设计模式之---观察者模式

   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，他的所有依赖者都会收到通知并自动更新。
   关于观察者，主题只需要知道观察者实现了某个接口。主题不需要知道观察者的具体类是谁，做了什么或者其他的什么细节，任何时候都可以添加新的观察者或者删除观察者。下面就以一个例子来实现观察者模式。
   首先新建一个主题、观察者以及显示的接口

public interface Subject {

    public void registerObserver(Observe observe);
    public void removeObserver(Observe observe);
    public void notifyObserver();
}
public interface Observe {

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

    public void display();
}

然后通过实现上述的接口实现一个数据源

public class WeatherData implements Subject {

    private ArrayList<Observe> observes;
    private float temperature;
    private float humidity;
    private float pressure;

    public WeatherData() {
        observes = new ArrayList<Observe>();
    }

    @Override
    public void registerObserver(Observe observe) {
        observes.add(observe);
    }

    @Override
    public void removeObserver(Observe observe) {
        if(observes.contains(observe)){
            observes.remove(observe);
        }
    }

    @Override
    public void notifyObserver() {
        for (int i = 0; i < observes.size(); i++) {
            Observe observe = observes.get(i);
            observe.update(temperature, humidity, pressure);
        }
    }

    public void measurementChanged(){
        notifyObserver();
    }

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

数据源完成时候就是要展示数据，也就是实现观察者

public class CurrentConditionsDisplay implements Observe, DisplayElement {

    private float temperature;
    private float humidity;
    private Subject weatherData;

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

    @Override
    public void display() {
        System.out.println("当前气温：："+temperature+"摄氏度，相对湿度为："+humidity+"%");
    }

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

}

public class ForecastDisplay implements Observe,DisplayElement{

    private float temperature;
    private float humidity;
    private float pressure;
    private Subject weatherData;

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

    @Override
    public void display() {
        System.out.println("天气不错，可以出去郊游！");
    }

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

}

public class StatisticsDisplay implements Observe,DisplayElement{
    private float temperature;
    private WeatherData weatherData;
    private List<Float> list;

    public StatisticsDisplay(WeatherData weatherData) {
        list = new ArrayList<Float>();
        this.weatherData = weatherData;
        weatherData.registerObserver(this);
    }

    @Override
    public void display() {
        // 排序
        Collections.sort(list);
        float temp = 0;
        for (int i = 0; i < list.size(); i++) {
            temp+=list.get(i);
        }
        int vagtemp = (int) (temp/list.size());
        System.out.println("气温最大值/平均值/最小值："+list.get(list.size()-1)+"/"+vagtemp+"/"+list.get(0)+" 摄氏度");
    }

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

}

上述三个观察者都是跟着weatherData获取数据
测试程序：

“`
public class WeatherTest {

public static void main(String[] args) {
    WeatherData weatherData = new WeatherData();
    CurrentConditionsDisplay conditionsDisplay = new CurrentConditionsDisplay(weatherData);
    StatisticsDisplay display = new StatisticsDisplay(weatherData);
    ForecastDisplay forecastDisplay = new ForecastDisplay(weatherData);

    weatherData.setMeasurements(20, 75, 30.4f);
    weatherData.setMeasurements(32, 70, 29.4f);
    weatherData.setMeasurements(18, 90, 31.4f);
}

}

“`