• 常见设计模式——观察者模式


    Intent

    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。

    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。

    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。

    public interface Subject {
        void resisterObserver(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 resisterObserver(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 class StatisticsDisplay implements Observer {
    
        public StatisticsDisplay(Subject weatherData) {
            weatherData.resisterObserver(this);
        }
    
        @Override
        public void update(float temp, float humidity, float pressure) {
            System.out.println("StatisticsDisplay.update: " + temp + " " + humidity + " " + pressure);
        }
    }
    public class CurrentConditionsDisplay implements Observer {
    
        public CurrentConditionsDisplay(Subject weatherData) {
            weatherData.resisterObserver(this);
        }
    
        @Override
        public void update(float temp, float humidity, float pressure) {
            System.out.println("CurrentConditionsDisplay.update: " + temp + " " + humidity + " " + pressure);
        }
    }
    public class WeatherStation {
        public static void main(String[] args) {
            WeatherData weatherData = new WeatherData();
            CurrentConditionsDisplay currentConditionsDisplay = new CurrentConditionsDisplay(weatherData);
            StatisticsDisplay statisticsDisplay = new StatisticsDisplay(weatherData);
    
            weatherData.setMeasurements(0, 0, 0);
            weatherData.setMeasurements(1, 1, 1);
        }
    }
    CurrentConditionsDisplay.update: 0.0 0.0 0.0
    StatisticsDisplay.update: 0.0 0.0 0.0
    CurrentConditionsDisplay.update: 1.0 1.0 1.0
    StatisticsDisplay.update: 1.0 1.0 1.0

    JDK

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  • 原文地址:https://www.cnblogs.com/codestarer/p/13635532.html
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