Lets code a module

mymodule.info

name = My Cool Module
description = Provides an area handler for views

core = 7.x

dependencies[] = views
dependencies[] = xautoload (>= 7.x-5.0)

mymodule.module

/**
 * Implements hook_views_api().
 */
function mymodule_views_api() {
  return [
    'api' => 3
  ];
}

• Create two files in here to make this a module that we can enable... the .info file and the .module file.
• Views basically needs to know what version of the API we're using.
• Views will now look for a file in our module called modulename.views.inc
• create that file modulename.views.inc

mymodule.views.inc

/**
 * Implements hook_views_data().
 */
function mymodule_views_data() {
  return [
    'views' => [
      'mymodule_area' => [
        'title' => t('My Custom Area'),
        'help' => t('Adds a custom area handler.'),
        'area' => [
          'handler' => 'Drupal\mymodule\Views\Handlers\Area\MyCoolHandler'
        ]
      ]
    ]
  ];
}

• Views will load this file before it tries to invoke any hooks that it defines, so this is a good place to put your hook_views_data hook.
• This hook basically registers any new data that views should know about.
• So if your module defines a whole new table in the database, you could use this hook to tell views about that table and how to access data from it.
• Now if you're creating new database tables that you want Views to know about, then that table should probably be an Entity, so that you get the API's surrounding Entities when working with that data.
• And if you use the Entity API module when creating that Entity, you get the Views integration for free, you don't have to define a hook_views_data for that custom table.
• Keep in mind that Entities don't have to fieldable, and you don't have to provide a UI for your entities, but in my opinion I can't think of a good reason to not make custom tables that you add to the Drupal database not an Entity, if not simply for the CRUD operations.
• We are defining an Area Handler here
• Point out the PSR4 - and explain

Checkout our video on PSR4

Directory structure follows naming convention

• Basically, the way this works is it's going to look for the MyCoolHandler class in this directory structure...
• Lets go create our directory structure for this handler, and the actual class

namespace Drupal\mymodule\Views\Handlers\Area;

/**
 * Defines a new handler class for views.
 */
class MyCoolHandler {

}

Time to write a custom field.

We're going to create a psuedo field

• Like I said earlier, if you setup new tables using the Entity API module, you won't need define every column
• I'm going to create a pseudo field on the node table to show you how to define it yourself.

'node' => [
  'my_custom_field' => [
    'title' => t('My Custom Field'),
    'help' => t('Showing how to write fields at SandCamp'),
    'real field' => 'created',
    'field' => [
      'handler' => 'Drupal\mymodule\Views\Handlers\Field\MyCustomField'
    ],
    'filter' => [
      'handler' => 'Drupal\mymodule\Views\Handlers\Filter\MyCustomFilter'
    ]
  ],
]

• You can see that I specified a field called my_custom_field, which doesn't exist on the node table, so that would immediately give me an SQL error, so I specified the 'real field' key to set what actual column I want to query. Now the SQL won't fail.

namespace Drupal\mymodule\Views\Handlers\Field;

class MyCustomField extends \views_handler_field {

  function render($values) {
    $value = $this->get_value($values);
    return date('Y-m-d', $value);
  }

}

• Lets change how the field is rendered. Lets go in the handler that we defined and override the render method.

Custom Filter

function option_definition() {
  $options = parent::option_definition();

  $options['even_odd'] = array('default' => 'odd');

  return $options;
}

function options_form(&$form, &$form_state) {
  $form['even_odd'] = [
    '#type' => 'radios',
    '#title' => t('Even or Odd'),
    '#default_value' => $this->options['even_odd'],
    '#options' => ['odd' => t('Only odd numbers'), 'even' => t('Just even numbers')],
    '#description' => t('Demonstrating how to create your own options form')
  ];

  parent::options_form($form, $form_state);
}

function admin_summary() {
  return $this->options['even_odd'] == 'even' ? t('Just even numbers') : t('Just odd numbers');
}

Change the query

function query() {
  $this->ensure_my_table();
  $operator = $this->options['even_odd'] == 'even' ? '=' : '>';
  $this->query->add_where_expression($this->options['group'], "($this->table_alias.$this->real_field % 2) $operator 0");
}