观察者模式--Java设计模式.doc

观察者模式-Java设计模式观察者模式定义：定义了对象之间的一对多的依赖，这样一来，当一个对象发生改变状态的时候，它的所有依赖者都会收到通知并自动更新。参考如下图：观察者设计模式也叫发布-订阅模式。也可以称作：出版者+订阅者 = 观察者模式在Android中观察者模式的实例有：广播机制，ContentObserver的注册方式，一个是同步的观察者模式，一个是异步的观察者模式。当两个对象之间松耦合，它们依然可以交互，但是不清楚彼此的细节，观察者提供了一种对象的设计，可以降低之间的耦合，避免对象的双向依赖。举例：在Android中很多的Listener是观察者模式，比如点击事件的OnClickListener，就是为了避免对象的双向依赖。-先来看看Java源码中的观察者模式：Observer观察者接口：package com.daming.java.observer;public interface Observer void update(Observable observable, Object arg); 1 2 3 4 5 6对象Observable 类的实现package com.daming.java.observer;import java.util.Vector;public class Observable private boolean changed = false; private Vector obs; public Observable() obs = new Vector(); public synchronized void addObserver(Observer o) if (o = null) throw new NullPointerException(); if (!obs.contains(o) obs.addElement(o); public synchronized void deleteObserver(Observer o) obs.removeElement(o); public void notifyObservers() notifyObservers(null); 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) arrLocali).update(this, arg); public synchronized void deleteObservers() obs.removeAllElements(); protected synchronized void setChanged() changed = true; protected synchronized void clearChanged() changed = false; public synchronized boolean hasChanged() return changed; /* * Returns the number of observers of this Observable object. * * return the number of observers of this object. */ public synchronized int countObservers() return obs.size(); 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70MyObserver 观察者对象的实现package com.daming.java.observer;public class MyObserver implements Observer Override public void update(Observable observable, Object arg) System.out.println(observable : + observable + arg : + arg); 1 2 3 4 5 6 7 8 9MyObserver2 观察者对象的实现package com.daming.java.observer;public class MyObserver2 implements Observer Override public void update(Observable observable, Object arg) System.out.println(observable2 : + observable + arg2 : + arg); 1 2 3 4 5 6 7 8 9TestObserver类的实现package com.daming.java.observer;public class TestObserver /* * param args */ public static void main(String args) Observable observable = new Observable(); MyObserver myObserver = new MyObserver(); MyObserver2 myObserver2 = new MyObserver2(); observable.addObserver(myObserver); observable.addObserver(myObserver2); observable.setChanged(); observable.notifyObservers(14); observable.setChanged(); observable.notifyObservers(I am daming); 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21Log输入结果：observable2 :com.daming.java.observer.Observable14318bbarg2 :14observable :com.daming.java.observer.Observable14318bbarg :14observable2 :com.daming.java.observer.Observable14318bbarg2 :I am damingobservable :com.daming.java.observer.Observable14318bbarg :I am daming 1 2 3 4Java源码中上述代码中用的是推模式，当然源码中也有拉模式，即主动查询的模式。拉模式就像广播一样，通过onReceive（）方法拉起来一些动作的，还有像ContentObserver可以通过拉模式，当数据库发生改变的时候，通过onChange（）方法来调用一些操作。-接着我们来看看Android中异步的观察者模式。ContentObserver就是异步的观察者模式，异步的观察者有什么好处呢？不阻塞观察者的回调。在同步通知中会有阻塞问题，各个Observer的响应方法是串行的，如果有一个observer耗时的话就会阻塞其他observer接收者了，这样就有可能就会引起bug来，所以在设计的时候多考虑一些，是否考虑用异步的观察者模式，让并发处理快一些；我们通过一个简单的demo来学习下异步的观察者模式：先来看看Observer，这个Android中是抽象的类package cn.daming.observer.design;import android.os.Handler;import android.util.Log;public abstract class Observer private Handler mHandler; public Observer(Handler handler) mHandler = handler; public void onChange() public final void dispatchChange() Log.v(daming, Observer dispatchChange is mHandler= null : + (mHandler = null); if (mHandler = null) onChange(); else mHandler.post(new NotificationRunnable(); private final class NotificationRunnable implements Runnable Override public void run() Log.v(daming, NotificationRunnable dispatchChange is run . ); Observer.this.onChange(); 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35接着来看看ObserverService这个类的实现：package cn.daming.observer.design;import java.util.ArrayList;import java.util.List;import android.util.Log;public class ObserverService private int mState; private List mObservers = new ArrayList(); public final void registerObserver(Observer observer) if (!mObservers.contains(observer) mObservers.add(observer); public final void unregisterObserver(Observer observer) Log.v(daming, ObserverService unregisterObserver :); mObservers.remove(observer); public void notifyChange() for (Observer observer : mObservers) observer.dispatchChange(); public int getState() return mState; public void setState(int state) mState = state; notifyChange(); 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40最后我们来写测试类FirstActivity ：package cn.daming.observer.design;import android.app.Activity;import android.os.Bundle;import android.os.Handler;import android.util.Log;import android.view.View;import android.view.View.OnClickListener;import android.widget.Button;import com.daming.designtraning.R;public class FirstActivity extends Activity private ObserverService mObserverService; private Button mButton; private int mState = 0; Override protected void onCreate(Bundle savedInstanceState) super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); mButton = (Button) findViewById(R.id.button); mObserverService = new ObserverService(); mObserverService.registerObserver(mFirstObserver); mObserverService.setState(+mState); mButton.setOnClickListener(new OnClickListener() Override public void onClick(View v) mObserverService.setState(+mState); ); Override protected void onDestroy() super.onDestr