Change LVGL label text in code

KimBrussow · August 27, 2025, 6:57am

This should be real simple but I cannot find it.
I am using LVGL v9.1.0.. On one of my screens is a label on a button .
once that button is clicked it calls a function in platform IO (working great).
If the called function is running, I want to change the button label to .

Points to note: the button already exists as it calls a function.
I have tried to read as much LVGL but it reads like mud.
Would like to get the object by some method and then I can change text.

Any ideas??

UKHeliBob · August 27, 2025, 7:06am

As your topic does not relate directly to the installation or operation of the IDE it has been moved to the Programming category of the forum

embeddedkiddie · August 27, 2025, 8:23am

There are several possible approaches, depending on how you've created your widget and programmed your callbacks.

If the following examples don't solve your problem, please read "How to get the best out of this forum" and post any helpful information (especially any relevant code enclosed in <code /> tags).

Tip:

Use lv_obj_get_child() in the callback function to get the text label.
Or, make the text label object created by lv_label_create() as a global variable.

xfpd · August 27, 2025, 5:46pm

And your post...

embeddedkiddie · August 27, 2025, 9:46pm

Alternatively, in lvgl 9.3 and later you may be able to use lv_obj_set_id() and lv_obj_find_by_id().

github.com/lvgl/lvgl

v9.3/src/core/lv_obj_id_builtin.c

release


      
          void lv_obj_set_id(lv_obj_t * obj, void * id)
          {
              LV_ASSERT_NULL(obj);
              if(obj->id) lv_obj_free_id(obj);
              obj->id = id;
          }

github.com/lvgl/lvgl

v9.3/src/core/lv_obj.c

release


      
          #if LV_USE_OBJ_ID
          void * lv_obj_get_id(const lv_obj_t * obj)
          {
              LV_ASSERT_NULL(obj);
              return obj->id;
          }
          
          lv_obj_t * lv_obj_find_by_id(const lv_obj_t * obj, const void * id)
          {
              LV_LOG_WARN("DEPRECATED: IDs are used only to print the widget trees. To find a widget use obj_name");
          
              if(obj == NULL) obj = lv_display_get_screen_active(NULL);
              if(obj == NULL) return NULL;
          
              uint32_t i;
              uint32_t child_cnt = lv_obj_get_child_count(obj);
              for(i = 0; i < child_cnt; i++) {
                  lv_obj_t * child = obj->spec_attr->children[i];
                  if(lv_obj_id_compare(child->id, id) == 0) return child;
              }

このファイルは切り詰められています。オリジナルを表示

Also you may be able to use lv_obj_find_by_name() in lvgl 9.3 and later.

Finding widgets

(LV_USE_OBJ_NAME must be set to 1 in lv_conf.h)

KimBrussow · August 28, 2025, 12:12am

I am trying to migrate to LVGL 9.3 where there are more features as you described, However getting several compile errors:
lib/ui/eez-flow.cpp: In function 'int32_t eez::flow::anim_callback_get_opacity(lv_anim_t*)':
lib/ui/eez-flow.cpp:3837:113: error: invalid conversion from 'int' to 'lv_part_t' [-fpermissive]

lib/ui/eez-flow.cpp: In function 'void eez::flow::executeLVGLComponent(FlowState*, unsigned int)':
lib/ui/eez-flow.cpp:4019:49: error: too many arguments to function 'void lv_dropdown_set_selected(lv_obj_t*, uint32_t)'
4019 | lv_dropdown_set_selected(target, intValue, LV_ANIM_OFF);
| ^~~~~~~

lib/ui/eez-flow.cpp: In function 'int32_t eez::flow::anim_callback_get_opacity(lv_anim_t*)':
lib/ui/eez-flow.cpp:3837:113: error: invalid conversion from 'int' to 'lv_part_t' [-fpermissive]

lib/ui/eez-flow.cpp: In function 'void eez::flow::objGetStyleOpa(FlowState*, unsigned int, const eez::ListOfAssetsPtreez::Property&, uint32_t)':
lib/ui/eez-flow.cpp:4408:54: error: invalid conversion from 'int' to 'lv_part_t' [-fpermissive]
4408 | int32_t opa = (int32_t)lv_obj_get_style_opa(obj, 0);

Any thoughts?

xfpd · August 28, 2025, 12:47am

Post your error log in a code block.

KimBrussow · August 28, 2025, 2:35am

lib/ui/eez-flow.cpp: In function 'int32_t eez::flow::anim_callback_get_opacity(lv_anim_t*)':
lib/ui/eez-flow.cpp:3837:113: error: invalid conversion from 'int' to 'lv_part_t' [-fpermissive]

 622 | static inline lv_opa_t lv_obj_get_style_opa(const lv_obj_t * obj, lv_part_t part)
      |                                                                   ~~~~~~~~~~^~~~
lib/ui/eez-flow.cpp: In function 'void eez::flow::executeLVGLComponent(FlowState*, unsigned int)':
Compiling .pio\build\esp32dev\lib216\Network\NetworkUdp.cpp.o
lib/ui/eez-flow.cpp:4019:49: error: too many arguments to function 'void lv_dropdown_set_selected(lv_obj_t*, uint32_t)'
 4019 |                         lv_dropdown_set_selected(target, intValue, LV_ANIM_OFF);

Compiling .pio\build\esp32dev\lib995\WiFi\AP.cpp.o 112 | void lv_dropdown_set_selected(lv_obj_t * obj, uint32_t sel_opt); | ^~~~~~~~~~~~~~~~~~~~~~~~ lib/ui/eez-flow.cpp: In function 'void eez::flow::objGetStyleOpa(FlowState*, unsigned int, const eez::ListOfAssetsPtreez::Property&, uint32_t)': lib/ui/eez-flow.cpp:4408:54: error: invalid conversion from 'int' to 'lv_part_t' [-fpermissive] 4408 | int32_t opa = (int32_t)lv_obj_get_style_opa(obj, 0); | ^ | | | int

lib/lvgl/src/core/lv_obj_style_gen.h:622:77: note:   initializing argument 2 of 'lv_opa_t lv_obj_get_style_opa(const lv_obj_t*, lv_part_t)'
  622 | static inline lv_opa_t lv_obj_get_style_opa(const lv_obj_t * obj, lv_part_t part)
      |                                                                   ~~~~~~~~~~^~~~
*** [.pio\build\esp32dev\lib82a\ui\eez-flow.cpp.o] Error 1

embeddedkiddie · August 28, 2025, 3:10am

I have very little experience with EEZ Studio and am completely unfamiliar with EEZ Flow.

However, looking at the error messages, these errors require a fix in EEZ Studio, so I think you should actually request a fix in Issues.

1. Errors related to `lv_part_t`:

lib/ui/eez-flow.cpp: In function 'int32_t eez::flow::anim_callback_get_opacity(lv_anim_t*)':
lib/ui/eez-flow.cpp:3837:113: error: invalid conversion from 'int' to 'lv_part_t' [-fpermissive]

lib/ui/eez-flow.cpp:4408:54: error: invalid conversion from 'int' to 'lv_part_t' [-fpermissive]
4408 | int32_t opa = (int32_t)lv_obj_get_style_opa(obj, 0);

There are no practical issues with 32-bit MCUs, but the Arduino IDE does not seem to allow them.

You may be able to downgrade these to a warning level by adding -fpermissive to your platform.txt.

Alternatively, the original code:

static int32_t anim_callback_get_opacity(lv_anim_t * a) { return lv_obj_get_style_opa((lv_obj_t *)a->user_data, 0); }

should be modifed:

static int32_t anim_callback_get_opacity(lv_anim_t * a) { return lv_obj_get_style_opa((lv_obj_t *)a->user_data, LV_PART_MAIN); }

or

static int32_t anim_callback_get_opacity(lv_anim_t * a) { return lv_obj_get_style_opa((lv_obj_t *)a->user_data, (lv_part_t)0; }

2. Error related to `lv_dropdown_set_selected()`

lib/ui/eez-flow.cpp:4019:49: error: too many arguments to function 'void lv_dropdown_set_selected(lv_obj_t*, uint32_t)'
4019 | lv_dropdown_set_selected(target, intValue, LV_ANIM_OFF);
| ^~~~~~~

Since the API of lv_dropdown_set_selected() has only 2 arguments in LVGL 9.3 and up:

void lv_dropdown_set_selected(lv_obj_t *obj, uint32_t sel_opt)

So the original code:

#if LVGL_VERSION_MAJOR >= 9 && LVGL_VERSION_MINOR >= 3
	lv_dropdown_set_selected(target, intValue, LV_ANIM_OFF);
#else 
	lv_dropdown_set_selected(target, intValue);
#endif

should be:

#if LVGL_VERSION_MAJOR >= 9 && LVGL_VERSION_MINOR >= 3
	lv_dropdown_set_selected(target, intValue);
#else 
	lv_dropdown_set_selected(target, intValue);
#endif

Note that the LVGL API specification changes frequently.

KimBrussow · August 30, 2025, 9:01am

I was finally able to solve this, took a bit but here it is:
USING lvgl v9.3

in the screens.c file I had to add

objects.lblbut_scan = obj;
lv_obj_set_pos(obj, -1, 0);
lv_obj_set_name(obj, “scan_button_label”); << added to set the name

in my main.ccp code had to get reference to the object
Getting the button required
lv_obj_t * objbutton = lv_obj_get_child_by_name(lv_screen_active(), "scan_button");
lv_obj_t * objbuttontxt = lv_obj_get_child_by_name(objbutton, "scan_button_label");

by doing that I was able to
lv_label_set_text(objbuttontxt, “scanning…”);
more code was required but there are 2 steps needed

Topic		Replies	Views
Codebeispiel: LCD Menü & variable ändern Deutsch	1	2444	May 6, 2021
Changing the text on an LCD with a button Programming	2	1646	May 5, 2021
Modificare Testo stampato su LCD Software	9	4638	May 7, 2021
Changing the label of an HTML button (I think) Programming	10	1326	May 5, 2021
Tekst veranderen op een LCD scherm Nederlands	4	2649	May 6, 2021

Change LVGL label text in code

1. Errors related to lv_part_t:

2. Error related to lv_dropdown_set_selected()

Related topics

1. Errors related to `lv_part_t`:

2. Error related to `lv_dropdown_set_selected()`