观察者模式

《Head First设计模式》Eric Freeman;Elisabeth Freeman;Kathy Sierra;Bert Bates
中国电力出版社
ISBN：9787508353937

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，它的所有依赖者都会收到通知并自动更新。

设计原则-为了交互对象之间的送耦合设计而努力。对于观察者模式而言，改变主题或观察者其中一方，并不会影响另一方。因为两者是送耦合的，所以只要他们之间的接口仍被遵守，我们就可以自由地改变他们。

天气预报主题(WeatherData)需要监控temperature，humidity和pressure三个数据，如果发生变化，需要通知相应的观察者。

示例代码:
Subject.java

package net.dp.observer;

public interface Subject {
    void registerObserver(Observer o);

    void removeObserver(Observer o);

    void notifyObservers();
}

Observer.java

package net.dp.observer;

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

DisplayElement.java

package net.dp.observer;

public interface DisplayElement {
    void display();
}

WeatherData.java

package net.dp.observer;

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 (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

package net.dp.observer;

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");
    }
}

ForecastDisplay.java

package net.dp.observer;

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");
        }
    }
}

StatisticsDisplay.java

package net.dp.observer;

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);
    }
}

HeatIndexDisplay.java

package net.dp.observer;

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

    public HeatIndexDisplay(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);
    }
}

WeatherStation.java

package net.dp.observer;

public class WeatherStation {

    public static void main(String[] args) {
        WeatherData weatherData = new WeatherData();

        new CurrentConditionsDisplay(weatherData);
        new StatisticsDisplay(weatherData);
        new ForecastDisplay(weatherData);
        new HeatIndexDisplay(weatherData);

        weatherData.setMeasurements(80, 65, 30.4f);
        System.out.println("======================");
        weatherData.setMeasurements(82, 70, 29.2f);
        System.out.println("======================");
        weatherData.setMeasurements(78, 90, 29.2f);
    }
}

《Head First Design Pattern》源代码下载 http://zhangjunhd.blog.51cto.com/113473/69908

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