How should this arc act?

How-to
1

I’m creating an arc as an indicator only. I mapped a custom knob to use. The image does show up, and when I do “basically” the same thing on another arc, the custom image moves as it should (but that’s an input arc). So, this arc does not have an update callback, there’s no need I don’t think.
I have bound the arcs value to another variable. (which used to also work)

The question is, with a simple arc, with a imaged mapped to the knob, will it use/move the custom image, relative with the bound value? The default knob works. But so far, not the mapped knob when I use a bound value. Which kinda makes sense, since there’s nothing to update the knob. But the default knob works, and all I’m (supposed to be) doing is changing the image.
But, as I mentioned, it USED to work. At least I’m pretty sure it did. It broke somewhere down the line, when I got other stuff working…

So I’m wondering, before I spend more time on it, should it work or not? The built in knob works just fine, but I just can’t get the custom image knob to work. The other arc with a callback works, but this one doesn’t really need one.

2

Hi!

Yes it should work with an image as the knob. See the example below where I have two arcs. The right arc has an image as the knob and when moving the left arc both of the arcs move using the same subject for binding the arcs value.

lv_obj_t * screen_arcs_create(void)
{
    static lv_style_t style_knob;
    static bool style_inited = false;

    if (!style_inited) {
        lv_style_init(&style_knob);
        lv_style_set_bg_image_src(&style_knob, img_knob);
        lv_style_set_pad_all(&style_knob, 10);

        style_inited = true;
    }

    lv_obj_t * lv_obj_0 = lv_obj_create(NULL);
    lv_obj_set_style_pad_all(lv_obj_0, 50, 0);

    lv_obj_t * lv_arc_0 = lv_arc_create(lv_obj_0);
    lv_obj_set_width(lv_arc_0, 250);
    lv_obj_set_height(lv_arc_0, 250);
    lv_arc_bind_value(lv_arc_0, &subject_arc_value);
    lv_obj_set_align(lv_arc_0, LV_ALIGN_TOP_MID);
    lv_obj_add_style(lv_arc_0, &style_knob, LV_PART_KNOB);
    
    lv_obj_t * lv_arc_1 = lv_arc_create(lv_obj_0);
    lv_arc_bind_value(lv_arc_1, &subject_arc_value);
    lv_obj_set_align(lv_arc_1, LV_ALIGN_TOP_LEFT);

    return lv_obj_0;
}

This will generate the following result:
arc_example

3

Thank you kindly for that answer.
I thought I had it working like that, it was automatically changing the value similar to what you have. But somewhere down the line, it stopped.
That’s why I had the question, maybe it wasn’t doing what I thought it was.
I’ll have to go back and look at it again. I have it working, but I am manually setting the knob in the call back. Works, but…
Thanks.

4

I’ve been meaning to ask you…

You did a “style init check”, at the top of your example.

Is that normally used? Does lvgl have do something like double init if you’re not careful?
Just curious as to why you added that.

5 
There is a common delete function for all object types. 
It deletes the object and all of its children.

void lv_obj_del(lv_obj_t * obj);

then for example you remove screen and call show it next time = object is created, but styles exist…

6

This is a newer pattern generated by the LVGL Editor to support reusable components.

When more instances of the component are created, the same style is reused instead of being re-initialized each time. LVGL doesn’t automatically guard against double init, so this check ensures the style is set up once and shared across new components, keeping things efficient and consistent.

7

Ok, I think I got that. Makes sense.
Thanks.