How to create own Event. How can I call a method when the value of a sensor changes?

Question

I am currently working with the LeJOS Ev3 library and have programmed the following listener:

    import lejos.hardware.port.Port;
    import robots.ev3.drive.parts.LightSensor;

    public abstract class LightListener {
        protected int trigger;
        protected boolean dark;

        public LightListener(int triggerLevel) { trigger = triggerLevel; }
        public void notify(LightSensor s) {
            // Depending on the value that s measures, either the method
            // dark or bright is called
        }
        public void initialValue(int level) {
            dark = level < trigger;
        }

        abstract public void bright(Port port, int level);
        abstract public void dark(Port port, int level);
    }

As you can see, there is a method called notify that is supposed to call different methods, depending on what the sensor is measuring. I would like to be able to call the method notify only when the value the sensor is measuring changes, without using a while-loop that constantly checks if the value has changed. But I couldn't think of or find the way to achieve this.

The only thing that occurred to me, is to start a thread that would run this code:

    while(true) {
        waitForSensorValueToChange();
        listener.notify(sensor);
    }

But, again, I don't even know if it is possible to wait for an event to happen (without constantly checking the values).

EDIT - Since it has been repeatedly been suggested to me, I will mention that I already added a method on my LightSensor called addLightListener(LightListener listener). This does not solve my problem with avoiding to poll.

Answer 1

Not familiar with that library but your two basic options are polling (what you've tried so far) or go with an observer pattern.

In the observer pattern, you notify any classes that have registered to the provider when the event happens.

class ObserveMe {
    List<Observer> observers = new ArrayList<>();

    public updateSensorValue(){
        //do what you need to
        for(Observer observer : observers){
            observer.notify();
        }
    }

    public void addObserver(Observer o){
        this.observers.add(o);
    }
}

interface Observer {
    public void notify();
}