Java中的观察者设计模式

Question

I have the following notification sent for an Observer and Observable design pattern

private void changeEnvironment()
    {
        Random random = new Random();
        season = SEASON[random.nextInt(3)];
        dayInTime = DAY_IN_TIME[random.nextInt(2)];
        temperature = random.nextInt(120);
        setChanged();
        notifyObservers(season);
        notifyObservers(dayInTime);
        notifyObservers(temperature);
    }

However, in my:

public void update(Observable o, Object arg) {

    }

How do I check whether it's season, dayInTime, or temperature?

Answer 1

As per your code, the arg in update(Observable o, Object arg) will be one of season, dayInTime or temperature. The argument to notifyObservers() is what gets passed to update() .

You probably want to include more metadata in a custom 'event' object and use that as an argument for your calls to notifyObservers() .

Answer 2

You can use the observer pattern in pull mode , which means that observable will pass itself to the observer 's update() .So , observer can get properties they want from the observable 's getters.

I assume temperature , dayInTime , season in your subject are both int

So , you Observable (ie Subject) will looks like :

public class Subject extends Observable{

    private int temperature;
    private int dayInTime;
    private int season;

    private void changeEnvironment() {
        Random random = new Random();
        this.season = SEASON[random.nextInt(3)];
        this.dayInTime = DAY_IN_TIME[random.nextInt(2)];
        this.temperature = random.nextInt(120);
        setChanged();

         /** Notify all of the subscribed observers about its changes
          It will call each observers' update(this, null)***/
        notifyObservers();
    }


    public int getTemperature(){....}
    public int getdayInTime(){.....}
    public int getSeason(){....}

}

Then your observer can get temperature, dayInTime , season by using the corresponding getters of your Observable :

public void update(Observable o, Object arg) {

   Subject = (Subject) o;
   int temperature = o.getTemperature();
   int dayInTime= o.getdayInTime();
   int season= o.getSeason();
}

Answer 3

You can call notifyObservers("season"); and in the Observer's update(Observable o, Object arg) , the second parameter will be the string "season" .

Or declared an enum MyAttr {Season, DayInTime, Temperature} and call notifyObservers(MyAttr.Season);

Answer 4

If season , dayInTime and temperature should be send to observer at the same time, it may be better to call:

notifyObservers(season, dayInTime, temperature)

and Observer can have a method update(Object season, Object dayInTime, Object temperature) , now you know whether it's season, dayInTime, or temperature.