Issue #3446009 by andileco: Make column selector a select list based on the headers

Closes #3446009

src/Plugin/views/field/ViewsCsvField.php

@@ -7,6 +7,7 @@ use Drupal\Core\Form\FormStateInterface;
 use Drupal\Core\Render\Markup;
 use Drupal\views\Plugin\views\field\FieldPluginBase;
 use Drupal\views\ResultRow;
+use Drupal\views_csv_source\Plugin\views\query\ViewsCsvQuery;
 
 /**
  * Base field handler for views_csv_source.
@@ -66,13 +67,28 @@ class ViewsCsvField extends FieldPluginBase {
    */
   public function buildOptionsForm(&$form, FormStateInterface $form_state) {
     parent::buildOptionsForm($form, $form_state);
+
     $form['key'] = [
       '#title' => $this->t('Column Selector'),
       '#description' => $this->t('Choose a column'),
-      '#type' => 'textfield',
       '#default_value' => $this->options['key'],
       '#required' => TRUE,
     ];
+
+    // Get the query.
+    $query = $this->view->query;
+    assert($query instanceof ViewsCsvQuery, 'Query must be of type ViewsCsvQuery');
+
+    // Ensure the query is of the correct type.
+    if ($headers = $query->getCsvHeader()) {
+      $form['key']['#type'] = 'select';
+      $form['key']['#options'] = array_combine($headers, $headers);
+    }
+    else {
+      // Fall back to text value if CSV has not been selected.
+      $form['key']['#type'] = 'textfield';
+    }
+
     $form['trusted_html'] = [
       '#title' => $this->t('Trusted HTML'),
       '#description' => $this->t('This field is from a trusted source and contains raw HTML markup to render here. Use with caution.'),