设计模式之观察者模式

观察者模式（Observer）

定义

定义对象间的一种一对多的依赖关系，当一个对象的状态发生改变时，所有依赖于它的对象都可以得到通知并自动更新。

使用场景

主要角色

1. Subject（主题）
2. ConcreteSubject（具体主题）
3. Observer（观察者）
4. ConcreteObserver（具体观察者）

类图

示例代码

public interface Subject {
    void addObserver(Observer observer);

    void removeObserver(Observer observer);

    void notifyObservers();
}

public class WeatherStation implements Subject {
    private List<Observer> observers = new ArrayList<>();
    private String temperature;
    private String condition;

    public void setWeatherData(String temperature, String condition) {
        this.temperature = temperature;
        this.condition = condition;
        notifyObservers();
    }

    @Override
    public void addObserver(Observer observer) {
        observers.add(observer);
    }

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

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

public interface Observer {
    void update(String temperature, String condition);
}

public class TemperatureDisplay implements Observer {
    @Override
    public void update(String temperature, String condition) {
        System.out.println("Temperature Display: Current Temperature is " + temperature);
    }
}

public class WeatherConditionDisplay implements Observer {
    @Override
    public void update(String temperature, String condition) {
        System.out.println("Weather Condition Display: Current Weather Condition is " + condition);
    }
}

public class Client {
    public static void main(String[] args) {
        // 创建主题
        WeatherStation weatherStation = new WeatherStation();

        // 创建观察者
        Observer temperatureDisplay = new TemperatureDisplay();
        Observer weatherConditionDisplay = new WeatherConditionDisplay();

        // 注册观察者
        weatherStation.addObserver(temperatureDisplay);
        weatherStation.addObserver(weatherConditionDisplay);

        // 模拟天气数据更新
        weatherStation.setWeatherData("25°C", "Sunny");
    }
}

Temperature Display: Current Temperature is 25°C
Weather Condition Display: Current Weather Condition is Sunny