Java-设计模式-观察者模式

观察者模式原理

Internet气象站项目,普通的OO设计方案有问题。

  1. Internet气象站项目:
  • 提供温度、气压和湿度的接口
  • 测量数据更新时需试试通知给第三方
  • 需要设计开放型API,便于其他第三方公司也能接入气象站获取数据
  1. WeatherData类
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在第三方获取数据的时候,datachange()函数里就要去通知相应的类,造成一个代码的重复。那么在这里的一个解决的办法就是将被通知的类抽象,在气象站类里进行注册,气象站会向在列表中的对象发放通知提醒,这样气象站就不需要重新编译停下来。


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原理

  1. 观察者模式就像定牛奶
  • 奶站 subject
  • 用户 observer
  1. Subject:登记注册、移除和通知
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  1. Observer:接收输入
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  1. 观察者模式:对象之间多对一依赖的一种设计方案,被依赖的对象为Subject,依赖的对象为Observer,Subject通知Observer变化。

用观察者重新设计的方案

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首先定义subject接口,用于注册删除通知其他观察者

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

然后定义Observer接口,用于接收更新的数据

public interface Observer {
    public void update(float mTemperatrue,float mPressure,float mHumidity);
}

定义subject的气象站实现类 用于发送数据 注册观察者
在这里用了一个ArrayList数组来保存观察者,相当于一个注册表的功能

public class WeatherDataSt implements Subject{
    
    private float mTemperatrue;
    private float mPressure;
    private float mHumidity;
    private ArrayList mObservers;
    public WeatherDataSt()
    {
        mObservers=new ArrayList();
    }
    
    public float getTemperature()
    {
        return mTemperatrue;
        
    }
    
    public float getPressure()
    {
        return mPressure;
        
    }
    
    public float getHumidity()
    {
        return mHumidity;
        
    }
    public void dataChange()
    {
        notifyObservers();
    }

    

    
    
    
    public void setData(float mTemperatrue,float mPressure,float mHumidity)
    {
        this.mTemperatrue=mTemperatrue;
        this.mPressure=mPressure;
        this.mHumidity=mHumidity;
        dataChange();
    }

    @Override
    public void registerObserver(Observer o) {
        // TODO Auto-generated method stub
        mObservers.add(o);
    }

    @Override
    public void removeObserver(Observer o) {
        // TODO Auto-generated method stub
        if(mObservers.contains(o))
        {mObservers.remove(o);}
    }

    @Override
    public void notifyObservers() {
        // TODO Auto-generated method stub
        for(int i=0,len=mObservers.size();i

定义两个观察者实现类,update用于接收数据,display用于打印

public class ForcastConditions implements Observer{
    private float mTemperatrue;
    private float mPressure;
    private float mHumidity;
    @Override
    public void update(float mTemperatrue, float mPressure, float mHumidity) {
        // TODO Auto-generated method stub
        this.mTemperatrue=mTemperatrue;
        this.mPressure=mPressure;
        this.mHumidity=mHumidity;
        
        display();
    }
    public void display()
    {
        System.out.println("**明天温度:"+(mTemperatrue+Math.random())+"**");
        System.out.println("**明天气压:"+(mPressure+10*Math.random())+"**");
        System.out.println("**明天湿度:"+(mHumidity+Math.random())+"**");
    }
}

public class CurrentConditions implements Observer {

    private float mTemperatrue;
    private float mPressure;
    private float mHumidity;

    @Override
    public void update(float mTemperatrue, float mPressure, float mHumidity) {
        // TODO Auto-generated method stub
        this.mHumidity = mHumidity;
        this.mPressure = mPressure;
        this.mTemperatrue = mTemperatrue;
        display();
    }
    public void display() {
        System.out.println("***Today mTemperatrue:" + mTemperatrue + "***");
        System.out.println("***Today mPressure:" + mPressure + "***");
        System.out.println("***Today mHumidity:" + mHumidity + "***");
    }
}

下民是一个测试类

public class InternetWeather {

    public static void main(String[] args) {
        
        CurrentConditions mCurrentConditions;
        ForcastConditions mForcastConditions;
        WeatherDataSt mWeatherDataSt;
        
        mWeatherDataSt=new WeatherDataSt();
        mCurrentConditions=new CurrentConditions();
        mForcastConditions=new ForcastConditions();
        
        mWeatherDataSt.registerObserver(mCurrentConditions);
        mWeatherDataSt.registerObserver(mForcastConditions);
        
        mWeatherDataSt.setData(30, 150, 40);
        mWeatherDataSt.removeObserver(mCurrentConditions);
        mWeatherDataSt.setData(40, 250, 50);
    }
}

其实观察者模式的想法就是基于插销原理,实现一个主类一直运行,然后获取数据的接口类一个可插拔解耦。

Java中的观察者模式

Java中其实也有已经实现的观察者的类,那么subject类集成Observable类,变成一个主站,其他两个接收数据的子类实现Observer接口,成为一个观察者,主站可以调用nofityObservers方法来通知观察者。那么观察者这里接收数据,java给了两个选择,一种是主站推数据,一种是观察者拉数据,选择自己需要的数据进行获取。
下面贴出示例代码 不做解释

public class WeatherData extends Observable{
    private float mTemperatrue;
    private float mPressure;
    private float mHumidity;
    
    public float getTemperature()
    {
        return mTemperatrue;
        
    }
    
    public float getPressure()
    {
        return mPressure;
        
    }
    
    public float getHumidity()
    {
        return mHumidity;
        
    }
    
    
    public void dataChange()
    {
        this.setChanged();
        this.notifyObservers(new Data(getTemperature(),getPressure(),getHumidity()));
        
    }
    
    
    public void setData(float mTemperatrue,float mPressure,float mHumidity)
    {
        this.mTemperatrue=mTemperatrue;
        this.mPressure=mPressure;
        this.mHumidity=mHumidity;
        dataChange();
    }
    
    public class Data
    {
        public float mTemperatrue;
        public float mPressure;
        public float mHumidity;
        public Data(float mTemperatrue,float mPressure,float mHumidity)
        {
            this.mTemperatrue=mTemperatrue;
            this.mPressure=mPressure;
            this.mHumidity=mHumidity;
        }
    }
    
}

public class ForcastConditions implements Observer  {
    
    private float mTemperatrue;
    private float mPressure;
    private float mHumidity;
    @Override
    public void update(Observable arg0, Object arg1) {
        // TODO Auto-generated method stub
        this.mTemperatrue=((Data)(arg1)).mTemperatrue;
        this.mPressure=((Data)(arg1)).mPressure;
        this.mHumidity=((Data)(arg1)).mHumidity;
        display();
    }
    
    public void display()
    {
        System.out.println("***Tomorrow mTemperatrue:" +(mTemperatrue+1)+"***");
        System.out.println("***Tomorrow mPressure:" +(mPressure+1)+"***");
        System.out.println("***Tomorrow mHumidity:" +(mHumidity+1)+"***");
    }
    
    
}
public class ForcastConditions implements Observer  {
    
    private float mTemperatrue;
    private float mPressure;
    private float mHumidity;
    @Override
    public void update(Observable arg0, Object arg1) {
        // TODO Auto-generated method stub
        this.mTemperatrue=((Data)(arg1)).mTemperatrue;
        this.mPressure=((Data)(arg1)).mPressure;
        this.mHumidity=((Data)(arg1)).mHumidity;
        display();
    }
    
    public void display()
    {
        System.out.println("***Tomorrow mTemperatrue:" +(mTemperatrue+1)+"***");
        System.out.println("***Tomorrow mPressure:" +(mPressure+1)+"***");
        System.out.println("***Tomorrow mHumidity:" +(mHumidity+1)+"***");
    }
}
public class CurrentConditions implements Observer {
    
    private float mTemperatrue;
    private float mPressure;
    private float mHumidity;
    @Override
    public void update(Observable arg0, Object arg1) {
        // TODO Auto-generated method stub
        this.mTemperatrue=((Data)(arg1)).mTemperatrue;
        this.mPressure=((Data)(arg1)).mPressure;
        this.mHumidity=((Data)(arg1)).mHumidity;
        display();
    }
    
    public void display()
    {
        System.out.println("***Today mTemperatrue:" +mTemperatrue+"***");
        System.out.println("***Today mPressure:" +mPressure+"***");
        System.out.println("***Today mHumidity:" +mHumidity+"***");
    }   
}
public class InternetWeather {
    public static void main(String[] args) {
        CurrentConditions mCurrentConditions;
        ForcastConditions mForcastConditions;
        WeatherData mWeatherData;
         
        mCurrentConditions=new CurrentConditions();
        mForcastConditions=new ForcastConditions();
        mWeatherData=new WeatherData();
        
        mWeatherData.addObserver(mCurrentConditions);
        mWeatherData.addObserver(mForcastConditions);
        mWeatherData.setData(30, 150, 40);
        
        mWeatherData.deleteObserver(mCurrentConditions);
        mWeatherData.setData(35, 150, 60);
        
    }
}

观察者模式关键点

  • 松耦合 高内聚 隔离影响
  • java内置观察者使用注意点。Observable是一个类,只能继承,而java又是单继承的,所以要注意这一点。

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