Question

I’m starting out a new project, and thus far I’ve made a simple app that has an image which is drawn onto a Canvas, which you can drag around the screen with your finger. The idea is to draw lots of stuff onto this Canvas for a game. However the dragging is extremely laggy, while dragging with my finger the screen updates the image maybe 2-3 times a second. It’s very noticeably slow, so I’m assuming I’m doing it very wrong (I’m on a Nexus S).

I only have the very barebones of the app, just a SurfaceView containing a thread to update the Canvas with this one image. Here’s the code:

// In GameThread

private void doDraw(Canvas c) {
    if(canvasInit){
        Bitmap background = BitmapFactory.decodeResource(mContext.getResources(), R.drawable.bg);
        c.drawBitmap(background, offsetX, offsetY, null);           
    }
}


public boolean doTouchDown(MotionEvent event) {
    if(drag == false){
        //initialize drag
        startX = event.getX();
        startY = event.getY();
        drag = true;
    }else{
        //caluclate new position
        offsetX = Math.max(Math.min(0, offsetX + (event.getX() - startX)), mCanvasWidth - BACKGROUND_WIDTH); 
        offsetY = Math.max(Math.min(0, offsetY + (event.getY() - startY)), mCanvasHeight- BACKGROUND_HEIGHT);
        startX = event.getX();
        startY = event.getY();
    }
    return true;
}

public boolean doTouchUp(MotionEvent event){
    drag = false;
    return true;
}

Answer 1

You seem to be loading the Bitmap every time you draw. You can try moving the resource loading to some kind of initialisation method/ SurfaceView constructor and compare the performance?

Bitmap background;
...

private void init() {
  ...
  background = BitmapFactory.decodeResource(mContext.getResources(), R.drawable.bg);
  ...
}

private void doDraw(Canvas c) {
    if(canvasInit){
        c.drawBitmap(background, offsetX, offsetY, null);           
    }
}