设计模式：观察者

现在，我们要模拟气象站观测天气，然后将天气预报给不同的对象去显示的情况。

首先，模拟新建一个气象站。其中WeatherData 看作为一个天气发布器，由WeatherStation 获取信息并传送给WeatherData。由WeatherData去通知各个对象显示。在新建发布器之后，就应该新建发布器要通知的信息的那些显示器。然后注册给WeatherData，让WeatherData知道当有新数据过来的时候通知那些对象进行相应处理。

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);
}
}

接下来是天气发布器的接口。定义和观察者相关的一些方法。

public interface Subject {
public void registerObserver(Observer o);
public void removeObserver(Observer o);
public void notifyObservers();
}

发布器的实现：每次有新值过来的时候调用measurementsChanged（）方法。

measurementsChanged（）调用notifyObservers（）方法。通知各个显示器进行处理。

import java.util.ArrayList;

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 (Observer observer : observers) {
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();
}

public float getTemperature() {
return temperature;
}

public float getHumidity() {
return humidity;
}

public float getPressure() {
return pressure;
}

}

接下来要实现气温更新（即观察者，这里的观察者定义为一个行为。）。

public interface Observer {
public void update(float temp, float humidity, float pressure);
}

接下来是显示器。

public interface DisplayElement {
public void display();
}

最后是各种显示器的实现。

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);
}

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");
}
}

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;

7fe0
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);
}
}

package designpatterns.observer.weather;

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");
}
}
}

我感觉这么些表述意义不明。 观察者应该为

WeatherData

他观察气象站气温改变，然后通知各个显示器显示。由它获取数据，由它分发数据。