观察者模式

在观察者设计模式中，一个对象（被观察者）维护一系列依赖于它的对象（观察者），当被观察者的状态发生变化时，它会通知所有的观察者。使用观察者模式可以实现解耦，被观察者不需要关心观察者的具体实现，观察者只需注册到被观察者即可。

场景描述：

我们可以将一个气象站作为"被观察者"（WeatherStation），它会定期检测温度、湿度和气压。多个显示面板作为"观察者"（Observer），当气象站更新天气信息时，所有的观察者都会收到通知并显示最新数据。

代码实现

1. 定义 Subject 接口（被观察者）

import java.util.ArrayList;
import java.util.List;

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

2. 定义 Observer 接口（观察者）

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

3. 实现 WeatherStation 类（具体的被观察者）

class WeatherStation implements Subject {
    private List<Observer> observers;
    private float temperature;
    private float humidity;
    private float pressure;

    public WeatherStation() {
        observers = new ArrayList<>();
    }

    @Override
    public void registerObserver(Observer o) {
        observers.add(o);
    }

    @Override
    public void removeObserver(Observer o) {
        observers.remove(o);
    }

    @Override
    public void notifyObservers() {
        for (Observer observer : observers) {
            observer.update(temperature, humidity, pressure);
        }
    }

    public void setMeasurements(float temperature, float humidity, float pressure) {
        this.temperature = temperature;
        this.humidity = humidity;
        this.pressure = pressure;
        measurementsChanged();
    }

    public void measurementsChanged() {
        notifyObservers();
    }
}

4. 实现具体的观察者类

4.1 CurrentConditionsDisplay（当前天气显示面板）

class CurrentConditionsDisplay implements Observer {
    private float temperature;
    private float humidity;

    @Override
    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 + "C degrees and " + humidity + "% humidity.");
    }
}

4.2 StatisticsDisplay（天气统计显示面板）

class StatisticsDisplay implements Observer {
    private float maxTemp = 0.0f;
    private float minTemp = 200;
    private float tempSum = 0.0f;
    private int numReadings;

    @Override
    public void update(float temperature, float humidity, float pressure) {
        tempSum += temperature;
        numReadings++;

        if (temperature > maxTemp) {
            maxTemp = temperature;
        }

        if (temperature < minTemp) {
            minTemp = temperature;
        }

        display();
    }

    public void display() {
        System.out.println("Avg/Max/Min temperature = " + (tempSum / numReadings) + "/" + maxTemp + "/" + minTemp);
    }
}

4.3 ForecastDisplay（天气预报显示面板）

class ForecastDisplay implements Observer {
    private float currentPressure = 29.92f;
    private float lastPressure;

    @Override
    public void update(float temperature, 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");
        }
    }
}

5. 测试代码

public class WeatherStationDemo {
    public static void main(String[] args) {
        WeatherStation weatherStation = new WeatherStation();

        CurrentConditionsDisplay currentDisplay = new CurrentConditionsDisplay();
        StatisticsDisplay statisticsDisplay = new StatisticsDisplay();
        ForecastDisplay forecastDisplay = new ForecastDisplay();

        // 注册观察者
        weatherStation.registerObserver(currentDisplay);
        weatherStation.registerObserver(statisticsDisplay);
        weatherStation.registerObserver(forecastDisplay);

        // 模拟气象站数据变化
        weatherStation.setMeasurements(28, 65, 30.4f);
        weatherStation.setMeasurements(27, 70, 29.2f);
        weatherStation.setMeasurements(26, 90, 29.2f);
    }
}

输出结果

Current conditions: 28.0C degrees and 65.0% humidity.
Avg/Max/Min temperature = 28.0/28.0/28.0
Forecast: Improving weather on the way!
Current conditions: 27.0C degrees and 70.0% humidity.
Avg/Max/Min temperature = 27.5/28.0/27.0
Forecast: Watch out for cooler, rainy weather
Current conditions: 26.0C degrees and 90.0% humidity.
Avg/Max/Min temperature = 27.0/28.0/26.0
Forecast: More of the same

代码解释

1. Subject接口 ：WeatherStation实现了Subject接口，负责维护观察者列表并通知所有观察者。
2. Observer接口 ：不同的显示面板实现了Observer接口，每当气象站数据更新时，这些观察者会自动收到通知。
3. WeatherStation类：当气象站的测量数据（温度、湿度、气压）发生变化时，会通知所有注册的观察者。
4. Observer实现类 ：CurrentConditionsDisplay、StatisticsDisplay和ForecastDisplay是具体的观察者，它们在接收到通知后会更新自己的显示内容。

总结

观察者模式很好地解耦了数据源（气象站）和观察者（显示面板）。通过这种模式，当气象站的数据变化时，所有观察者都会自动收到通知并更新显示，扩展性好且易于维护。