Java观察者模式

• 小时候我们都看过动画片<<葫芦啦>>,孩子们，不要管我！快打妖精!(这是爷爷的经典台词)看着动画化片,做着英雄梦。问题来了,电视中的画面用代码如何实现呢?

• 与其临渊羡鱼，不如退而结网。让我们一步一步来实现

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1. 首先定义爷爷

import java.util.Observable;

/**
 * Created by zhangfeng on 2016/10/17.
 * 爷爷,消息发布者
 */

public class Grandpa extends Observable {
    private String mName;

    public Grandpa(String name) {
        mName = name;
    }

    /**
     * 向葫芦娃发出求救消息
     * @param msg
     */
    public void sendMsg(String msg) {
        setChanged();
        notifyObservers(msg);
    }

    public String getName() {
        return mName;
    }

    public void setName(String name) {
        mName = name;
    }
}

爷爷是消息发布者,当有情况时会调用 sendMsg()方法给你葫芦娃发消息,它继承 java的Observable

public class Observable {

    List<Observer> observers = new ArrayList<Observer>();

    boolean changed = false;

    public Observable() {
    }

    public void addObserver(Observer observer) {
        if (observer == null) {
            throw new NullPointerException("observer == null");
        }
        synchronized (this) {
            if (!observers.contains(observer))
                observers.add(observer);
        }
    }

    protected void clearChanged() {
        changed = false;
    }

    public int countObservers() {
        return observers.size();
    }

    public synchronized void deleteObserver(Observer observer) {
        observers.remove(observer);
    }

    public synchronized void deleteObservers() {
        observers.clear();
    }

    public boolean hasChanged() {
        return changed;
    }

    public void notifyObservers() {
        notifyObservers(null);
    }

    @SuppressWarnings("unchecked")
    public void notifyObservers(Object data) {
        int size = 0;
        Observer[] arrays = null;
        synchronized (this) {
            if (hasChanged()) {
                clearChanged();
                size = observers.size();
                arrays = new Observer[size];
                observers.toArray(arrays);
            }
        }
        if (arrays != null) {
            for (Observer observer : arrays) {
                observer.update(this, data);
            }
        }
    }

    protected void setChanged() {
        changed = true;
    }
}

Observable 里保存着一个 Observer 的集合，然后提供了添加、删除、提醒等方法。其中重要的是 notifyObservers(…) 方法，当确定出事了（changed 为 true）时 ，挨个通知葫芦娃

Observer 是一个接口，规定了葫芦娃收到消息的方法 update(…)

public interface Observer {
    void update(Observable observable, Object data);
}

2. 定义葫芦娃 LagenariaSicerariaChildren，葫芦娃需要按规矩来，实现 Observer 接口:

import android.util.Log;
import java.util.Observable;
import java.util.Observer;

/**
 * Created by zhangfeng on 2016/10/17.
 * 葫芦娃,消息接收者
 */

public class LagenariaSicerariaChildren implements Observer {
    String mName;

    public LagenariaSicerariaChildren(String name){
        mName = name;
    }

    /**
     * 接收到爷爷的求救消息
     * @param observable
     * @param data
     */
    @Override
    public void update(Observable observable, Object data) {
        Grandpa writer;
        if (observable instanceof Grandpa) {
            writer = (Grandpa) observable;
            Log.i("----",getName() + "收到" + writer.getName() + " 的求救: " + data.toString());
        }
    }

    public String getName() {
        return mName;
    }

    public void setName(String name) {
        mName = name;
    }
}

3. 葫芦娃去爷爷那登记

//爷爷出厂
Grandpa grandpa = new Grandpa("爷爷");
//种下一颗葫芦籽
LagenariaSicerariaChildren penhuo = new LagenariaSicerariaChildren("喷火娃");
//再种一颗葫芦籽
LagenariaSicerariaChildren penshui = new LagenariaSicerariaChildren("喷水娃");
//葫芦娃认亲(爷爷)
grandpa.addObserver(penhuo);
grandpa.addObserver(penshui);
//爷爷有危险向葫芦娃发出求救
grandpa.sendMsg("孩儿们,蛇精来了,快来救我!");

4. 最后结果

喷水娃收到爷爷 的求救: 孩儿们,蛇精来了,快来救我!
喷火娃收到爷爷 的求救: 孩儿们,蛇精来了,快来救我!