Android: Canvas not being redrawn

Question

There are multiple similar questions like mine, but these questions didn't help me.

I'm making a game. The game thread, SurfaceView and Activity is already finished and works so far. The problem is that the canvas is not redrawn. At startup, it draws the icon on the background, but at every tick, the icon doesn't move (It should move once a second). I want to mention, that I never needed to call postInvalidate. I have a working example where I never called it, but it doesn't work in my current example (I don't want to go into it deeper, since I actually don't need to call it). I copied the current code from my working example, the concept and the way of implementation is exactly the same, but my current code doesn't refresh the canvas. When I log the drawing positions in onDraw method, I see that it's coordinates are updated every second as expected, so I can be sure it's a canvas drawing problem. I have searched for hours but I didn't find what's different to my working example (except that I'm using another Android version and I don't extend thread but implement Runnable, because it's a bad style to extend thread. Nevertheless, I also extended thread to see if there is any difference, but it doesn't help). I already tried to clean the canvas by using canvas.drawColor(Color.BLACK), but that didn't help either. I already tried to use background colors instead of a background image which changes randomly every tick, but it didn't change but stays always the same. I figured out that the canvas at the very first call has a density of (for example) 240. After the second tick, the canvas density is always 0. I know that the density will not help me here, but maybe it's an important information for someone.

Here are the important classes....

game layout, R.layout.game

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/gameContainer"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical" >

    <com.mydomain.mygame.base.game.GameSurface
        android:id="@+id/gameSurface"
        android:layout_width="match_parent"
        android:layout_height="0dp"
        android:layout_weight="1.0"
        android:background="@drawable/background_game" >
    </com.mydomain.mygame.base.game.GameSurface>

<!-- ...more definitions -->

</LinearLayout>

GameActivity (contains layout)

public class GameActivity extends Activity
{
    @SuppressWarnings("unused")
    private GameSurface gameSurface;

    @Override
    protected void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.game);

        gameSurface = (GameSurface)findViewById(R.id.gameSurface);
        //TODO on button click -> execute methods
    }
}

GameSurface (log in onDraw shows updated coordinates every tick)

public class GameSurface extends SurfaceView implements SurfaceHolder.Callback
{
    private GameThread thread;
    protected final static int TICK_FREQUENCY = 100;// ms, stays always the same. It's a technical constant which doesn't change

    private static final String TAG = GameSurface.class.getSimpleName();

    public GameSurface(Context context, AttributeSet attrs)
    {
        super(context, attrs);

        ShapeManager.INSTANCE.init(context);

        SurfaceHolder holder = getHolder();
        holder.addCallback(this);

        setFocusable(true); // make sure we get key events

        thread = new GameThread(holder, this);
    }

    public void updateStatus()
    {
        GameProcessor.INSTANCE.updateShapes();
    }

    @Override
    protected void onDraw(Canvas canvas)
    {
        for (Shape shape : GameProcessor.INSTANCE.getShapes())
        {
            Log.i(TAG, "getX()=" + shape.getX() + ", getY()=" + shape.getY());
            canvas.drawBitmap(shape.getBitmap(), shape.getX(), shape.getY(), null);
        }
    }

    @Override
    public void surfaceChanged(SurfaceHolder holder, int format, int width, int height)
    {
        //will never invoked since we only operate in landscape
    }

    @Override
    public void surfaceCreated(SurfaceHolder holder)
    {
        // start the thread here so we don't busy-wait in run
        thread.setRunning(true);
        new Thread(thread).start();
    }

    @Override
    public void surfaceDestroyed(SurfaceHolder holder)
    {
        Log.i(TAG, "executing surfaceDestroyed()...");  
        thread.setRunning(false);
    }
}

GameThread

public class GameThread implements Runnable
{
    private SurfaceHolder surfaceHolder;
    private boolean running = false;
    private GameSurface gameSurface;
    private long lastTick;

    public GameThread(SurfaceHolder surfaceHolder, GameSurface gameSurface)
    {
        this.surfaceHolder = surfaceHolder;
        this.gameSurface = gameSurface;

        lastTick = System.currentTimeMillis();
    }

    @Override
    public void run()
    {
        Canvas canvas;
        while (running)
        {
            canvas = null;
            if (System.currentTimeMillis() > lastTick + GameSurface.TICK_FREQUENCY)
            {
                long timeDifference = System.currentTimeMillis() - (lastTick + GameSurface.TICK_FREQUENCY);

                try
                {
                    canvas = surfaceHolder.lockCanvas(null);
                    synchronized (surfaceHolder)
                    {
                        gameSurface.updateStatus();
                        gameSurface.draw(canvas);
                    }
                }
                finally
                {
                    if (canvas != null)
                    {
                        surfaceHolder.unlockCanvasAndPost(canvas);
                    }
                }
                lastTick = System.currentTimeMillis() - timeDifference;
            }
        }
    }

    public void setRunning(boolean running)
    {
        this.running = running;
    }
}

Any ideas why this code doesn't update my canvas? I can't explain it. I do not post ShapeManager and GameProcessor since they don't have anything to do with the problem (and they only load and control the current states and speed of the game).

[UPDATE]

I figured out that onDraw() is invoked before the game thread has started. That means that canvas is passed to this method before thread is using it. The interesting thing is that, after the thread has started, it always uses the same canvas, but it's not the canvas reference which is passed the very first time. Although canvas = surfaceHolder.lockCanvas(null); is assigned every tick, it's always the same reference, but it's not the original reference. In a working example of mine, the reference is always the same, since I create the bitmaps at constructor initialization time. I can't do that in my current implementation, since I have to do calculations with values I get from onMeasure() which is invoked much later than the constructor.

I tried to somehow pass the original canvas to the thread, but the reference still changes. Meanwhile I think this is the problem, but I don't know how to solve it yet.

Answer 1

As often happens, I found the solution on my own. Obviously it's really a problem that it draws to different canvas instances. I'm still not sure why this happens. Didn't have the problem before. Nevertheless, I can avoid drawing to canvas by setting setWillNotDraw(true); in my GameSurface constructor and I must not invoke gameSurface.draw(canvas) in my thread, but gameSurface.postInvalidate() instead.