Custom module development in Drupal 8

name: Drupal 8 Demo
description: 'Drupal 8 demo module'
type: module
core: 8.x

name: Drupal 8 Demo
description: 'Drupal 8 demo module'
type: module
core: 8.x

dependencies:
  - node
test_dependencies:
  - test_module

configure: configure.route

demo.route:
  path: '/demo'
  defaults:
    _controller: '\Drupal\demo\Controller\DemoController::demo'
    _title: 'Demo'
  requirements:
    _permission: 'access content'

demo.route:
  path: '/demo'
  defaults:
    _controller: '\Drupal\demo\Controller\DemoController::demo'
    _title: 'Demo'
  requirements:
    _permission: 'access content'

demo.param:
  path: '/admin/content/demo/{node}'
  defaults:
    _controller: '\Drupal\demo\Controller\DemoController::param'
    _title: 'Demo'
  requirements:
    _permission: 'access content'

<?php

namespace Drupal\demo\Controller;

class DemoController {

  public function demo() {
    return ['#markup' => t('Demo')];
  }

}

services:
  demo.service:
    class: Drupal\demo\DemoService

services:
  demo.service:
    class: Drupal\demo\DemoService
    arguments: ['@entity_type.manager']

<?php

namespace Drupal\demo\Plugin\Block;

use Drupal\Core\Block\BlockBase;

/**
 * Demo block.
 *
 * @Block(
 *   id = "demo_block",
 *   admin_label = @Translation("Demo block"),
 * )
 */
class DemoBlock extends BlockBase {
  
  public function build() {
    return ['#markup' => $this->t('Demo block')];
  }
}