设计模式(Observer Pattern)

一 定义：

Observer Pattern 基本精神为一个被观察者对应多个观察者，当被观察者的状态改变时候，会通知观察者，属于行为型模式。

二 应用：

Java源码中实现了一个 Observer Pattern。

java.util.Observable 被观察对象，维护了一个观察者列表(Vector<Observer> obs)，还有一个同步方法。

java.util.Observer 观察对象，void update(Observable o, Object arg)，接口内仅此一个方法。

三 实例演示(该实例为Head First设计模式模式中实例)：

需求：设计一个气象站,当被观察对象数据改变时，观察者对象会接收到通知。

// 观察者对象接口

public interface Observer {
    /**
     * This method is used to update the data about the weather
     *
     * @param temp
     * @param humidity
     * @param pressure
     */
    public void update(float temp,float humidity,float pressure);
}

// 被观察者(主题)接口

public interface Subject {
    /**
     * This method is used to register an observable object
     * @param o observable object
     */
    public void registerObserver(Observer o);
    /**
     * This method is used to remove an observable object
     * @param o observable object
     */
    public void removeObserver(Observer o);
    /**
     * This method is used to notify all observable objects
     *
     */
    public void notifyObservers();
}

// 看板接口

public interface DisplayElement {
    /**
     * This method is used to show the data
     *
     */
    public void display();
}

// 天气数据实现类

public class WeatherData implements Subject{
    private float temp;
    private float humidity;
    private float pressure;
    private List observers;

    public WeatherData() {
        observers = new ArrayList();
    }

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

    @Override
    public void removeObserver(Observer o) {
        int i = observers.indexOf(o);
        if (i >= 0){
            observers.remove(i);
        }
    }

    @Override
    public void notifyObservers() {
        for (int i = 0;i < observers.size();i++){
            Observer observer = (Observer)observers.get(i);
            observer.update(temp,humidity,pressure);
        }
    }

    public void measurementsChanged(){
        notifyObservers();
    }

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

// 观察者对象实现类

public class CurrentConditionDisplay implements Observer,DisplayElement {
    private float temp;
    private float humidity;
    private float pressure;
    private Subject WeatherData;

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

    @Override
    public void update(float temp, float humidity, float pressure) {
        this.temp = temp;
        this.humidity = humidity;
        this.pressure = pressure;
        display();
    }

    @Override
    public void display() {
        System.out.println("Current conditions " + temp + "F degree and " + humidity + " pressure are " + pressure);
    }
}

// 测试类

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

        weatherData.setMeasurements(80,65,30.4f);
    }
}

四 引用与参考

Head First设计模式   中文版    

观察者模式 | 菜鸟教程

五 小提示

因为本人水平有限,标红的部分可能会有错误或待补充之处,希望大家提出宝贵意见,感激不尽。