观察者模式定义: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,它的所有依赖者都会收到通知并自动更新。
简单定义:让对象能够在状态改变时被通知。
OO原则:
为了交互对象之间的松耦合设计而努力。
以书中的WeatherStation为例:
Subject.java(定义接口)
package headfirst.observer.weather; public interface Subject { public void registerObserver(Observer o); public void removeObserver(Observer o); public void notifyObservers(); }
Observer.java
package headfirst.observer.weather; public interface Observer { public void update(float temp, float humidity, float pressure); }
DisplayElement.java
package headfirst.observer.weather; public interface DisplayElement { public void display(); }
WeatherData.java(天气数据类,布告板在此进行注册和删除观察者)
package headfirst.observer.weather; import java.util.*; public class WeatherData implements Subject { private ArrayList<Observer> observers; private float temperature; private float humidity; private float pressure; public WeatherData() { observers = new ArrayList<Observer>(); } public void registerObserver(Observer o) { observers.add(o); } public void removeObserver(Observer o) { int i = observers.indexOf(o); if (i >= 0) { observers.remove(i); } } 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; } }
CurrentConditionsDisplay.java(布告板类)
package headfirst.observer.weather; public class CurrentConditionsDisplay implements Observer, DisplayElement { private float temperature; private float humidity; public CurrentConditionsDisplay(Subject 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"); } }StatisticsDisplay.java
package headfirst.observer.weather; public class StatisticsDisplay implements Observer, DisplayElement { private float maxTemp = 0.0f; private float minTemp = 200; private float tempSum= 0.0f; private int numReadings; public StatisticsDisplay(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); } }
ForecastDisplay.java
package headfirst.observer.weather; public class ForecastDisplay implements Observer, DisplayElement { private float currentPressure = 29.92f; private float lastPressure; public ForecastDisplay(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"); } } }
WeatherStation.java
package headfirst.observer.weather; public class WeatherStation { public static void main(String[] args) { WeatherData weatherData = new WeatherData(); new CurrentConditionsDisplay(weatherData); new StatisticsDisplay(weatherData); new ForecastDisplay(weatherData); weatherData.setMeasurements(80, 65, 30.4f); weatherData.setMeasurements(82, 70, 29.2f); weatherData.setMeasurements(78, 90, 29.2f); } }
Current conditions: 80.0F degrees and 65.0% humidity Avg/Max/Min temperature = 80.0/80.0/80.0 Forecast: Improving weather on the way! Current conditions: 82.0F degrees and 70.0% humidity Avg/Max/Min temperature = 81.0/82.0/80.0 Forecast: Watch out for cooler, rainy weather Current conditions: 78.0F degrees and 90.0% humidity Avg/Max/Min temperature = 80.0/82.0/78.0 Forecast: More of the same
原文:http://blog.csdn.net/viviwen123/article/details/26481171