How to check if list has been scrolled to bottom

How-to
1

Hi

I am trying to figure out how to get something like a terms of service type prompt working where you have to “read” everything to enable a button.
I currently have a list which can contain an arbitrary number of messages which the user should read before they proceed.
Then there is a button at the bottom of the screen to confirm they read the messages.
I was wanting to enable the button at the bottom only when the list had been scrolled to the bottom (all the buttons in the list had been shown).
I was looking for something like getting the list scroll position or checking visibility of objects.
But I haven’t found a way of doing that so far.
Any suggestions on how to achieve that?

I have a couple of other ideas if I can’t figure out how to do that:
-Put the confirmation button in the list at the bottom so if the messages go off the screen they have to scroll down to get the confirmation button.
-Get the user to press the messages in the list to acknowledge each one before the confirmation button is enabled.

Thanks

2

Hi,

You can use the LV_EVENT_DRAG_END event and lv_page_on_edge().

While I was testing it I’ve found a bug in library and fixed it in the master branch. So you should update lvgl from GitHub.

Here is my test code:

static void list_event(lv_obj_t * list, lv_event_t e)
{
    if(e == LV_EVENT_DRAG_BEGIN) {
        printf("start\n");
    }
    if(e == LV_EVENT_DRAG_END) {
       printf("end: %d\n", lv_page_on_edge(list, LV_PAGE_EDGE_BOTTOM));
    }
}

...

    lv_obj_t* list = lv_list_create(lv_scr_act(), NULL);
    lv_list_add_btn(list, NULL, "a");
    lv_list_add_btn(list, NULL, "b");
    lv_list_add_btn(list, NULL, "c");
    lv_list_add_btn(list, NULL, "d");
    lv_list_add_btn(list, NULL, "e");
    lv_list_add_btn(list, NULL, "f");
    lv_list_add_btn(list, NULL, "g");
    lv_list_add_btn(list, NULL, "h");
    lv_obj_set_event_cb(list, list_event);
3

Wow thanks for the quick response.

I didn’t even look at lv_page_* so that was my bad.

Should lv_page_on_edge be returning true if the list isn’t scrollable?
For example change your test to only add 1 button.

4

I think it should (pretty sure I wrote it) but I never tested that feature.

5

This is what I am getting.

image

6

Drag it up as if you were scrolling (even though it doesn’t appear visually scrollable) and see if it shows top: 0 bottom: 1. I’m guessing that it thinks it is scrollable by a few pixels.

7

I can’t drag it so I guess it isn’t scrollable. I’m just using the default style.
My debugger isn’t working at the moment so sorry I’m not that helpful.

8

Right, debugging working.
The default size of the page doesn’t match up with the y2 of the scrl you get when you only put one thing in it.

Extra info:

  • Create a list
    list has size 2 * LV_DPI, 3 * LV_DPI
    list’s scrl has size 2 * LV_DPI, (not sure where y comes from but it != 3 * LV_DPI)

  • obj_set_size(list, h, v)
    list has size h, v
    list’s scrl has size h, vertical is unchanged

  • list_add_btn
    list’s scrl vertical dimension changes to match up with the size of the button I guess

So page_on_edge(bottom) is never true without me going and changing sizes.

I’m not very familiar with the lvgl source yet so I’m not really sure if this is intentional.

9

Yes, it’s intentional because it’s really on the top. You can check this

    if(lv_page_get_scrl_height(list) < lv_obj_get_height_fit(list)) {
        /*Always visible*/
    }

Or a more generic method:

    if(e == LV_EVENT_DRAG_END) {
       lv_obj_t * btn = lv_list_get_prev_btn(list, NULL);
       if(btn->coords.y2 <= list->coords.y2) printf("visible\n");
       else  printf("not visible\n");
    }

Instead of “events” you can create an lv_task to periodically check the position of the last element.

10

Cool thanks.
I figured that first example but I haven’t looked into lv_task yet.

As it turns out the requirements were changed so I probably don’t need this functionality anymore.

Thanks for all the help guys.

11

You’re welcome!