Android drawing on surfaceview and canvas

Question

I have a "simple" problem. I try to draw on surfaceview. Layout-XML:

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:keepScreenOn="true"
    android:orientation="vertical" >

    <TextView
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:text="@string/hello" />

    <SurfaceView
        xmlns:android="http://schemas.android.com/apk/res/android"
        android:id="@+id/imagesurface"
        android:layout_height="fill_parent"
        android:layout_width="fill_parent"
        android:background="#00ff00">
    </SurfaceView>

</LinearLayout>

The Activity is a SurfaceHolder.Callback:

public class OpenCvonAndroidGTDforHOGActivity extends Activity implements SurfaceHolder.Callback{

    private SurfaceHolder _surfaceHolder;
    private SurfaceView _surfaceView;

    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        _surfaceView = (SurfaceView)findViewById(R.id.imagesurface);
        _surfaceHolder = _surfaceView.getHolder();
        _surfaceHolder.addCallback(this);
        _surfaceView.setWillNotDraw(false);

    }

    protected void onDraw(Canvas canvas) {
        canvas.drawRGB(255, 0, 255);            
    }

    public void surfaceCreated(SurfaceHolder holder) {
        Canvas canvas = null;
        try {
            canvas = holder.lockCanvas();
            synchronized(holder) {
                onDraw(canvas);
            }
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            if (canvas != null) {
                holder.unlockCanvasAndPost(canvas);
            }
        }
    }


    public void showToast(String msg) {
        Toast.makeText(getApplicationContext(), msg, Toast.LENGTH_SHORT).show();
    }
}

If i call

_surfaceview.setBackground(Color.RED) in onDraw(...) it works. But

canvas.drawRGB(255, 0, 255) doesn't work :(

Answer 1

Following Snippet will help you.

public class SurfaceDemo extends Activity implements SurfaceHolder.Callback {

    private static final String TAG = "Svetlin SurfaceView";

    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        SurfaceView view = new SurfaceView(this);
        setContentView(view);
        view.getHolder().addCallback(this);
    }

    @Override
    public void surfaceCreated(SurfaceHolder holder) {
        tryDrawing(holder);
    }

    @Override
    public void surfaceChanged(SurfaceHolder holder, int frmt, int w, int h) { 
        tryDrawing(holder);
    }

    @Override
    public void surfaceDestroyed(SurfaceHolder holder) {}

    private void tryDrawing(SurfaceHolder holder) {
        Log.i(TAG, "Trying to draw...");

        Canvas canvas = holder.lockCanvas();
        if (canvas == null) {
            Log.e(TAG, "Cannot draw onto the canvas as it's null");
        } else {
            drawMyStuff(canvas);
            holder.unlockCanvasAndPost(canvas);
        }
    }

    private void drawMyStuff(final Canvas canvas) {
        Random random = new Random();
        Log.i(TAG, "Drawing...");
        canvas.drawRGB(255, 128, 128);
    }
}

Answer 2

SurfaceView is composed by a View in your current layout and a surface under your layout. If you set a background to the view, you won't see anything of what is happening on the surface.