How to get text from button and perform task

How-to
1

LVGL v8.3.0-dev
Visual Studio Code / PlatformIO

Hi,

I am trying to learn LVGL but I am hindered by my lack of knowledge of C++

I have set up a screen and it has four (4) buttons relating to four different actions. With the current code I get the following error.

.. operand types are incompatible ("lv_obj_t *" and "const char *")

I clearly do not understand what “lv_obj_t” is and how it is used.

I would like to know how I can compare the output from “lv_obj_t * label = lv_obj_get_child(btn, 0);” and use it in my code perhaps as an int or String.

This is my callback function;

static void my_event_cb(lv_event_t * event)
{
    lv_obj_t * btn = lv_event_get_target(event);
    lv_obj_t * label = lv_obj_get_child(btn, 0);   //I know this returns something like 1070173192

    if (label == "1070173192"){    //This is what I am trying to do but it is not working for me
      //Turn on light 1
    }
    if (label == "1070173193"){
      //Turn on light 2
    }
    //operand types are incompatible ("lv_obj_t *" and "const char *")
}
2

Well lv_obj_t is a generic lvgl struct and you can’t really compare it with a char *. If you want the labels text you should try lv_label_get_text(label) == "...".

3

Hi @dpunter ,

Here is a quick example, I am not sure of your aims but this should cover everything I hope:

#define NUM_BUTTONS		4

static lv_obj_t		*buttons[NUM_BUTTONS];

static void btn_cb( lv_event_t *event ) {

	lv_obj_t *btn = lv_event_get_target(event);
	lv_obj_t *label = lv_obj_get_child(btn, 0);  // Get a pointer to the label object containing button text

	if( btn == buttons[0] ) {            // Compare the object pointers to discover which object was clicked
		printf( "1 Clicked!\n");
	} else if( btn == buttons[1] ) {
		printf( "2 Clicked!\n");
	} else if( btn == buttons[2] ) {
		printf( "3 Clicked!\n");
	} else if( btn == buttons[3] ) {
		printf( "4 Clicked!\n");
	}
	printf( "Button with label %s Clicked!\n", lv_label_get_text(label));    // Get the label text
}

static void create_screen( lv_obj_t *parent ) {

	lv_obj_t	*btn_txt;

	for( uint8_t i = 0; i < NUM_BUTTONS; i++ ) {

		buttons[i] = lv_btn_create( parent );
		btn_txt = lv_label_create( buttons[i] );
		lv_label_set_text_fmt( btn_txt, "Button %d", i+1  );
		lv_obj_set_pos( buttons[i], 20 + (120 * i), 20 );
		lv_obj_add_event_cb(buttons[i], btn_cb, LV_EVENT_CLICKED, NULL);
	}
}

create_screen( lv_scr_act() );

Kind Regards,

Pete

4

Pete,

Fantastic, that works great.

Now I will go and study the code provided and try to learn something!