Add custom field type

Ref: https://www.packtpub.com/sites/default/files/9781849516808-Chapter-3.pdf

Create custom field type
1. in .module, implement hook_field_info
    'default_widget'=> 'widget_name'
    which we will implement later in hook_field_widget_info.

   1.1 implement hook_field_validate & hook_field_is_empty as required.

2. in .module, implement hook_field_widget_info to make drupal aware of our custom widget
   $widgets['widget_name'] = array( ...)

3. implement hook_field_widget_form to actually add the form components to the node form

Save and install module.

Use the custom field type
1. Admin > Structure , select content type and manage fields.
Now you can see the custom field type available in the "field type" pull down list .