Drupal 8 example module to override page template from within a custom module

README.md

Drupal 8 module to demonstrate how to override a page template (twig) from within a custom module.

Made successful from response to stack exchange question: https://drupal.stackexchange.com/questions/233935/override-page-template-and-preserve-administration-navigation-bar

HelloController.php

<?php

// goes in src/Controller/HelloController.php

namespace Drupal\my_module\Controller;

use Drupal\Core\Controller\ControllerBase;

class HelloController extends ControllerBase {
    public function content() {
        return array(
            '#type' => 'markup',
            '#markup' => $this->t('Hello World!')
        );
    }
}

my_module.info.yml

name: Overriding page module
description: "Module with a template that overrides the default page.html.twig"
package: mystuff

type: module
core: 8.x

my_module.module

<?php

function my_module_theme_suggestions_page(array $variables) {
  $path = explode('/', trim(\Drupal::service('path.current')->getPath(), '/'));
  if (isset($path[0]) && $path[0] == 'hello') {
    return 'page--my-module';
  }
} 

function my_module_theme() {
    $theme['page--my-module'] = [
        'template' => 'page--my_module',
    ];

    return $theme;
}

my_module.routing.yml

my_module.content:
  path: '/hello'
  defaults:
    _controller: '\Drupal\my_module\Controller\HelloController::content'
    _title: 'Hello World'
  requirements:
    _permission: 'access content'

page--my_module.html.twig

{# goes in templates/page--my_module.html.twig #}

<h1 style="color:pink;">WHY DOESN'T THIS TEMPLATE GET USED!?</h1>