观察者模式（一）--《Head First DesignPattern》

观察者模式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，它的所有依赖着都会受到通知并且自动更新。

我们先看下类图：

首先我们自己创建Subject接口，定义了注册观察者，移除观察者和通知观察者三个函数。

package headfirst.observer.weather;

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

而WeatherData实现了这个接口，内部维护的是一个ArrayList的Observer。

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);
        }
    }
    
    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();
    }
    
    public float getTemperature() {
        return temperature;
    }
    
    public float getHumidity() {
        return humidity;
    }
    
    public float getPressure() {
        return pressure;
    }
}

DisplayElement接口只包含了一个方法，也就是display()。但布告板需要显示时，调用此方法。

package headfirst.observer.weather;

public interface DisplayElement {
    public void display();
}

Observer接口定义了update函数，当Subject的内容发生改变时，会调用update函数来通知观察者更新状态值。

package headfirst.observer.weather;

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

观察者需要存储Subject的引用，通过这个引用来进行注册。

package headfirst.observer.weather;
    
public class CurrentConditionsDisplay implements Observer, DisplayElement {
    private float temperature;
    private float humidity;
    private Subject weatherData;
    
    //构造函数的参数为Subject
    public CurrentConditionsDisplay(Subject weatherData) {
        this.weatherData = weatherData;
        //把自己注册给Subject
        weatherData.registerObserver(this);
    }
    
    //更新时调用相应的display函数
    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");
    }
}
        

其他两个差不多:

ForecastDisplay

package headfirst.observer.weather;

import java.util.*;

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");
        }
    }
}

StatisticsDisplay

package headfirst.observer.weather;

import java.util.*;

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;
        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);
    }
}

HeadDisplay

package headfirst.observer.weather;

public class HeatIndexDisplay implements Observer, DisplayElement {
    float heatIndex = 0.0f;
    private WeatherData weatherData;

    public HeatIndexDisplay(WeatherData weatherData) {
        this.weatherData = weatherData;
        weatherData.registerObserver(this);
    }

    public void update(float t, float rh, float pressure) {
        heatIndex = computeHeatIndex(t, rh);
        display();
    }
    
    private float computeHeatIndex(float t, float rh) {
        float index = (float)((16.923 + (0.185212 * t) + (5.37941 * rh) - (0.100254 * t * rh) 
            + (0.00941695 * (t * t)) + (0.00728898 * (rh * rh)) 
            + (0.000345372 * (t * t * rh)) - (0.000814971 * (t * rh * rh)) +
            (0.0000102102 * (t * t * rh * rh)) - (0.000038646 * (t * t * t)) + (0.0000291583 * 
            (rh * rh * rh)) + (0.00000142721 * (t * t * t * rh)) + 
            (0.000000197483 * (t * rh * rh * rh)) - (0.0000000218429 * (t * t * t * rh * rh)) +
            0.000000000843296 * (t * t * rh * rh * rh)) -
            (0.0000000000481975 * (t * t * t * rh * rh * rh)));
        return index;
    }

    public void display() {
        System.out.println("Heat index is " + heatIndex);
    }
}

接下来就是main函数了。

package headfirst.observer.weather;

import java.util.*;

public class WeatherStationHeatIndex {

    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);
        HeatIndexDisplay heatIndexDisplay = new HeatIndexDisplay(weatherData);

        weatherData.setMeasurements(80, 65, 30.4f);
        weatherData.setMeasurements(82, 70, 29.2f);
        weatherData.setMeasurements(78, 90, 29.2f);
    }
}

可以看到这里实现的Observer模式有两个要注意的地方

1. “推”模式，Subject内容一有变化，就主动向Observer推送消息。它是通过在Subject中的notifyObservers中对每个Observer调用update来实现的。如果要实现“拉”模式，因为在Observer保存了Subject的引用，所以可以通过定时的方式，向Subject拉取数据。
2. 可以看到update函数是依赖于具体实现的，可以看到参数是什么温度，湿度等等。这样的接口是无法面向所有的应用的。不过Java内置了观察者模式相关的接口，可以在下一篇看到相关的实现。