Modify available templates (in dropdown)

Question

I'm developing a WordPress theme with several templates. One template is called "Tabs". If a page with this template assigned has children pages, parts of their contents are shown in tabs on the parent page. Therefore I only want to allow certain templates for the children pages.

Is it possible to modify the list (dropdown) of available templates under certain conditions? Is there a hook to achieve this?

My filter/action should look like this (pseudocode):

if(parent_page->template == 'tabs')
   remove template != 'tab-content'

Answer 1

The list of available templates is generated by get_page_templates. By the end of this function you see a filter that allows you to modify the output. You can use that to change it under certain conditions like this:

add_filter ('theme_page_templates','wpse302574_conditional_templates', 10, 4);

function wpse302574_conditional_templates ($post_templates, $this, $post, $post_type) {
  $parent_id = wp_get_post_parent_id ($post->ID);
  if (get_page_template_slug ($parent_id) == 'slug_of_your_parent_template') {
    // remove unwanted templates from $post_templates
    }
  return $post_templates;
  }

( I didn't test this code, some debugging may be necessary )

Answer 2

You can do something like this

function wpdocs_filter_theme_page_templates( $page_templates, $this, $post ) {

      $parent_id = wp_get_post_parent_id( $post->ID );

      $parent_template = get_page_template_slug( $parent_id );

      if( 'template-parent.php' === $parent_template ){ // compare parent template

        foreach ($page_templates as $key => $value) {

          if( 'template-child.php' != $key ){ // compare child template

            unset( $page_templates[$key] ); // This will unset all the template except default template and child template

          }
        }

      }

      return $page_templates;
  }
  add_filter( 'theme_page_templates', 'wpdocs_filter_theme_page_templates', 20, 3 );

See https://developer.wordpress.org/reference/hooks/theme_page_templates/