WordPress: How to filter the Page Template that is being rendered.

wp-plugin-filtering-rendering-page-template.php

<?php
/**
 * template_include Filter callback
 * 
 * Include plugin's template if there's one chosen for the rendering page 
 *
 * @param string $template path
 * @return string $template path
 */
public function add_template_filter ( $template ) {

      $user_selected_template = get_page_template_slug($post->ID);

      // We need to check if the selected template
      // is inside the plugin folder
      $file_name = pathinfo($user_selected_template, PATHINFO_BASENAME);
      $template_dir = $this->template_dir;

      if (file_exists($template_dir . $file_name)) {
          $is_plugin = true;
      }

      // If selected template is not empty, it's not the Default Template
      // AND if it's a plugin template, we replace the normal flow to
      // include the selected template
      if ( $user_selected_template != '' AND $is_plugin ) {
          $template = $user_selected_template;
      }       

      return $template;
 }