Drupal 7 Views 3 custom field handler

mymodule.info

dependencies[] = ctools

; Views Handlers
files[] = views/mymodule_handler_handlername.inc

mymodule.module

<?php

/**
 * Implements hook_views_api().
 */
function mymodule_views_api() {
  return array(
    'api' => 3,
    'path' => drupal_get_path('module', 'mymodule') . '/views',
  );
}

mymodule.views.inc

<?php

// This file must be at mymodule/views directory.

/**
 * @file
 * Views definitions for mymodule module.
 */

/**
 * Implements hook_views_data_alter().
 */
function mymodule_views_data_alter(&$data) {
  $data['node']['handlername'] = array(
    'title' => t('Name of my handler'),
    'help' => t('Description of my handler.'),
    'field' => array(
      'handler' => 'mymodule_handler_handlername',
    ),
  );
}

mymodule_handler_handlername.inc

<?php

// This file must be at mymodule/views directory.

/**
 * @file
 * Definition of mymodule_handler_handlername.
 */

/**
 * Description of what my handler does.
 */
class mymodule_handler_handlername extends views_handler_field {
  /**
   * Add some required fields needed on render().
   */
  function construct() {
    parent::construct();
    $this->additional_fields['field_something'] = array(
      'table' => 'field_data_field_something',
      'field' => 'field_something_value',
    );
  }

  /**
   * Loads additional fields.
   */
  function query() {
    $this->ensure_my_table();
    $this->add_additional_fields();
  }

  /**
   * Default options form.
   */
  function option_definition() {
    $options = parent::option_definition();

    $options['option_a'] = array('default' => '');
    $options['option_b'] = array('default' => '');

    return $options;
  }

  /**
   * Creates the form item for the options added.
   */
  function options_form(&$form, &$form_state) {
    parent::options_form($form, $form_state);

    $form['option_a'] = array(
      '#type' => 'textfield',
      '#title' => t('Label'),
      '#default_value' => $this->options['option_a'],
      '#description' => t('Some description.'),
      '#weight' => -10,
    );

    $form['option_b'] = array(
      '#type' => 'textfield',
      '#title' => t('Label'),
      '#default_value' => $this->options['option_b'],
      '#description' => t('Some description.'),
      '#weight' => -9,
    );
  }

  /**
   * Renders the field handler.
   */
  function render($values) {
    // This ensures that even if Views changes the field name within the query, the handler will still work.
    // Inspect the $this->aliases array to find the key of the field that you need.
    $value = $this->get_value($values, 'field_something');

    return format_string('<div class="@option_a" data-tag="@field_something" data-follow_name="@option_b"></div>', array(
      '@field_something' => $value,
      '@option_a' => $this->options['option_a'],
      '@option_b' => $this->options['option_b'],
    ));
  }
}

Good point @robme. Updated to use hook_views_data_alter().

Added https://gist.github.com/juampynr/27df316e7dab89f02a6d18bbff252bc2 to show a sample Views filter handler in Drupal 7.

Thanks for this example. Very clear