設計模式 觀察者模式

觀察者模式

1. 定義

定義了對象之間一對多的關係，這樣一樣，當一個對象改變狀態時，它的所有依賴者都會收到通知並且自動更新。

2. 建造一個氣象臺

氣象臺實現接口

1. 主題 Subject 作爲 被觀察者 接口，實現 註冊觀察者 registerObserver()， 除去觀察者 removeObserver()，通知所有觀察者
2. Observer 作爲 觀察者接口，實現 update(float temp , float humidity, float pressure) 方法
3. DisplayElement 作爲 觀察者顯示的接口，實現 display()方法

• 被觀察者 接口

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

• 觀察者接口

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

public interface DisplayElement {
    public void display();
}

氣象臺實現類

被觀察者 WeatherData

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

    //構造函數時候創建
    private ArrayList<Observer> observers;

    // 所有觀察者
    public WeatherData(){
        observers = new ArrayList<Observer>();

    }

    //註冊觀察者
    public void registerObserver(Observer o) {
        observers.add(o);
    }

    //刪除觀察者
    public void removeObserver(Observer o) {
        int i = observers.indexOf(o);
        if(i >= 0){
            observers.remove(i);
        }
    }
    
    //通知觀察者
    public void notifyObserver() {
        for(int i = 0 ; i < observers.size() ; i++){
            Observer o = (Observer) observers.get(i);
            o.update(temp,humidity,pressure);

        }
    }

    public void setMeasurement(float temp , float humidity , float pressure){
        this.temp = temp;
        this.humidity = humidity;
        this.pressure = pressure;


        measurementChanged();
    }

    public void measurementChanged(){
        notifyObserver();
    }

    public float getTemp() {
        return temp;
    }

    public void setTemp(float temp) {
        this.temp = temp;
    }

    public float getHumidity() {
        return humidity;
    }

    public void setHumidity(float humidity) {
        this.humidity = humidity;
    }

    public float getPressure() {
        return pressure;
    }

    public void setPressure(float pressure) {
        this.pressure = pressure;
    }


}

事實情況 觀察者 CurrentCondition

public class CurrentCondition implements Observer , DisplayElement {
    private float temp;
    private float humidity;
    private Subject weatherDate;

    public CurrentCondition(Subject weatherData){
        this.weatherDate = weatherData;
        weatherData.registerObserver(this);

    }

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

    }

    public void display(){
        System.out.println("CurrentCondition : " + "temperature = " + temp +"℃ and " + "humidity = " + humidity + "%");
    }
}

最大最小溫度情況 觀察者 MaxMinCondition

public class MaxMinCondition implements Observer , DisplayElement{
    float maxTemp ;
    float minTemp ;
    Subject weatherData;

    public MaxMinCondition(Subject weatherData){
        this.weatherData = weatherData;
        maxTemp = 0;
        minTemp = 0;
        weatherData.registerObserver(this);
    }

    public void update(float temp, float humidity, float pressure) {
        if (maxTemp == 0 && minTemp == 0){
            maxTemp = temp;
            minTemp = temp;
        }
        else if(temp > maxTemp){
            maxTemp = temp;
        }else if (temp < minTemp){
            minTemp = temp;
        }
        display();
    }

    public void display(){
        System.out.println("MaxTemplate is " + maxTemp +  "and MinTemplate is " + minTemp);
    }
}