《Head First Design Patterns》：第2章，观察者（Observer）模式

观察者模式的定义

观察者模式定义了对象之间的一对多依赖，这样，当一个对象改变状态的时候，它的所有依赖者都会收到通知并自动更新。

场景

Weather-O—Rama的Internet气象观测站

场景描述

该气象站建立在该公司的专利WeatherData类上，WeatherData类负责跟物理气象站联系，以获得最新的天气状况（温度、湿度、气压），然后实时更新各种显示设备。目前的显示设备有三种：a)显示目前状况（温度、湿度、气压）的设备，b)显示气象统计的设备，c)显示天气预报的设备；WeatherData类还需要具备方便未来有其它显示设备以插拔的方式接入的功能。

已有WeatherData类，其中

1. getTemperature()、getHumidity()、getPressure()是气象站的开发人员已经实现好的方法；
2. 我们只需要实现measurementsChanged()，该方法一旦气象测量有更新就会被调用；

不好的设计

public class WeatherData {
    // instance variable declarations

    public void measurementsChanged() {
        float temp = getTemperature();
        float humidity = getHumidity();
        float pressure = getPressure();

        currentConditionsDisplay.update(temp, humidity, pressure);
        statisticsDisplay.update(temp, humidity, pressure);
        forecastDisplay.update(temp, humidity, pressure);
    }

    // other WeatherData methods here
}

解决方案一

相关接口：

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

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

public interface DisplayElement {
    public void display();
}

实现WeatherData：

public class WeatherData implements Subject {
    private List<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) {
        observers.remove(o);
    }
    
    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;
    }
}

实现显示设备：

public class CurrentConditionsDisplay implements Observer, DisplayElement {
    private float temperature;
    private float humidity;
    private WeatherData weatherData;
    
    public CurrentConditionsDisplay(WeatherData 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;
        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);
    }
}

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");
        }
    }
}

观测站运行：

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);
        
        weatherData.removeObserver(forecastDisplay);
        weatherData.setMeasurements(62, 90, 28.1f);
    }
}

解决方案二：使用Java内置观察者模式

注意点：

1. java.util.Observable是类；
2. java.util.Observable中的setChanged()方法；

参考

1. https://wickedlysmart.com/head-first-design-patterns/