How to change view's field (Custom-Text field specifically) value based on other field value using hook_view_pre_render?

Question

I have created a taxonomy-term view. I have added two fields in a view: term-id and custom-text field.

I want to check if term-id has children and depending upon the result, I want to set markup inside custom-text field.

I am able to get term-id in hook_views_pre_render with following code, but I am not able to set the value of custom-text field

function mymodule_views_pre_render(\Drupal\views\ViewExecutable $view) {
 if ($view->id() == 'test_view' && $view->current_display == 'block_2') {
    foreach($view->result as $value){
        $tid = $value->_entity->get('tid')->value;
        kint($tid);

        //trying to set value for custom text field, with foll. line of code 
        //'nothing' is the field-name you get for any custom-text field 
        $value->_entity->set('nothing', '<h1>This is my custom mark-up</h1>');
    }
 }
}

I referred the code from these following 2 sources, but they are not dealing with custom-text field:

1. How to change views field value using a hook

2. https://api.drupal.org/comment/62573#comment-62573

Or should I use template_preprocess_views_view_fields(), if yes then how can I get term-id value to check if it has children terms.

Any help will be appreciated.

Answer 1

You can't set a value for "nothing" aka "Global: Custom text" in hook_views_pre_render() because _entity is the entity object. The entity (Taxonomy Term in your case, and Node in my test case) does not contain this field.

Therefore, you need to use template_preprocess_views_view_field

function THENAME_preprocess_views_view_field(&$vars) {
  if(isset($vars['view']) && ($vars['view']->id() == 'test_view') && ($vars['view']->current_display == 'block_2')) {
    if (isset($vars['view']->field) && (count($vars['view']->field) > 0)) {
      if ($vars['field']->field == 'nothing') {
        $tid = $vars['row']->_entity->get('tid')->value; // get tid
        if ($vars['field']->field == 'nothing') {
          $vars['output'] = [
            '#markup' => "<h1>This is my custom mark-up with term id: $tid</h1>",
          ];
        }
      }
    }
  }
}

Answer 2

I referred the code from this source https://drupal.stackexchange.com/a/262718/97205 which was also helpful and figured out following solution:

function THEMENAME_preprocess_views_view_fields(&$variables) {
  $view = $variables['view'];
  $field = $variables['field'];

  if ($view->id() == 'test_view' && $view->current_display == 'block_2') {
    foreach ($view->field as $id => $field) {

      if ($field->field == 'nothing') {
        $tid = $variables['row']->tid; //get tid
        $variables['fields']['nothing']->content = "<h1>Custom markup goes here $tid</h1>";
        //kint($tid);
      }
    }
  }
}

But solution from @NoSssweat is more appropriate.