观察者模式小结-CFANZ编程社区

最近在阅读经典的著作<<head first design pattern>>中文版,觉得老外的著作真是太牛了,十分生动.先是复习了观察者模式,下面用书中
的例子小结之.
观察者模式的要点在于,有主题和观察者,主题是"一"方,观察者有很多个,是"多"方,当主题一方有变化时,会通知观察者.
书中的例子是有一个weatherdata对象,有温度,湿度,气压等参数,其中要通知三个不同的观察者,三个观察者要以不同的方式去展示这些数据.

首先是主题接口(subject.java)

package headfirst.observer.weather;
public interface Subject {
  public void registerObserver(Observer o);
  public void removeObserver(Observer o);
  public void notifyObservers();
 }

这里实现了注册,移除,和通知各观察者的接口.
观察者接口:


package headfirst.observer.weather;
public interface Observer {
  public void update(float temp, float humidity, float pressure);
 }

 观察者的显示方式接口,因为各观察者会以不同的方式实现该接口package headfirst.observer.weather;
public interface DisplayElement {
  public void display();
 }

 在weatherdata中,实现subject接口package headfirst.observer.weather;
import java.util.*;
public class WeatherData implements Subject {
  private ArrayList observers;
  private float temperature;
  private float humidity;
  private float pressure;
  
  public WeatherData() {
   observers = new ArrayList();
  }
  
  public void registerObserver(Observer o) {
   observers.add(o);
  }
  
  public void removeObserver(Observer o) {
   int i = observers.indexOf(o);
   if (i >= 0) {
    observers.remove(i);
   }
  }
  

 //通知每个观察者
  public void notifyObservers() {
   for (int i = 0; i < observers.size(); i++) {
    Observer observer = (Observer)observers.get(i);
    observer.update(temperature, humidity, pressure);
   }
  }
  
  public void measurementsChanged() {
   notifyObservers();
  }
  
  public void setMeasurements(float temperature, float humidity, float pressure) {
   this.temperature = temperature;

this.humidity = humidity;
  this.pressure = pressure;
  measurementsChanged();
}

// other WeatherData methods here

public float getTemperature() {
  return temperature;
}

public float getHumidity() {
  return humidity;
}

public float getPressure() {
  return pressure;
}
}

建立其中一个观察者

package headfirst.observer.weather;
  
 public class CurrentConditionsDisplay implements Observer, DisplayElement {
  private float temperature;
  private float humidity;
  private Subject weatherData;
  

 //注册观察者
  public CurrentConditionsDisplay(Subject weatherData) {
   this.weatherData = weatherData;
   weatherData.registerObserver(this);
  }
  
  public void update(float temperature, float humidity, float pressure) {
   this.temperature = temperature;
   this.humidity = humidity;
   display();
  }
  
  public void display() {
   System.out.println("Current conditions: " + temperature 
    + "F degrees and " + humidity + "% humidity");
  }
 }

客户端:

package headfirst.observer.weather;
import java.util.*;
public class WeatherStation {
 public static void main(String[] args) {
   WeatherData weatherData = new WeatherData();
  
   CurrentConditionsDisplay currentDisplay = 
    new CurrentConditionsDisplay(weatherData);
   StatisticsDisplay statisticsDisplay = new StatisticsDisplay(weatherData);
   ForecastDisplay forecastDisplay = new ForecastDisplay(weatherData);  weatherData.setMeasurements(80, 65, 30.4f);
   weatherData.setMeasurements(82, 70, 29.2f);
   weatherData.setMeasurements(78, 90, 29.2f);
  }
 }