Question

I have been developing the application, and I need that drawingg is executed in another thread. Now my code is:

public class PainterView extends View implements DrawingListener {

    //private GestureDetector detector;
    private Context context;
    private Painter painter;
    private Bitmap background;
    private Bitmap bitmap;
    private Paint bitmapPaint;
    private Path path;
    private Paint paint;

    private float x;
    private float y;

    private boolean isErasing=false;
    private boolean isTextDrawing=false;

    private ExecutorService pool;

    public PainterView(Context context, Painter painter) {

        super(context);
        this.context = context;
        this.painter = painter;
        pool=Executors.newFixedThreadPool(3);
        //detector = new GestureDetector(context, new GestureListener());
    }

    @Override
    protected void onSizeChanged(int w, int h, int oldw, int oldh) {
        super.onSizeChanged(w, h, oldw, oldh);
    }

    @Override
    protected void onDraw(final Canvas canvas) {
        if (bitmap != null) {
            pool.submit(new Runnable() {

                @Override
                public void run() {
                    // TODO Auto-generated method stub
                    synchronized (PainterView.this) {
                        canvas.drawBitmap(background, 0, 0, bitmapPaint);
                        canvas.drawBitmap(bitmap, 0, 0, bitmapPaint);
                        canvas.drawPath(path, paint);
                    }
                }

            });

        }
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        //detector.onTouchEvent(event);
        x = event.getX();
        y = event.getY();
        switch (event.getAction()) {
            case MotionEvent.ACTION_DOWN:
                painter.touchStart(x, y);
                break;
            case MotionEvent.ACTION_MOVE:
                painter.touchMove(x, y);
                break;
            case MotionEvent.ACTION_UP:
                painter.touchUp();
                break;
        }
        return true;
    }

    @Override
    public void onPictureUpdate(Bitmap background, Bitmap bitmap, Paint bitmapPaint, Path path, Paint paint) {
        this.background=background;
        this.bitmap = bitmap;
        this.bitmapPaint = bitmapPaint;
        this.path = path;
        this.paint = paint;
        invalidate();
    }

    public void setPainter(Painter painter) {
        this.painter = painter;
    }
}

I thought that if I uses ExecutorService then the application can draw in another thread, but it doesn’t work – when I draw the screen of device blinkes. So, please, tell me, how can I use multithreading for drawing using SurfaceHolder and other elements? I need to make as few as possible changes in my code.

Answer 1

You can only draw in the main UI thread. You should use SurfaceView, since it was made specifically to support drawing from secondary threads.

One of the purposes of this class is to provide a surface in which a
secondary thread can render into the screen. If you are going to use
it this way, you need to be aware of some threading semantics.

source

See also this video: Learn Android Tutorial 1.28- Introduction to the SurfaceView