甲方提供了一个气象站的接口,气象站上面装有:温度感应装置、湿度感应装置、气压感应装置。编程
如今咱们是乙方,须要设计一个 WeatherData 对象,从气象站获取数据,而且利用这些数据,更新三个布告板(当前情况、气象统计、天气预报)。ide
经过简单地分析,咱们能够很快肯定一套解决方案:this
WeatherData 提供一个 measurementsChanged() 方法,当这个方法被调用了,去实时获取气象站的数据,而后更新到三个布告板上。spa
public class BadWeatherData { @Getter private float temperature; @Getter private float humidity; @Getter private float pressure; private CurrentConditionsDisplay currentConditionDisplay; private StatisticsDisplay statisticsDisplay; private ForecastDisplay forecastDisplay; public BadWeatherData() { // some initialized function for displays } // We don't care how it be called, we only know is when it is called, we will update displays. public void measurementsChanged() { // We don't care how it gets data float temperature = getTemperature(); float humidity = getHumidity(); float pressure = getPressure(); currentConditionDisplay.update(temperature, humidity, pressure); statisticsDisplay.update(temperature, humidity, pressure); forecastDisplay.update(temperature, humidity, pressure); } }
显然,这是一个扩展性不好的解决方案,它有以下问题:设计
定义了对象之间的一对多依赖,这样一来,当一个对象改变状态时,它的全部依赖者都会收到通知并自动更新。code
观察者模式 = 主题+观察者(Subject + Observer)。server
public interface Subject { void registerObserver(Observer observer); void removeObserver(Observer observer); void notifyObservers(); }
public interface Observer { void update(float temperature, float humidity, float pressure); }
public interface DisplayElement { void display(); }
@Data public final class WeatherData implements Subject { private float temperature; private float humidity; private float pressure; private List<Observer> observers = new ArrayList<>(); public void measurementsChanged() { notifyObservers(); } @Override public void registerObserver(Observer observer) { observers.add(observer); } @Override public void removeObserver(Observer observer) { observers.remove(observer); } @Override public void notifyObservers() { observers.forEach(observer -> observer.update(temperature, humidity, pressure)); } }
public class CurrentConditionsDisplay implements Observer, DisplayElement { private float temperature; private float humidity; private Subject weatherData; public CurrentConditionsDisplay(Subject weatherData) { this.weatherData = weatherData; weatherData.registerObserver(this); } @Override public void display() { System.out.println("Current conditions: " + temperature + "F degrees and " + humidity + "% humidity"); } @Override public void update(float temperature, float humidity, float pressure) { this.temperature = temperature; this.humidity = humidity; display(); } }
public class ForecastDisplay implements Observer, DisplayElement { private float temperature; private float humidity; private float pressure; private Subject weatherData; public ForecastDisplay(Subject weatherData) { this.weatherData = weatherData; weatherData.registerObserver(this); } @Override public void display() { System.out.println("Will show forecast related data"); } @Override public void update(float temperature, float humidity, float pressure) { this.temperature = temperature; this.humidity = humidity; this.pressure = pressure; display(); } }
public class StatisticsDisplay implements Observer, DisplayElement { private float temperature; private Subject weatherData; public StatisticsDisplay(Subject weatherData) { this.weatherData = weatherData; weatherData.registerObserver(this); } @Override public void display() { System.out.println("Statistics will show average temperature"); } @Override public void update(float temperature, float humidity, float pressure) { this.temperature = temperature; display(); } }