How can I use Thread.sleep()?

Question

I am creating a Memo Game for a school project and I have a problem with my Thread.sleep.

When the player returns two cards, the program checks if they are identical. If they are not, then they turn face down. Since the program does it almost instantly, I'd like to pause so that the player has time to see the second card. The problem is that the break is before the card turns

if (deck1.contains(carte)) {
    if (carte.getEtat() == 0) {
        carte.setEtat(1);
        cartesRetournees1.add(carte);
        majAffichage(carte);

        try {
            Thread.sleep(500);
        } catch (InterruptedException ie) {
            ie.printStackTrace();
        }
    }
    if (cartesRetournees1.size() == 2) {
        int nouveauEtat = 0;
        if (cartesRetournees1.get(0).getMotif() == cartesRetournees1.get(1).getMotif()) {
            nouveauEtat = -1;
            this.nbPairsTrouvees++;
        }

        cartesRetournees1.get(0).setEtat(nouveauEtat);
        cartesRetournees1.get(1).setEtat(nouveauEtat);

        majAffichage(cartesRetournees1.get(0));
        majAffichage(cartesRetournees1.get(1));
        cartesRetournees1.remove(1);
        cartesRetournees1.remove(0);

        if (nbPairsTrouvees == this.plateau.size()) System.out.println("GAGNE !");
    }
}

On my game, Thread.sleep works before majAffichage(carte) and I don't understand why. I dont know how to resolve my problem and if you guys can help me I would be very grateful :)

Answer 1

Its because of how the UI and drawing works. Inside the android framework on the main thread, there's a giant event loop processing messages. One of those messages is a draw message. The screen is only drawn when that message is processed. Messages can only be processed at the top of the loop. So if the thread sleeps, it never processes new messages during that sleep, and the screen won't update.

If you want to do something on a delay like that, there are two options:

1)Instead of sleeping, post a Runnable to the main looped with postDelayed, so the runnable is run at a time you specify in the future.

2)Use a second Thread and run all your game logic in there, passing messages to it when UI events occur and passing events back to the main thread when it needs to draw.

Answer 2

First, you don't pause the Main Thread but the Background Thread because you don't want the freeze the entire app.

Second, use System.sleep() instead of Thread.sleep() cause the first one doesn't throw an exception.

Answer 3

Thank you for your answer ! I have created a second Thread.

package com.example.jeudumemo;

import android.graphics.Canvas;

public class GameLoopThread extends Thread
{
    private final static int FRAMES_PER_SECOND = 30;

    private final static int SKIP_TICKS = 1000 / FRAMES_PER_SECOND;

    private final Game view; 
    private boolean running = false; 

    public GameLoopThread(Game view) {
        this.view = view;
    }

    public void setRunning(boolean run) {
        running = run;
    }

    @Override
    public void run()
    {
        long startTime;
        long sleepTime;

        while (running)
        {
            startTime = System.currentTimeMillis();

            synchronized (view.getHolder()) {view.update();}

            Canvas c = null;
            try {
                c = view.getHolder().lockCanvas();
                synchronized (view.getHolder()) {view.draw(c);}
            }
            finally
            {
                try {
                    if (c != null) {
                        view.getHolder().unlockCanvasAndPost(c);
                    }
                } catch(IllegalArgumentException iae) { iae.printStackTrace(); }
            }
            sleepTime = SKIP_TICKS-(System.currentTimeMillis() - startTime);
            try {
                if (sleepTime >= 0) {sleep(sleepTime);}
            }
            catch (Exception e) {}
        } 
    } 

} 

and I call this in my main class :

@Override
    public void surfaceCreated(SurfaceHolder holder) {
        if(gameLoopThread.getState()==Thread.State.TERMINATED) {
            gameLoopThread=new GameLoopThread(this);
        }
        gameLoopThread.setRunning(true);
        gameLoopThread.start();
    }

    @Override
    public void surfaceChanged(SurfaceHolder holder, int format, int width, int height) {

    }

    @Override
    public void surfaceDestroyed(SurfaceHolder holder) {
        boolean retry = true;
        gameLoopThread.setRunning(false);
        while (retry) {
            try {
                gameLoopThread.join();
                retry = false;
            }
            catch (InterruptedException e) {}
        }
    }

public void controleJeu(Carte carte) {
        if(deck1.contains(carte)) {
            if (carte.getEtat() == 0) {
                carte.setEtat(1);
                cartesRetournees1.add(carte);
                majAffichage(carte);

            }
            if (cartesRetournees1.size() == 2) {
                try {
                    gameLoopThread.sleep(500);
                }catch(InterruptedException ie) { ie.printStackTrace(); }
                int nouveauEtat = 0;
                if (cartesRetournees1.get(0).getMotif() == cartesRetournees1.get(1).getMotif()) {
                    nouveauEtat = -1;
                    this.nbPairsTrouvees++;
                }

                cartesRetournees1.get(0).setEtat(nouveauEtat);
                cartesRetournees1.get(1).setEtat(nouveauEtat);

                majAffichage(cartesRetournees1.get(0));
                majAffichage(cartesRetournees1.get(1));
                cartesRetournees1.remove(1);
                cartesRetournees1.remove(0);
            }
        }
        else if(deck2.contains(carte)) {
            if (carte.getEtat() == 0) {
                carte.setEtat(1);
                cartesRetournees2.add(carte);
                majAffichage(carte);
            }
            if (cartesRetournees2.size() == 2) {
                try {
                    gameLoopThread.sleep(500);
                }catch(InterruptedException ie) { ie.printStackTrace(); }
                int nouveauEtat = 0;
                if (cartesRetournees2.get(0).getMotif() == cartesRetournees2.get(1).getMotif()) {
                    nouveauEtat = -1;
                    this.nbPairsTrouvees++;
                }

                cartesRetournees2.get(0).setEtat(nouveauEtat);
                cartesRetournees2.get(1).setEtat(nouveauEtat);

                majAffichage(cartesRetournees2.get(0));
                majAffichage(cartesRetournees2.get(1));
                cartesRetournees2.remove(1);
                cartesRetournees2.remove(0);
            }
        }
        if (nbPairsTrouvees == nbDeck*6) { showPopupVictoire(this); this.victory = true; }
        System.out.println(nbDeck);
    }

My problem is still perduring. I think I didn't use my GameLoopThread as it must be used. Any remarks ?