Observer mode

1、 demand

2、 How observer mode works + The class diagram design

When subject When the data in the , All of them will be informed obervers;
It's actually subject One was maintained oberver Of list,observer Can dynamically join subject Of list, Or remove from it

3、 The observer pattern implements the above requirements

/** * @author houChen * @date 2022/6/19 21:51 * @Description: */
public interface Subject {
    

    public void registerObserver(Observer o);
    public void removeObserver(Observer o);
    public void notifyObservers();

}

/** * @Description: * 1、 Contains the latest weather information  * 2、  Have a collection of observers , Use ArrayList  Conduct management  * 3、 When the data is updated , Notify all access parties of the latest information  */
public class WeatherData implements Subject {
    
    private float temperatrue;
    private float pressure;
    private float humidity;
    // Observers gather 
    private ArrayList<Observer> observers;

    public WeatherData() {
    
        this.observers = new ArrayList<Observer>();
    }

    public float getTemperature() {
    
        return temperatrue;
    }

    public float getPressure() {
    
        return pressure;
    }

    public float getHumidity() {
    
        return humidity;
    }

    public void setData(float temperature, float pressure, float humidity) {
    
        this.temperatrue = temperature;
        this.pressure = pressure;
        this.humidity = humidity;
        this.notifyObservers();
    }

    // Register an observer 
    @Override
    public void registerObserver(Observer o) {
    
        this.observers.add(o);
    }

    // Remove an observer 
    @Override
    public void removeObserver(Observer o) {
    
        if (this.observers.contains(o)) {
    
            this.observers.remove(o);
        }
    }

    // Traverse all observers , And give notice 
    @Override
    public void notifyObservers() {
    
        for (int i = 0; i < this.observers.size(); i++) {
    
            this.observers.get(i).update(this.temperatrue, this.pressure, this.humidity);
        }
    }
}

/** * @author houChen * @date 2022/6/19 21:55 * @Description:  Observer interface  */
public interface Observer {
    

    public void update(float temperature,float pressure,float humidity);

}

/** * @author houChen * @date 2022/6/19 21:58 * @Description:  Concrete observer  */
public class CurrentCondition implements Observer {
    

    //  temperature , pressure , humidity 
    private float temperature;
    private float pressure;
    private float humidity;

    // to update   Weather conditions , By  WeatherData  To call , I use push mode 
    public void update(float temperature, float pressure, float humidity) {
    
        this.temperature = temperature;
        this.pressure = pressure;
        this.humidity = humidity;
        display();
    }

    // Show 
    public void display() {
    
        System.out.println("***Today mTemperature: " + temperature + "***");
        System.out.println("***Today mPressure: " + pressure + "***");
        System.out.println("***Today mHumidity: " + humidity + "***");
    }
}

/** * @author houChen * @date 2022/6/19 22:14 * @Description: */
public class Client {
    
    public static void main(String[] args) {
    
        WeatherData weatherData = new WeatherData();
        // Create an observer 
        CurrentCondition currentCondition = new CurrentCondition();
        // Registered observer 
        weatherData.registerObserver(currentCondition);
        weatherData.setData(10, 100, 30.3f);
    }
}

4、 Observer mode benefits

• After the observer pattern is designed , Will manage users in a collective way （Observer）, Including registration removal and notification
• such , When we add observers , There is no need to modify the core class WeatherData,

JDK Source code ： Observale Class uses the observer pattern