3、观察者模式

观察者模式

常用于：对象间多对一依赖。被依赖的对象为Subject，依赖的对象为Observer，Subject通知Observer变化。

生活中的例子对应：

1>订牛奶
奶站--Subject：是一个接口，实现功能：注册、移除、通知
用户--Observer：也是一个接口，实现功能：接收输入



2>气象播报

气象站--Subject

公告牌--Observer

以气象播报为例：

vs1：什么都不用，想当然直接写出来的情况：

提供数据接口的气象站 WeatherData.java :

public class WeatherData {private float mTemperature;private float mPressure;private float mHumidity;private CurrentConditions mCurrentConditions = null;public WeatherData(CurrentConditions currentConditions){mCurrentConditions = currentConditions;}public float getTemperature() {return mTemperature;}public float getPressure() {return mPressure;}public float getHumidity() {return mHumidity;}public void dataChanged(){mCurrentConditions.update(getTemperature(), getPressure(), getHumidity());}public void setData(float temperature, float pressure, float humidity){mTemperature = temperature;mPressure = pressure;mHumidity = humidity;dataChanged();}}

接收气象数据的公告牌 CurrentConditions.java：

public class CurrentConditions {private float mTemperature;private float mPressure;private float mHumidity;public void update(float temperature, float pressure, float humidity){mTemperature = temperature;mPressure = pressure;mHumidity = humidity;display();}public void display(){System.out.println("Today's temperature:"+ mTemperature);System.out.println("Today's pressure:"+ mPressure);System.out.println("Today's humidity:"+ mHumidity);}}

写个类（WeatherTest.java）测试一下：

public class WeatherTest {public static void main(String[] args) {CurrentConditions mCurrentConditions; WeatherData mWeatherData;mCurrentConditions = new CurrentConditions();mWeatherData = new WeatherData(mCurrentConditions);mWeatherData.setData(30, 150, 40);}}

运行结果：

Today's temperature:30.0Today's pressure:150.0Today's humidity:40.0

很显然可以运行，但是当增多一个接收信息的公告牌时，改动很麻烦（flag）。

vs2：采用装饰者模式

先定义一个接口 Subject.java：

public interface Subject {public void registerObserver(Observer o);public void removeObserver(Observer o);public void notifyObservers();}

创建注册、移除、通知三个方法

再定义一个接口 Observer.java：

public interface Observer {public void update(float temperature, float pressure, float humidity);}

创建更新数据的update（）方法，接收传进来的数据。

提供数据接口的气象站 WeatherData.java 很明显要实现 Subject.java 接口:

public class WeatherData implements Subject {private float mTemperature;private float mPressure;private float mHumidity;private ArrayList<Observer> mObservers = null;public WeatherData(){mObservers = new ArrayList<Observer>();}public float getTemperature() {return mTemperature;}public float getPressure() {return mPressure;}public float getHumidity() {return mHumidity;}public void dataChanged(){notifyObservers();}public void setData(float temperature, float pressure, float humidity){mTemperature = temperature;mPressure = pressure;mHumidity = humidity;dataChanged();}@Overridepublic void registerObserver(Observer o) {mObservers.add(o);}@Overridepublic void removeObserver(Observer o) {mObservers.remove(o);}@Overridepublic void notifyObservers() {for(int i = 0; i < mObservers.size(); i++){//这里有两种方式通知：//vs1：直接把参数传过去mObservers.get(i).update(getTemperature(), getPressure(), getHumidity());//vs2：只是通知观察者们有新消息了，需要什么参数观察者们自己去取（更适合扩展），取回来的数据再进行解析}}}

而作为接收气象数据的公告牌有两个，两个都要实现Observer.java接口：

CurrentConditions.java：

public class CurrentConditions implements Observer {private float mTemperature;private float mPressure;private float mHumidity;@Overridepublic void update(float temperature, float pressure, float humidity) {mTemperature = temperature;mPressure = pressure;mHumidity = humidity;display();}public void display(){System.out.println("Today's temperature:"+ mTemperature);System.out.println("Today's pressure:"+ mPressure);System.out.println("Today's humidity:"+ mHumidity);}}

ForecastConditions.java：

public class ForecastConditions implements Observer {private float mTemperature;private float mPressure;private float mHumidity;@Overridepublic void update(float temperature, float pressure, float humidity) {mTemperature = temperature;mPressure = pressure;mHumidity = humidity;display();}public void display(){System.out.println("明天温度:"+ (mTemperature+Math.random()));System.out.println("明天气压:"+ (mPressure+Math.random()));System.out.println("明天湿度:"+ (mHumidity+Math.random()));}}

这样一来，就用装饰者模式重新实现了一下这个气象播报例子，写个类（WeatherTest2.java）测试一下：

public class WeatherTest2 {public static void main(String[] args) {CurrentConditions mCurrentConditions;ForecastConditions mForecastConditions;WeatherData mWeatherData;mWeatherData = new WeatherData();mCurrentConditions = new CurrentConditions();mForecastConditions = new ForecastConditions();mWeatherData.registerObserver(mCurrentConditions);mWeatherData.registerObserver(mForecastConditions);mWeatherData.setData(30, 150, 40);}}

运行结果：

Today's temperature:30.0Today's pressure:150.0Today's humidity:40.0明天温度:30.641740621608005明天气压:150.4129630982695明天湿度:40.283201107445485

改动一下WeatherTest2.java再测试一下：

public class WeatherTest2 {public static void main(String[] args) {CurrentConditions mCurrentConditions;ForecastConditions mForecastConditions;WeatherData mWeatherData;mWeatherData = new WeatherData();mCurrentConditions = new CurrentConditions();mForecastConditions = new ForecastConditions();mWeatherData.registerObserver(mCurrentConditions);mWeatherData.registerObserver(mForecastConditions);mWeatherData.removeObserver(mCurrentConditions);mWeatherData.setData(35, 160, 50);}}

运行结果：

明天温度:35.69158101675214明天气压:160.32248284833398明天湿度:50.3754990869303

嗯，全部都运行成功了。

vs3：Java内置观察者

其实，还有第三种方式，更简便的方式，不过有一点局限性就是了。

按我自己的理解，如果可以用Java内置观察者的话，就直接用。
如果不可以的话（比如要继承其他的第三方接口，又要继承Observable时），就自己定义吧。

Java内部有一个 Java内置观察者，使用了观察者模式。

Subject对应的是Observable，也实现了注册、移除、通知三个功能。

Observer对应的还是Observer，也还是一个接口。（之所以还是接口，是因为观察者可能会选择直接通过参数传递过来数据，也可能仅仅想被通知一下，然后自己去取数据）

唯一不同的是Observable是一个类而不是接口，所以观察者需要extends（而不是implements）Observable（java.util.Observable）类

提供数据接口的气象站 WeatherData.java，继承了Observable类:

public class WeatherData extends Observable {private float mTemperature;private float mPressure;private float mHumidity;public float getTemperature() {return mTemperature;}public float getPressure() {return mPressure;}public float getHumidity() {return mHumidity;}public void dataChanged(){//使通知观察者时更具灵活性（比如想实现超过0.5度的变化才通知时，就可以在超过0.5度的逻辑中写下这句代码）this.setChanged();//vs1：直接推送信息给观察者this.notifyObservers(new Data(getTemperature(), getPressure(), getHumidity()));//vs2：仅仅通知观察者，需要什么让观察者自己去取}public void setData(float temperature, float pressure, float humidity){mTemperature = temperature;mPressure = pressure;mHumidity = humidity;dataChanged();}public class Data {public float mTemperature;public float mPressure;public float mHumidity;public Data(float temperature, float pressure, float humidity){mTemperature = temperature;mPressure = pressure;mHumidity = humidity;}}}

很明显接收气象数据的另两个公告牌就要实现Observer接口了：

CurrentConditions.java:

public class CurrentConditions implements Observer {private float mTemperature;private float mPressure;private float mHumidity;@Overridepublic void update(Observable o, Object arg) {//Object是notifyObservers()方法传递过来的参数，传过来时事Data类，所以现在也要转换成Data类this.mTemperature = ((Data)(arg)).mTemperature;this.mPressure = ((Data)(arg)).mPressure;this.mHumidity = ((Data)(arg)).mHumidity;display();}public void display(){System.out.println("Today's temperature:"+ mTemperature);System.out.println("Today's pressure:"+ mPressure);System.out.println("Today's humidity:"+ mHumidity);}}

ForecastConditions.java：

public class ForecastConditions implements Observer {private float mTemperature;private float mPressure;private float mHumidity;@Overridepublic void update(Observable o, Object arg) {//Object是notifyObservers()方法传递过来的参数，传过来时事Data类，所以现在也要转换成Data类this.mTemperature = ((Data)(arg)).mTemperature;this.mPressure = ((Data)(arg)).mPressure;this.mHumidity = ((Data)(arg)).mHumidity;display();}public void display(){System.out.println("Tomorrow's temperature:"+ (mTemperature+1));System.out.println("Tomorrow's pressure:"+ (mPressure+1));System.out.println("Tomorrow's humidity:"+ (mHumidity+1));}}

没错，这样就写完了，写个类（WeatherTest3.java）测试一下吧：

public class WeatherTest3 {public static void main(String[] args) {CurrentConditions mCurrentConditions;ForecastConditions mForecastConditions;WeatherData mWeatherData;mWeatherData = new WeatherData();mCurrentConditions = new CurrentConditions();mForecastConditions = new ForecastConditions();mWeatherData.addObserver(mCurrentConditions);mWeatherData.addObserver(mForecastConditions);mWeatherData.setData(30, 150, 40);mWeatherData.deleteObserver(mCurrentConditions);mWeatherData.setData(35, 150, 60);}}

运行结果：

Tomorrow's temperature:31.0Tomorrow's pressure:151.0Tomorrow's humidity:41.0Today's temperature:30.0Today's pressure:150.0Today's humidity:40.0Tomorrow's temperature:36.0Tomorrow's pressure:151.0Tomorrow's humidity:61.0

咦，这里发现一个问题，我先加了一个CurrentConditions类的Observer，再加了一个ForecastConditions类的Observer，为什么先输出的是Tomorrow的数据（ForecastConditions类的）呢？

原来自己定义Subject、Observer的话，是先注册先通知，先进先出原则
Java内置观察者刚好相反，是先注册后通知，先进后出原则。

所以这里才会反过来显示。

气象站Subject有新数据过来了，调用顺序：dataChange()-->notifyObservers()-->update() [update observers中相关数据]
公告牌Observer

嗯，至此，例子就全部整理完了。

下面简单说下观察者模式的特点：

特点么，

低耦合：
对象之间。
只需要知道要调用的对方的那个函数即可，其他诸如怎么实现的，数据怎么传过来的都不用管。
打个比方：Subject挂了，Observer照样运行，只不过可能显示不了信息了而已。


高内聚：
对象内部。