static void anim_completed_cb(lv_anim_t* a)
{
  LV_LOG_USER("completed cb");
  lv_anim_completed_cb_t completed = a->user_data;
  if (completed) {
    completed(a);
  }
}

static void scroll_event_cb(lv_event_t* e)
{
  if (e->code == LV_EVENT_SCROLL_BEGIN) {

    lv_anim_t * a = lv_event_get_scroll_anim(e);
    if (a) {
      lv_anim_set_user_data(a, a->completed_cb);
      a->completed_cb = anim_completed_cb;
      a->duration = 20000;
    }
    LV_LOG_USER("LV_EVENT_SCROLL_BEGIN");
  } else if (e->code == LV_EVENT_SCROLL_END) {
    LV_LOG_USER("LV_EVENT_SCROLL_END");
  }
}

static void btn_cb(lv_event_t* e)
{
  lv_obj_t* cont = lv_event_get_user_data(e);
  lv_obj_scroll_to(cont, 0, 320, LV_ANIM_OFF);
  LV_LOG_USER("scroll to 320");
}

static void lv_scroll_demo(void)
{
  lv_obj_t* cont = lv_obj_create(lv_scr_act());
  lv_obj_set_size(cont, LV_PCT(100), LV_PCT(100));
  lv_obj_set_scroll_snap_y(cont, LV_SCROLL_SNAP_CENTER);
  lv_obj_add_event_cb(cont, scroll_event_cb, LV_EVENT_SCROLL_BEGIN, NULL);
  lv_obj_add_event_cb(cont, scroll_event_cb, LV_EVENT_SCROLL_END, NULL);

  lv_obj_t* obj = lv_obj_create(cont);
  lv_obj_set_size(obj, LV_PCT(100), LV_PCT(100));
  lv_obj_set_style_bg_color(obj, lv_color_hex(0xff0000), 0);

  obj = lv_obj_create(cont);
  lv_obj_set_size(obj, LV_PCT(100), LV_PCT(100));
  lv_obj_set_style_bg_color(obj, lv_color_hex(0x0000ff), 0);
  lv_obj_set_y(obj, LV_VER_RES);

  lv_obj_t* btn = lv_obj_create(lv_layer_sys());
  lv_obj_center(btn);
  lv_obj_add_flag(btn, LV_OBJ_FLAG_CLICKABLE);
  lv_obj_add_event_cb(btn, btn_cb, LV_EVENT_CLICKED, cont);
}