Java设计模式(二)：观察者设计模式

梦里梦外; 2022-04-03 19:24 206阅读 0赞

# 1. 应用场景 #

某个实例的变化将影响其他多个对象。 观察者模式多用于实现订阅功能的场景，例如微博的订阅，当我们订阅了某个人的微博账号，当这个人发布了新的消息，就会通知我们。

# 2.概念 #

定义对象之间的一对多依赖，当一个对象状态改变时，它的所有依赖都会收到通知并且自动更新状态。主题（Subject）是被观察的对象，而其所有依赖者（Observer）称为观察者。

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzIxMTI1MTgz_size_16_color_FFFFFF_t_70]

# 3. Class Diagram #

主题（Subject）具有注册和移除观察者、并通知所有观察者的功能，主题是通过维护一张观察者列表来实现这些操作的。

观察者（Observer）的注册功能需要调用主题的 registerObserver() 方法。

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzIxMTI1MTgz_size_16_color_FFFFFF_t_70 1]

# 4. Implementation #

天气数据布告板会在天气信息发生改变时更新其内容，布告板有多个，并且在将来会继续增加。

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzIxMTI1MTgz_size_16_color_FFFFFF_t_70 2]

public interface Subject { 
        void registerObserver(Observer o);
    
        void removeObserver(Observer o);
    
        void notifyObserver();
    }
    
    public class WeatherData implements Subject { 
        private List<Observer> observers;
        private float temperature;
        private float humidity;
        private float pressure;
    
        public WeatherData() { 
            observers = new ArrayList<>();
        }
    
        public void setMeasurements(float temperature, float humidity, float pressure) { 
            this.temperature = temperature;
            this.humidity = humidity;
            this.pressure = pressure;
            notifyObserver();
        }
    
        @Override
        public void registerObserver(Observer o) { 
            observers.add(o);
        }
    
        @Override
        public void removeObserver(Observer o) { 
            int i = observers.indexOf(o);
            if (i >= 0) { 
                observers.remove(i);
            }
        }
    
        @Override
        public void notifyObserver() { 
            for (Observer o : observers) { 
                o.update(temperature, humidity, pressure);
            }
        }
    }
    
    public interface Observer { 
        void update(float temp, float humidity, float pressure);
    }
    
    public interface DisplayElement { 
        public void display();
    }
    
    public class StatisticsDisplay implements Observer, DisplayElement { 
    	private float maxTemp = 0.0f;
    	private float minTemp = 200;
    	private float tempSum= 0.0f;
    	private int numReadings;
    	private WeatherData weatherData;
    
    	public StatisticsDisplay(WeatherData weatherData) { 
    		this.weatherData = weatherData;
    		weatherData.registerObserver(this);
    	}
    
    	public void update(float temp, float humidity, float pressure) { 
    		tempSum += temp;
    		numReadings++;
    
    		if (temp > maxTemp) { 
    			maxTemp = temp;
    		}
     
    		if (temp < minTemp) { 
    			minTemp = temp;
    		}
    
    		display();
    	}
    
    	public void display() { 
    		System.out.println("Avg/Max/Min temperature = " + (tempSum / numReadings)
    			+ "/" + maxTemp + "/" + minTemp);
    	}
    }
    
    public class CurrentConditaionsDisplay implements Observer,DisplayElement { 
        private WeatherData weatherData;
        private float temperature;
        private float humidity;
    
        public CurrentConditaionsDisplay(WeatherData weatherData) { 
            this.weatherData = weatherData;
            weatherData.registerObserver(this);
        }
    
        @Override
        public void display() { 
            System.out.println("temperature: "+temperature+" humidity: "+ humidity);
        }
    
        @Override
        public void update(float temp, float humidity, float preesure) { 
            this.humidity=humidity;
            this.temperature=temp;
            display();
        }
    }
    
    public class ForecastDisplay implements Observer, DisplayElement { 
    	private float currentPressure = 29.92f;  
    	private float lastPressure;
    	private WeatherData weatherData;
    
    	public ForecastDisplay(WeatherData weatherData) { 
    		this.weatherData = weatherData;
    		weatherData.registerObserver(this);
    	}
    
    	public void update(float temp, float humidity, float pressure) { 
            lastPressure = currentPressure;
    		currentPressure = pressure;
    
    		display();
    	}
    
    	public void display() { 
    		System.out.print("Forecast: ");
    		if (currentPressure > lastPressure) { 
    			System.out.println("Improving weather on the way!");
    		} else if (currentPressure == lastPressure) { 
    			System.out.println("More of the same");
    		} else if (currentPressure < lastPressure) { 
    			System.out.println("Watch out for cooler, rainy weather");
    		}
    	}
    }
    
    public class WeatherStation { 
        public static void main(String[] args) { 
            WeatherData weatherData=new WeatherData();
            CurrentConditaionsDisplay conditaionsDisplay=new CurrentConditaionsDisplay(weatherData);
            StatisticsDisplay statisticsDisplay=new StatisticsDisplay(weatherData);
            ForecastDisplay forecastDisplay=new ForecastDisplay(weatherData);
            weatherData.setMeasurements(80,65,30.4f);
            weatherData.setMeasurements(82,75,35.4f);
            weatherData.setMeasurements(82,75,35.4f);
        }
    }
    
    运行结果：
    
    temperature: 80.0 humidity: 65.0
    Avg/Max/Min temperature = 80.0/80.0/80.0
    Forecast: Improving weather on the way!
    temperature: 82.0 humidity: 75.0
    Avg/Max/Min temperature = 81.0/82.0/80.0
    Forecast: Improving weather on the way!
    temperature: 82.0 humidity: 75.0
    Avg/Max/Min temperature = 81.333336/82.0/80.0
    Forecast: More of the same

\#5. JDK

*  [java.util.Observer][]
 *  [java.util.EventListener][]
 *  [javax.servlet.http.HttpSessionBindingListener][]
 *  [RxJava][]

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[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzIxMTI1MTgz_size_16_color_FFFFFF_t_70 1]: /images/20220403/ca5c1f1045184d3dba5c6b95e0b6c6f6.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzIxMTI1MTgz_size_16_color_FFFFFF_t_70 2]: /images/20220403/7b3df779918c415aac1f764a41133fdd.png
[java.util.Observer]: http://docs.oracle.com/javase/8/docs/api/java/util/Observer.html
[java.util.EventListener]: http://docs.oracle.com/javase/8/docs/api/java/util/EventListener.html
[javax.servlet.http.HttpSessionBindingListener]: http://docs.oracle.com/javaee/7/api/javax/servlet/http/HttpSessionBindingListener.html
[RxJava]: https://github.com/ReactiveX/RxJava