16. The observer pattern

Observer Pattern

First, application examples

1) Weather Station can be measured daily temperature, humidity, pressure, etc. by notice published in the form of out (such as publish to your own website or third party).

2) the need to design an open API, to facilitate access to other third parties can obtain weather data.

3) provide temperature, pressure and humidity of the interface

4) When updating, to be able to real-time measurement data to a third party notification

Second, the Observer pattern

1. Basic Introduction

Many-to-one design dependency between objects, the object is dependent Subject, object dependent Observer, the Subject Observer change notification.

2. Description FIG class

3. code implementation

public class Client {
    public static void main(String[] args) {
        //创建一个WeatherData
        WeatherData weatherData = new WeatherData();
        
        //创建观察者
        CurrentConditions currentConditions = new CurrentConditions();
        BaiduSite baiduSite = new BaiduSite();
        
        //注册到weatherData
        weatherData.registerObserver(currentConditions);
        weatherData.registerObserver(baiduSite);
        
        //测试
        System.out.println("通知各个注册的观察者, 看看信息");
        weatherData.setData(10f, 100f, 30.3f);
        
        weatherData.removeObserver(currentConditions);
        //测试
        System.out.println();
        System.out.println("通知各个注册的观察者, 看看信息");
        weatherData.setData(10f, 100f, 30.3f);
    }
}

//接口, 让WeatherData 来实现
public interface Subject {
    public void registerObserver(Observer o);
    public void removeObserver(Observer o);
    public void notifyObservers();
}

/**
 * 类是核心
 * 1. 包含最新的天气情况信息 
 * 2. 含有 观察者集合，使用ArrayList管理
 * 3. 当数据有更新时，就主动的调用   ArrayList, 通知所有的（接入方）就看到最新的信息
 */
public class WeatherData implements Subject {
    private float temperatrue;
    private float pressure;
    private float humidity;
    //观察者集合
    private ArrayList<Observer> observers;
    
    //加入新的第三方

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

    public float getTemperature() {
        return temperatrue;
    }

    public float getPressure() {
        return pressure;
    }

    public float getHumidity() {
        return humidity;
    }

    public void dataChange() {
        //调用 接入方的 update
        
        notifyObservers();
    }

    //当数据有更新时，就调用 setData
    public void setData(float temperature, float pressure, float humidity) {
        this.temperatrue = temperature;
        this.pressure = pressure;
        this.humidity = humidity;
        //调用dataChange， 将最新的信息 推送给 接入方 currentConditions
        dataChange();
    }

    //注册一个观察者
    @Override
    public void registerObserver(Observer o) {
        observers.add(o);
    }

    //移除一个观察者
    @Override
    public void removeObserver(Observer o) {
        if(observers.contains(o)) {
            observers.remove(o);
        }
    }

    //遍历所有的观察者，并通知
    @Override
    public void notifyObservers() {
        for(int i = 0; i < observers.size(); i++) {
            observers.get(i).update(this.temperatrue, this.pressure, this.humidity);
        }
    }
}

//观察者接口，有观察者来实现
public interface Observer {
    public void update(float temperature, float pressure, float humidity);
}

public class CurrentConditions implements Observer {
    // 温度，气压，湿度
    private float temperature;
    private float pressure;
    private float humidity;

    // 更新 天气情况，是由 WeatherData 来调用，我使用推送模式
    public void update(float temperature, float pressure, float humidity) {
        this.temperature = temperature;
        this.pressure = pressure;
        this.humidity = humidity;
        display();
    }

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

public class BaiduSite implements Observer {
    // 温度，气压，湿度
    private float temperature;
    private float pressure;
    private float humidity;

    // 更新 天气情况，是由 WeatherData 来调用，我使用推送模式
    public void update(float temperature, float pressure, float humidity) {
        this.temperature = temperature;
        this.pressure = pressure;
        this.humidity = humidity;
        display();
    }

    // 显示
    public void display() {
        System.out.println("===百度网站====");
        System.out.println("***百度网站 气温 : " + temperature + "***");
        System.out.println("***百度网站 气压: " + pressure + "***");
        System.out.println("***百度网站 湿度: " + humidity + "***");
    }
}

Third, the advantage

1) After the observer pattern design, it will be a way to manage a collection of users, including registration, removal and notification.

2) increase the observer (can be understood as a new bulletin board), will not have to modify the core classes WeatherData, comply with the principle of ocp