观察者模式

定义

定义一对多的依赖关系，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上发生变化时，会通知所有观察者对象，使它们能够自动更新

角色介绍

• Subject :抽象主题，也就是被观察（Observale）的角色，抽象主题角色把所有观察则对象的引用保存在一个集合里，每个主题都可以有任意数量的观察者，抽象主题提供一个接口，可以增加和删除观察者对象。
• ConcreteSubject: 具体主题，该角色将有关状态存入具体观察者对象，在具体主题的内部状态发生改变时，给所有注册过的观察者发出通知，具体主题角色又叫做具体被观察者（Concrete Observable）角色
• Observer:抽象观察者，该角色是观察者的抽象类，它定义了一个更新接口，以便在主题的状态发生变化时更新自身的状态。
• ConcreteObserver: 具体的观察者，该角色实现抽象观察者角色所定义的更新接口，以便在主题的状态发生变化时，更新自身的状态。

代码示例如下：

抽象被观察者

public interface Observable { //抽象被观察者

    void add(Observer observer); //添加观察者

    void remove(Observer observer); //删除观察者

    void notify(String observer); //通知观察者


}

具体被观察者

import java.util.ArrayList;
import java.util.List;
public class Postman implements Observable{ //快递员

    private List<Observer> personList = new ArrayList<>();//保存收件人的信息

    @Override
    public void add(Observer observer) {
        personList.add(observer);
    }

    @Override
    public void remove(Observer observer) {

        personList.remove(observer);
    }

    @Override
    public void notify(String message) {

        for (Observer observer:personList)
        observer.update(message);
    }
}

抽象观察者

public interface Observer { //抽象观察者

    public void update(String message); //更新方法
}

具体观察者

public class Boy implements Observer{

    private String name; //名字

    public Boy(String name){
        this.name = name;
    }

    @Override
    public void update(String message) {

        System.out.println(name + ",收到了信息：" + message+",高高兴兴的去取快递...");
    }
}


public class Girl implements Observer{

    private String name;

    public Girl(String name){
        this.name = name;
    }

    @Override
    public void update(String message) {

        System.out.println(name+",收到了信息：" + message + "让男朋友去取快递");
    }
}

测试一下：

public class Test {

    public static void main(String[] args){

        Observable postman = new Postman();

       Observer boy1 = new Boy("汤姆");
        Observer boy2 = new Boy("杰瑞");
        Observer girl1 = new Girl("露丝");
        postman.add(boy1);
        postman.add(boy2);
        postman.add(girl1);

        postman.notify("快递到了，请下楼取快递...");
    }
}

运行结果如下：

image.png

实际上jdk中内置了Observer和Observable
下面我们来模拟一下开发技术前线的发布过程：

import java.util.Observable;
import java.util.Observer;

public class Coder implements Observer{

    private String name;

    public Coder(String name){
        this.name = name;
    }

    @Override
    public void update(Observable o, Object arg) {
        System.out.println("Hi," + name + "技术博客更新了,内容：" + arg);
    }

    @Override
    public String toString() {
        return "码农：" + name;
    }
}

import java.util.Observable;

public class DevTechFrontier extends Observable{

    public void postNewPublication(String content){

        //标识状态或者内容发生改变
        setChanged();

        notifyObservers(content);
    }
}

public class TestOrginal {

    public static void main(String[] args){

        //被观察者
        DevTechFrontier devTechFrontier = new DevTechFrontier();
        //观察者
        Coder mrSimple = new Coder("mr.simple");
        Coder coder1 = new Coder("coder-1");
        Coder coder2 = new Coder("coder-2");
        Coder coder3 = new Coder("coder-3");

        //将观察者注册到可观察对象的观察者列表中
        devTechFrontier.addObserver(mrSimple);
        devTechFrontier.addObserver(coder1);
        devTechFrontier.addObserver(coder2);
        devTechFrontier.addObserver(coder3);

        devTechFrontier.postNewPublication("新一期的技术周刊发布啦。。。");


    }
}

我们来看一下jdk内置的Observable和Observer源码

public class Observable {
    private boolean changed = false;
    private Vector<Observer> obs;

    /** Construct an Observable with zero Observers. */

    public Observable() {
        obs = new Vector<>();
    }

    /**
     * Adds an observer to the set of observers for this object, provided
     * that it is not the same as some observer already in the set.
     * The order in which notifications will be delivered to multiple
     * observers is not specified. See the class comment.
     *
     * @param   o   an observer to be added.
     * @throws NullPointerException   if the parameter o is null.
     */
    public synchronized void addObserver(Observer o) {  //添加观察者
        if (o == null)
            throw new NullPointerException();
        if (!obs.contains(o)) {
            obs.addElement(o);
        }
    }

    /**
     * Deletes an observer from the set of observers of this object.
     * Passing <CODE>null</CODE> to this method will have no effect.
     * @param   o   the observer to be deleted.
     */
    public synchronized void deleteObserver(Observer o) {
        obs.removeElement(o);
    }

    /**
     * If this object has changed, as indicated by the
     * <code>hasChanged</code> method, then notify all of its observers
     * and then call the <code>clearChanged</code> method to
     * indicate that this object has no longer changed.
     * <p>
     * Each observer has its <code>update</code> method called with two
     * arguments: this observable object and <code>null</code>. In other
     * words, this method is equivalent to:
     * <blockquote><tt>
     * notifyObservers(null)</tt></blockquote>
     *
     * @see     java.util.Observable#clearChanged()
     * @see     java.util.Observable#hasChanged()
     * @see     java.util.Observer#update(java.util.Observable, java.lang.Object)
     */
    public void notifyObservers() {
        notifyObservers(null);
    }

    /**
     * If this object has changed, as indicated by the
     * <code>hasChanged</code> method, then notify all of its observers
     * and then call the <code>clearChanged</code> method to indicate
     * that this object has no longer changed.
     * <p>
     * Each observer has its <code>update</code> method called with two
     * arguments: this observable object and the <code>arg</code> argument.
     *
     * @param   arg   any object.
     * @see     java.util.Observable#clearChanged()
     * @see     java.util.Observable#hasChanged()
     * @see     java.util.Observer#update(java.util.Observable, java.lang.Object)
     */
    public void notifyObservers(Object arg) { //通知观察者
        /*
         * a temporary array buffer, used as a snapshot of the state of
         * current Observers.
         */
        Object[] arrLocal;

        synchronized (this) {
            /* We don't want the Observer doing callbacks into
             * arbitrary code while holding its own Monitor.
             * The code where we extract each Observable from
             * the Vector and store the state of the Observer
             * needs synchronization, but notifying observers
             * does not (should not).  The worst result of any
             * potential race-condition here is that:
             * 1) a newly-added Observer will miss a
             *   notification in progress
             * 2) a recently unregistered Observer will be
             *   wrongly notified when it doesn't care
             */
            if (!changed) 
                return;
            arrLocal = obs.toArray();
            clearChanged();
        }

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

    /**
     * Clears the observer list so that this object no longer has any observers.
     */
    public synchronized void deleteObservers() {
        obs.removeAllElements();
    }

    /**
     * Marks this <tt>Observable</tt> object as having been changed; the
     * <tt>hasChanged</tt> method will now return <tt>true</tt>.
     */
    protected synchronized void setChanged() {
        changed = true;
    }

    /**
     * Indicates that this object has no longer changed, or that it has
     * already notified all of its observers of its most recent change,
     * so that the <tt>hasChanged</tt> method will now return <tt>false</tt>.
     * This method is called automatically by the
     * <code>notifyObservers</code> methods.
     *
     * @see     java.util.Observable#notifyObservers()
     * @see     java.util.Observable#notifyObservers(java.lang.Object)
     */
    protected synchronized void clearChanged() {
        changed = false;
    }

    /**
     * Tests if this object has changed.
     *
     * @return  <code>true</code> if and only if the <code>setChanged</code>
     *          method has been called more recently than the
     *          <code>clearChanged</code> method on this object;
     *          <code>false</code> otherwise.
     * @see     java.util.Observable#clearChanged()
     * @see     java.util.Observable#setChanged()
     */
    public synchronized boolean hasChanged() {
        return changed;
    }

    /**
     * Returns the number of observers of this <tt>Observable</tt> object.
     *
     * @return  the number of observers of this object.
     */
    public synchronized int countObservers() {
        return obs.size();
    }



public interface Observer { //抽象观察者
    /**
     * This method is called whenever the observed object is changed. An
     * application calls an <tt>Observable</tt> object's
     * <code>notifyObservers</code> method to have all the object's
     * observers notified of the change.
     *
     * @param   o     the observable object.
     * @param   arg   an argument passed to the <code>notifyObservers</code>
     *                 method.
     */
    void update(Observable o, Object arg); //定义了一个update方法
}

我们在Android源码中，也会用到观察者模式，比如在一个列表中，我们添加了数据后，都会调用adapter.notifyDataSetChanged()
我们进入notifyDataSetChanged方法源码中看一下：

   public final void notifyDataSetChanged() {
            mObservable.notifyChanged();
        }

跟我们的示例代码原理一样，被观察者去通知更新