设计模式之观察者模式

观察者模式就是：

1、所有的观察者需要被通知，实现了一个接口，供主题调用

2、所有的主题都有一个观察者列表，主题通过调用观察者实现的接口通知观察者。

代码结构(inside代表使用java.util， self使用自定义的接口)：

interface DisplayElement:

public interface DisplayElement {
    public void display();
}

interface subject:

public interface Subject {
    public void registerObserver(Observer o) ;

    public void removeObserver(Observer o);

    public void notifyObserver();
}

interface Observer:

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

WeatherData

public class WeatherData implements Subject {

    private ArrayList observers;

    private float temperature;

    private float humidity;

    private float pressure;

    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 notifyObserver() {
        for (int i = 0; i< observers.size(); i++) {
            Observer observer = (Observer) observers.get(i);
            observer.update(temperature, humidity, pressure);
        }
    }

    public void measurementsChanged() {
        notifyObserver();
    }

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

　CurrentConditionsDisplay

public class CurrentConditionsDisplay implements Observer, DisplayElement {

    private float temperature;

    private float humidity;

    private Subject weatherData;

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

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

    @Override
    public void display() {
        System.out.println("Current conditions:" + temperature
        + "F degrees and "+ humidity + "% humidity");
    }
}

　　

使用jdk内置的观察者接口：

WeatherData

public class WeatherData extends Observable{

    private float temperature;
    private float humidity;
    private float pressure;

    public WeatherData() {}

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

CurrentConditionsDisplay

public class CurrentConditionsDisplay implements Observer, DisplayElement {

    Observable observable;
    private float temperature;
    private float humidity;

    public CurrentConditionsDisplay(Observable observable) {
        this.observable = observable;
        observable.addObserver(this);
    }

    @Override
    public void update(Observable obs, Object arg) {
        if(obs instanceof WeatherData){
            WeatherData weatherData = (WeatherData) obs;
            this.temperature = weatherData.getTemperature();
            this.humidity = weatherData.getHumidity();
            display();
        }
    }

    @Override
    public void display() {
        System.out.println("Current conditions:" + temperature
        + "F degrees and " + humidity + "% humidity");
    }
}

测试类：

WeatherStationSelf

public class WeatherStationSelf {

    @Test
    public void test() {
        WeatherData weatherData = new WeatherData();

        CurrentConditionsDisplay currentConditionsDisplay = new CurrentConditionsDisplay(weatherData);

        weatherData.setMeasurements(80, 65, 30.4f);
        weatherData.setMeasurements(82, 70, 29.2f);
        weatherData.setMeasurements(78, 90, 29.2f);
    }
}