sclass Canvas extends ImageSurface {
  O makeImg;
  bool updating;
  
  *() { zoomable = false; }
  *(O makeImg) { this(); this.makeImg = makeImg; }
  
  void update() { updateCanvas(this, makeImg); }
}

static Canvas jcanvas() { ret jcanvas(null, 0); }

// f: (int w, int h) -> BufferedImage
static Canvas jcanvas(O f) {
  ret jcanvas(f, 0); // 100
}

static Canvas jcanvas(fO f, final int updateDelay) {
  ret (Canvas) swing(func {
    final Canvas is = new Canvas(f);
    is.specialPurposed = true;
    final Runnable update = new Runnable {
      bool first = true;
      public void run {
        BufferedImage img = is.getImage();
        int w = is.getWidth(), h = is.getHeight();
        if (first || img.getWidth() != w || img.getHeight() != h) {
          updateCanvas(is, f);
          first = false;
        }
      }
    };
    onResize(is, r { awtLater(is, updateDelay, update) });
    bindToComponent(is, update); // first update
    ret is;
  });
}