Question

I have a surface view class which draws my bitmaps, and I am having a bit of trouble getting an object to be launched from the center bottom of the screen to where the user clicks on the screen, here is a code snippet below
The onTouch method that gets the coordinates from user touching on the screen, then the run method draws the bitmaps. The rock bitmap is what I need to move from the bottom of the screen to where the touch was made.

           public boolean onTouch(View arg0, MotionEvent event) {


    switch (event.getAction()) {
    case MotionEvent.ACTION_DOWN:
        x = event.getX();
        y = event.getY();
        sX = event.getX();
        sY = event.getY();
        fX = fY = dX = dY = sclX = sclY = aniX = aniY = 0;
        break;
    case MotionEvent.ACTION_UP:
        fX = event.getX();
        fY = event.getY();
        dX = fX - sX;
        dY = fY - sY;
        sclX = dX / 30;
        sclY = dY / 30;

        break;
    }
    return true;
}

        //the code that draws and updates the bitmaps on screen
        public void run() {
        while (running) {
            if (!sh.getSurface().isValid())
                continue;

            canvas = sh.lockCanvas();
            canvas.drawRGB(02, 02, 150);
            if (x != 0 && y != 0)
                canvas.drawBitmap(cross, x - (cross.getWidth() / 2), y- (cross.getHeight() / 2), null);

            canvas.drawBitmap(sling,(canvas.getWidth() / 2) - (sling.getHeight() / 2),canvas.getHeight() - sling.getHeight(), null);
            if (fX != 0 && fY != 0) {
                canvas.drawBitmap(rock, (canvas.getWidth()/2) - aniX,(canvas.getHeight()- sling.getHeight()) - aniY,null);

            }

            aniX = aniX + sclX;
            aniY = aniY + sclY;

            sh.unlockCanvasAndPost(canvas);
        }

    }

Answer 1

You are missing some vital thing in your onTouch().

public boolean onTouch(View arg0, MotionEvent event) {
    switch (event.getAction()) {
      case MotionEvent.ACTION_DOWN:
        x = event.getX();
        y = event.getY();
        sX = event.getX();
        sY = event.getY();
        fX = fY = dX = dY = sclX = sclY = aniX = aniY = 0;
        break;
      case MotionEvent.ACTION_UP:
        fX = event.getX();
        fY = event.getY();
        dX = fX - sX;
        dY = fY - sY;
        sclX = dX / 30;
        sclY = dY / 30;

        break;
    }
    return true;
}

This code has a logic issue. Consider you tap down on 100/100, sX and sY are set to 100 in the down case. In the up case, you get the up coordinates in fX and fY, which will be 100, too. This results in dX = fX - sX => dX = 100 - 100. So your sclX will be 0, too. This means no animation at all.

You should use the middle of the screen as a base for your calculations. Source is the screen center and the destination is your touch.