Question

I have an map activity for displaying a map in fullscreen mode:

public class MapActivity extends Activity {

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    requestWindowFeature(Window.FEATURE_NO_TITLE);
    setContentView(new Panel(this));
}

public class Panel extends SurfaceView implements SurfaceHolder.Callback {

public Panel(Context context) {
        super(context);
        getHolder().addCallback(this);

        // initiate new thread for drawing
        _thread = new CanvasThread(getHolder(), this);
}

// start thread 
@Override
public void surfaceCreated(SurfaceHolder holder) {
        CanvasThread.setRunning(true);
        _thread.start();
}

// start drawing objects on canvas
@Override
public void onDraw(Canvas canvas) {
        super.onDraw(canvas);
     /* several canvas operations ...
     */
}

// Thread specific class
static class CanvasThread extends Thread {
    private SurfaceHolder _surfaceHolder;
    private Panel _panel;
    private static boolean _run = false;

    public CanvasThread(SurfaceHolder surfaceHolder, Panel panel) {
        _surfaceHolder = surfaceHolder;
        _panel = panel;
    }

    public static void setRunning(boolean run) {
        _run = run;
    }

    @Override
    public void run() {
        Canvas c;
        Log.d("Thread", "start");
        while (_run) {
            c = null;
            try {
                c = _surfaceHolder.lockCanvas(null);
                synchronized (_surfaceHolder) {
                    Log.d("Thread", "Canvas c init");
                    _panel.onDraw(c);
                }
            } finally {
                // do this in a finally so that if an exception is thrown
                // during the above, we don't leave the Surface in an
                // inconsistent state
                if (c != null) {
                    _surfaceHolder.unlockCanvasAndPost(c);
                }
            }
        }
    }
}
}

Now, I’d like to use the same class to display my map in an other activity within an SurfaceView Element. Therefore I tried to take SurfaceView Element in a new Activity as follows:

....
Panel surface = new KarteActivity().new Panel(this);
    surface._thread.start();
    CanvasThread.setRunning(true);
    LinearLayout midLL = new LinearLayout(this);

    midLL.findViewById(R.id.surfaceView1);
    midLL.addView(surface);

This causes a nullpointer exception because the canvas is null. In fullscreen mode I have no errors.
What is my mistake?

How is the usual way to solve such problem?

Answer 1

Ok, finally I got the answer on my own 🙂 Here is the solution:

1. The activity which contains the SurfaceView element has to hand over the SurfaceHolder

   SurfaceView surface = (SurfaceView) findViewById(R.id.surfaceView1);
   SurfaceHolder surfaceHolder = surface.getHolder();
   Panel panel = new Panel(this,surfaceHolder);
   

2. The surface class has to initialize the surface in its contructor and the thread will start when the surface is created.

   public class Panel extends SurfaceView implements SurfaceHolder.Callback {
   
   public Panel(Context context, SurfaceHolder surfaceHolder) {
       super(context);
       surfaceHolder.addCallback(this);
   }
   
   // start thread 
   @Override
   public void surfaceCreated(SurfaceHolder surfaceHolder) {
       // initiate new thread for drawing
       _thread = new CanvasThread(surfaceHolder, this);
       _thread.setRunning(true);
       _thread.start();
   }
   
   // start drawing objects on canvas
   @Override
   public void onDraw(Canvas canvas) {
       super.onDraw(canvas);
    /* several canvas operations ...
    */
   }
   
   // Thread specific class
   class CanvasThread extends Thread {
       private SurfaceHolder _surfaceHolder;
       private Panel _panel;
       private boolean _run = false;
   
       public CanvasThread(SurfaceHolder surfaceHolder, Panel panel) {
           _surfaceHolder = surfaceHolder;
           _panel = panel;
       }
   
       public void setRunning(boolean run) {
           _run = run;
       }
   
       @Override
       public void run() {
           Canvas c;
           Log.d("Thread", "start");
           while (_run) {
               c = null;
               try {
                   c = _surfaceHolder.lockCanvas(null);
                   synchronized (_surfaceHolder) {
                       Log.d("Thread", "Canvas c init");
                       _panel.onDraw(c);
                   }
               } finally {
                   // do this in a finally so that if an exception is thrown
                   // during the above, we don't leave the Surface in an
                   // inconsistent state
                   if (c != null) {
                       _surfaceHolder.unlockCanvasAndPost(c);
                   }
               }
           }
       }
   }
   }
   

This worked for me but I decided to seperate the class Panel out of the class MapActivity (shown in my question)