Handling long operations in observer chains

If you have lengthy observer notification chains where observers notify other observers, making the trigger order unpredictable, and these chains include time-consuming operations like updating a map drawing, you can use the following approach to only update the drawing when the last observer in the chain is reached:

	import java.util.Observable;
	import java.util.Observer;
	import java.util.concurrent.atomic.AtomicInteger;
	public class DrawLast extends Observable implements Observer {
	// Global static AtomicInteger to ensure thread safety
	public static final AtomicInteger activeDrawingObserverCount = new AtomicInteger(0);
	@Override
	public void update(Observable o, Object arg) {
	// Increment count safely
	DrawLast.activeDrawingObserverCount.incrementAndGet();
	try {
	// Do observer-specific operations
	System.out.println("Observer " + this + " is updating...");
	// Notify other observers, assuming each follows the same pattern in this update()
	setChanged();
	notifyObservers();
	} finally {
	// Decrement count safely in a finally block
	int remainingObservers = DrawLast.activeDrawingObserverCount.decrementAndGet();
	if (remainingObservers == 0) {
	draw(); // Call draw only when all observers are done
	}
	}
	}
	private void draw() {
	System.out.println("Drawing now that all observers are done.");
	}
	public static void main(String[] args) {
	DrawLast observer1 = new DrawLast();
	DrawLast observer2 = new DrawLast();
	DrawLast observer3 = new DrawLast(); // Last observer in the chain
	// Set up observers to observe each other in a chain
	observer1.addObserver(observer2);
	observer2.addObserver(observer3);
	// Start the chain by notifying observer1
	observer1.setChanged();
	observer1.notifyObservers();
	}
	}

view raw DrawLast.java hosted with ❤ by GitHub