Introduce hidden field for addition position of paragraph

Looking very much forward to it. :-)

+++ b/src/Plugin/Field/FieldWidget/ParagraphsWidget.php
@@ -1529,7 +1581,26 @@ class ParagraphsWidget extends WidgetBase {
+      if (is_numeric($add_more_delta)) {
+        $field_path = array_merge($submit['element']['#field_parents'], [$submit['element']['#field_name']]);
+
+        // Create place for new paragraph on specific position.
+        static::createCustomPosition(
+          $submit['widget_state'],
+          $form_state,
+          $field_path,
+          intval($add_more_delta)
+        );
+      }
+      else {
+        $submit['widget_state']['items_count']++;
+      }

Would it be easier to just set add_more_delta to $max_delta (or maybe support an explicit NULL argument) to have it added at the end?

I'm also not quite sure yet about the name. createNewPosition() sounds very generic. I'm not quite sure how to call it, maybe something more specific to what we are doing, something involving the deltas...

1. +++ b/src/Plugin/Field/FieldWidget/ParagraphsWidget.php
   @@ -1581,26 +1590,15 @@ class ParagraphsWidget extends WidgetBase {
   +      static::defineDeltaPosition(
   +        $submit['widget_state'],
   +        $form_state,
   +        array_merge($submit['element']['#field_parents'], [$submit['element']['#field_name']]),
   +        NestedArray::getValue(
   +          $submit['element'],
   +          ['add_more', 'add_modal_form_area', 'add_more_delta', '#value']
   +        )
          );
   

   For readability, I personally prefer to keep function calls on a single line and instead move some of those long arguments to a variable on a separate line like $field_path and $add_more_delta (as before)

2. +++ b/tests/src/FunctionalJavascript/ParagraphsExperimentalAddWidgetTest.php
   @@ -225,7 +225,8 @@ class ParagraphsExperimentalAddWidgetTest extends JavascriptTestBase {
    
   -    // Add a nested paragraph with the add widget.
   +    // Add a nested paragraph with the add widget - use negative delta.
   +    $this->getSession()->executeScript("jQuery('input.paragraph-type-add-modal-delta').last().val(-100)");
        $page->find('xpath', '//*[@name="button_add_modal"]')->click();
   

   this is to test that a negative delta is ignored and used as 0?

3. +++ b/src/Plugin/Field/FieldWidget/ParagraphsWidget.php
   @@ -1522,6 +1535,54 @@ class ParagraphsWidget extends WidgetBase {
   +  /**
   +   * Create custom position for new Paragraph.
   +   *
   +   * @param array $widget_state
   +   *   Widget state as reference, so that it can be updated.
   +   * @param \Drupal\Core\Form\FormStateInterface $form_state
   +   *   Form state.
   +   * @param array $field_path
   +   *   Path to paragraph field.
   +   * @param int|mixed $delta
   +   *   Delta position in list of paragraphs, where new paragraph will be added.
   +   */
   +  protected static function defineDeltaPosition(array &$widget_state, FormStateInterface $form_state, array $field_path, $delta) {
   

   what about prepareDeltaPosition()?

   Or: updateWidgetStateDeltas()

   And the description could be something like

   Prepares the widget state to add a new paragraph at a specific position.

   and $delta could be $new_delta or $add_$delta?

   I also think it is int|null, not mixed?

4. +++ b/tests/src/FunctionalJavascript/ParagraphsExperimentalAddWidgetTest.php
   @@ -173,4 +165,219 @@ class ParagraphsExperimentalAddWidgetTest extends JavascriptTestBase {
   +
   +    // Add a Paragraph types.
   +    $this->addParagraphsType('test_1');
   +    $this->addParagraphsType('test_2');
   +    $this->addParagraphsType('test_3');
   +
   +    // Add a text field to the text_paragraph type.
   +    static::fieldUIAddNewField('admin/structure/paragraphs_type/test_1', 'text_1', 'Text', 'text_long', [], []);
   +    static::fieldUIAddNewField('admin/structure/paragraphs_type/test_2', 'text_2', 'Text', 'text_long', [], []);
   +    static::fieldUIAddNewField('admin/structure/paragraphs_type/test_3', 'text_3', 'Text', 'text_long', [], []);
   +
   

   do we really need 3 different text-ish paragraph types for this test? Defining them all with a different text adds quite bit of overhead.

   A nested does make sense to test nested scenarios.

5. +++ b/tests/src/FunctionalJavascript/ParagraphsExperimentalAddWidgetTest.php
   @@ -173,4 +165,219 @@ class ParagraphsExperimentalAddWidgetTest extends JavascriptTestBase {
   +    // There should be 3 paragraphs and last one should be "test_2" type.
   +    $base_paragraphs = $page->findAll('xpath', '//*[contains(@class, "paragraph-type-label") and not(ancestor::div[contains(@class, "paragraphs-nested")])]');
   +    $this->assertEquals(3, count($base_paragraphs), 'There should be 3 paragraphs.');
   +    $this->assertEquals('test_2', $base_paragraphs[2]->getText(), 'Last paragraph should be type "test_2".');
   

   ah, so you use that to make it easier to test the expected order. Fair enough, ignore that part then.

Crosspost...
Awesome, this reads so straight and shares so much code with duplicate. :-)

Agree, simple is beautiful, but the common approach would be to add unique text to each Paragraph. You could then easily check for expected text in items or on output. Is the current approach somehow easier?

Checked code in detail and the delta handling feels perfectly right. Didn't test though.

	+    foreach ($widget_state['original_deltas'] as $current_delta => $original_delta) {
+      $new_delta = $current_delta >= $delta ? $current_delta + 1 : $current_delta;
+

+++ b/src/Plugin/Field/FieldWidget/ParagraphsWidget.php
@@ -1522,6 +1535,54 @@ class ParagraphsWidget extends WidgetBase {
+    $original_deltas_size = count($widget_state['original_deltas']);
...
+    $new_original_deltas[$delta] = $original_deltas_size;

I think i would move this down to where the variable is used, at least after the loop.

@mtodor Will you look into consuming this new API directly with Thunderspecific JS or also try to help us bring the button to the actions first?

Awesome this is ready to unlock the next level. :-)

Thank you for the great work. Committed.