观察者模式（发布订阅模式）

推送模式：

• CurrentCondition

  • /**
     * 显示当前天气情况（是调用天气数据的本体
     */
    public class CurrentConditions {
        private float temperature;//温度
        private float pressure;//气压
        private float humidity;//湿度
    
        public void update(float temperature, float pressure, float humidity) {
            this.temperature = temperature;
            this.pressure = pressure;
            this.humidity = humidity;
            display();
        }
    
        public void display() {
            System.out.println("***Today mTemperature: " + temperature + "***");
            System.out.println("***Today mPressure: " + pressure + "***");
            System.out.println("***Today mHumidity: " + humidity + "***");
        }
    }
    

• WeatherData

  • public class WeatherData {
        private float temperatrue;
        private float pressure;
        private float humidity;
        private CurrentConditions currentConditions;
    
        public WeatherData(CurrentConditions currentConditions) {
            this.currentConditions = currentConditions;
        }
    
        public float getTemperature() {
            return temperatrue;
        }
    
        public float getPressure() {
            return pressure;
        }
    
        public float getHumidity() {
            return humidity;
        }
    
        public void dataChange() {
            currentConditions.update(getTemperature(), getPressure(), getHumidity());
        }
    
        public void setData(float temperature, float pressure, float humidity) {
            this.temperatrue = temperature;
            this.pressure = pressure;
            this.humidity = humidity;
            dataChange();
        }
    }
    
    

• InternetWeather

  • public class InternetWeather {
        public static void main(String[] args) {
            CurrentConditions currentConditions = new CurrentConditions();
            WeatherData weatherData = new WeatherData(currentConditions);
            weatherData.setData(30, 150, 40);
        }
    }
    

InternetWeather这个类的作用就是给WeatherData赋值，然后WeatherData推送给各个订阅者。

但是坏处是显而易见的。每当要加入一个第三方需要用到WeatherData的数据时，就要在WeatherData的dataChange方法里添加第三方，这违反了开闭原则。

运用观察者设计模式改进

• Subject（作为发布者的接口）

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

• Observer（作为观察者的接口）

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

• Tencent（观察者的具体实现类）

  • /**
     * 显示当前天气情况（是调用天气数据的本体
     */
    public class Tencent implements Observer {
        private float temperature;//温度
        private float pressure;//气压
        private float humidity;//湿度
    
        public void update(float temperature, float pressure, float humidity) {
            this.temperature = temperature;
            this.pressure = pressure;
            this.humidity = humidity;
            display();
        }
    
        public void display() {
            System.out.println("***Today mTemperature: " + temperature + "***");
            System.out.println("***Today mPressure: " + pressure + "***");
            System.out.println("***Today mHumidity: " + humidity + "***");
        }
    }
    

• Ali（观察者的具体实现类）

  • public class Ali implements Observer{
        @Override
        public void update(float temperature, float pressure, float humidity) {
            System.out.println(temperature+'\t'+pressure+'\t'+humidity);
        }
    }
    

• WeatherData（作为发布者的具体实现类）

  • public class WeatherData implements Subject{
        private float temperatrue;
        private float pressure;
        private float humidity;
    
        private ArrayList<Observer> observers;
    
        public WeatherData() {
            observers = new ArrayList<Observer>();
        }
    
        public float getTemperature() {
            return temperatrue;
        }
    
        public float getPressure() {
            return pressure;
        }
    
        public float getHumidity() {
            return humidity;
        }
    
        public void dataChange() {
            notifyObservers();
    
        }
    
        public void setData(float temperature, float pressure, float humidity) {
            this.temperatrue = temperature;
            this.pressure = pressure;
            this.humidity = humidity;
            dataChange();
        }
    
        @Override
        public void registerObserver(Observer o) {
            observers.add(o);
        }
    
        @Override
        public void removeObserver(Observer o) {
            if (observers.contains(o)){
                observers.remove(o);
            }
        }
    
        @Override
        public void notifyObservers() {
            for (int i = 0;i < observers.size();i++){
                observers.get(i).update(this.temperatrue,this.pressure,this.humidity);
            }
        }
    }
    

• Client

  • public class Client {
        public static void main(String[] args) {
            WeatherData weatherData = new WeatherData();
            Tencent tencent = new Tencent();
            weatherData.registerObserver(tencent);
            Ali ali = new Ali();
            weatherData.registerObserver(ali);
            weatherData.setData(12f,4f,5f);
        }
    }
    
    

运行Client，得到结果如下

***Today mTemperature: 12.0***
***Today mPressure: 4.0***
***Today mHumidity: 5.0***
 12.0	4.0	5.0

观察者模式的要害在于，Tencent类和Ali类共同实现了一个Observer类，这意味着它们两者成为了同样的类。这样就会产生共同的接口，于是发布者（Subject类）就可以通过调用Observer类的抽象方法来为每一个具体实例服务了。

还有一个要点是ArrayList，多个观察者的注册，需要用到ArrayList进行存储。