li si,teacher phone number:098765
student name:wang wu,teacher phone number:098765
student name:zhao si,teacher phone number:098765
teacher phone number:112356
teacher phone number:112356
teacher phone number:112356
teacher phone number:112356