• 术语

  Observer：观察者

  Subject：主题

• 角色

• Observer 观察者 百度 新浪 谷歌 Subject 气象局 WeatherData

UML类图

• 案例

  需求：气象站数据更新推送问题

  • 出现前

  package com.javaxl.design.observer.before;
  ​
  /**
   * @author 小李飞刀
   * @site www.javaxl.com
   * @company
   * @create  2020-02-26 18:08
   */
  public class WeatherData {
      double temperature;
      double humidity;
  ​
      public WeatherData(double temperature, double humidity) {
          this.temperature = temperature;
          this.humidity = humidity;
      }
  ​
      public void getWeatherInfo() {
          System.out.println("当前温度：" + temperature + ",当前湿度：" + humidity);
      }
  ​
      public void change(double temperature, double humidity) {
          this.temperature = temperature;
          this.humidity = humidity;
      }
  }
  ​
  ​
  ​
  public class Baidu {
      private WeatherData weatherData;
  ​
      public Baidu(WeatherData weatherData) {
          this.weatherData = weatherData;
      }
  ​
      public void getWeatherInfo() {
          System.out.print("百度网站温馨提示===>");
          weatherData.getWeatherInfo();
      }
  }
  ​
  class Sina {
      private WeatherData weatherData;
  ​
      public Sina(WeatherData weatherData) {
          this.weatherData = weatherData;
      }
  ​
      public void getWeatherInfo() {
          System.out.print("新浪网站温馨提示===>");
          weatherData.getWeatherInfo();
      }
  }
  ​
  ​
  public class Client {
      public static void main(String[] args) {
          WeatherData weatherData = new WeatherData(30,20);
          Baidu baidu = new Baidu(weatherData);
          Sina sina = new Sina(weatherData);
          baidu.getWeatherInfo();
          sina.getWeatherInfo();
          weatherData.change(10,10);
          baidu.getWeatherInfo();
          sina.getWeatherInfo();
      }
  }

  由第三方（百度、新浪）主动获取最新天气信息，这种方案需要每个第三方主动定时获取最新天气数据，涉及多个第三方；

  • 出现后

package com.javaxl.design.observer.after;
​
import java.util.ArrayList;
import java.util.List;
​
/**
 * @author 小李飞刀
 * @site www.javaxl.com
 * @company
 * @create  2020-02-26 19:14
 */
public interface Subject {
    void addObserver(Observer observer);
    void removeObserver(Observer observer);
    void notifyObservers();
}
​
class WeatherData implements Subject{
    double temperature;
    double humidity;
    List<Observer> Observers = new ArrayList<>();
​
    public WeatherData(double temperature, double humidity) {
        this.temperature = temperature;
        this.humidity = humidity;
    }
​
    public void update(double temperature, double humidity) {
        this.temperature = temperature;
        this.humidity = humidity;
//        气象局数据一改变，马上通知接入的第三方/观察者
        notifyObservers();
    }
​
    @Override
    public void addObserver(Observer observer) {
        Observers.add(observer);
        observer.update(this.temperature,this.humidity);
    }
​
    @Override
    public void removeObserver(Observer observer) {
        Observers.remove(observer);
    }
​
    @Override
    public void notifyObservers() {
        for (Observer observer : Observers) {
            observer.update(this.temperature,this.humidity);
        }
    }
}
​
​
public interface Observer {
    void display();
    void update(double temperature, double humidity);
}
​
class Baidu implements Observer{
    double temperature;
    double humidity;
​
    @Override
    public void display() {
        System.out.println("百度温馨提示：当前温度：" + temperature + ",当前湿度：" + humidity);
    }
​
    @Override
    public void update(double temperature, double humidity) {
        this.temperature = temperature;
        this.humidity = humidity;
        this.display();
    }
}
​
class Sina implements Observer{
    double temperature;
    double humidity;
​
    @Override
    public void display() {
        System.out.println("新浪温馨提示：当前温度：" + temperature + ",当前湿度：" + humidity);
    }
​
    @Override
    public void update(double temperature, double humidity) {
        this.temperature = temperature;
        this.humidity = humidity;
        this.display();
    }
}
​
public class Client {
    public static void main(String[] args) {
        WeatherData weatherData = new WeatherData(30, 20);
        Baidu baidu = new Baidu();
        Sina sina = new Sina();
        weatherData.addObserver(baidu);
        weatherData.addObserver(sina);
        weatherData.update(10, 10);
        weatherData.removeObserver(baidu);
        weatherData.update(12, 12);
    }
}

由气象局主动通知第三方，天气数据发生了改变；并且，第三方的接入可以控制（增加、删除、通知）；

• 注意事项和细节

  集合的方式来管理用户(Observer)，包括注册，移除和通知

• 应用

  JDK源码中Observable类

over......