How to change a label at runtime from ble char array

How-to
1

Description

I want to change the string of a label at runtime when a new string is written from ble.

What MCU/Processor/Board and compiler are you using?

nRF52 with ST7789

What LVGL version are you using?

8.3.5

What do you want to achieve?

I want to change the text which is displayed on my screen after the text is changed.

What have you tried so far?

I get a new char array from ble and can see it in the serial monitor.

Code to reproduce

The code block(s) should be formatted like:


String message = "Hello World";

void display_screen() {
  
  lv_obj_set_style_bg_color(lv_scr_act(), lv_color_black(), LV_PART_MAIN);

  //Create a white label, set its text and align it to the center
  lv_obj_t * label = lv_label_create(lv_scr_act());
  lv_label_set_text(label,message.c_str());
  //lv_obj_set_style_text_color(lv_scr_act(), lv_color_hex(0xffffff), LV_PART_MAIN);
  lv_obj_set_style_text_color(lv_scr_act(), lv_color_white(), LV_PART_MAIN);
  lv_obj_set_style_text_font(label,&lv_font_montserrat_24,0);
  lv_obj_align(label, LV_ALIGN_CENTER, 0, 0);

}

void ble_written(BLECentral& central, BLECharacteristic& characteristic) {
  char remoteCharArray[22];
  tempLen1 = characteristic.valueLength();
  memcpy(remoteCharArray, characteristic.value(), tempLen1);
  Serial.println(remoteCharArray);
}
2

The simplest would be to make your label global, so you can access (change) the label from anywhere.
You also have a potential problematic code in ble_written function:
When the string from characteristic is for ever reason longer than your buffer (the remoteCharArray), it might result in a crash.

The code would look like the following:

String message = "Hello World";

lv_obj_t*   gtLabel = NULL;      // make the label object global

void display_screen () 
{
  lv_obj_set_style_bg_color (lv_scr_act (), lv_color_black (), LV_PART_MAIN);

  // Create a white label, set its text and align it to the center
  gtLabel = lv_label_create (lv_scr_act ());
  lv_label_set_text (gtLabel, message.c_str ());

  //lv_obj_set_style_text_color(lv_scr_act(), lv_color_hex(0xffffff), LV_PART_MAIN);
  lv_obj_set_style_text_color (lv_scr_act (), lv_color_white (), LV_PART_MAIN);
  lv_obj_set_style_text_font (gtLabel, &lv_font_montserrat_24, 0);
  lv_obj_align (gtLabel, LV_ALIGN_CENTER, 0, 0);
}

#define MAX_REMOTE_CHAR  22

void ble_written (BLECentral& central, BLECharacteristic& characteristic) 
{
  char remoteCharArray[MAX_REMOTE_CHAR];

  remoteCharArray[0] = '\0';                                        
  strncat (remoteCharArray, characteristic.value (), MAX_REMOTE_CHAR - 1);

  if (gtLabel) {
    lv_label_set_text (gtLabel, remoteCharArray);
  }

  Serial.println (remoteCharArray);
}
3

I got this error:

error: invalid conversion from 'const unsigned char*' to 'const char*' [-fpermissive]
strncat(remoteCharArray, characteristic.value(), MAX_REMOTE_CHAR - 1);

and I’m not sure how to make the gtLabel global as the display_screen function is in one .cpp file and the ble_written is in another.

4

I didn’t find any description of BLECharacteristic.value (), so I didn’t know about the return value…
The fix would be (we need a cast):

strncat (remoteCharArray, (const char*) characteristic.value (), MAX_REMOTE_CHAR - 1);

Just a quick fix: within the file where your ble_written is (e.g. after all the includes, before your functions) write:
(The better way would be a proper include file, with all the extern and prototypes for the specific implementation file)

extern lv_obj_t*   gtLabel;

So now the compiler nows the type (how to handle the variable gtLabel).

1 Like
5

This is great, @robekras! Thank you. I am using Arduino, so I need to figure out the extern part, but will try to follow what you recommend by creating a globals.h or something similar. It’s giving me a ‘undefined reference’ error now but that’s probably an Arduino loading thing.

I could also create a function which is in my display.ccp and call it from the ble_written function in the ble.cpp, but this well off the intent of the original post, which you solved. Thank you again.

6

I’m getting something through, but need to deal with shorter strings, because I’m getting some junk on the screen.

7

Ok. Now I am getting both text layered on top of one another. @robekras, it’s not getting rid of the text before.

8

I don’t know if this is the best answer but it seems to work, despite a quick flash of the previous screen text:

void screen_message(char* blemessage)
{

  lv_obj_t * scr1 = lv_obj_create(NULL);
  lv_obj_set_style_bg_color(scr1, lv_color_black(), LV_PART_MAIN);

  lv_obj_t* gtLabel = lv_label_create(scr1);
  
  lv_label_set_text(gtLabel,blemessage);
  lv_obj_set_style_text_color(scr1, lv_color_white(), LV_PART_MAIN);
  lv_obj_set_style_text_font(gtLabel,&lv_font_montserrat_24,0);
  lv_obj_align(gtLabel, LV_ALIGN_CENTER, 0, 0);

  lv_scr_load(scr1);
}
9

Well that works the first time but not the 2nd.

10

@robekras got the solution. My code was wrong. I was calling a 2nd function in the loop causing this behavior.