Concept of a field
Fields are the data entry points to a web application. Usually, they provide HTML elements and may be responsible for any manipulation of data before it goes into and comes out of the application. The data captured from a single field can be simple or complex.
Assuming we want to create a field for country, all we need is a single HTML element – textfield or select options. An address field, on the other hand, is a collection of discrete data which may include standalone simple fields including a textfield representing postcode (provided by a core module) and a country field (maybe from a custom or contributed module).
In the Drupal world, when considering solutions the first phrase you may hear is, “there’s a module for that!”. However, for the task at hand, “you need a module for that!”. We are now going to write a module which provides this custom country field..
To create a new field in Drupal 7, a number of hooks need to be implemented in a custom module and these include the following:
- hook_field_info() – the field type definition as well as its settings.
- hook_field_schema() – the database schema for the field structure.
- hook_field_widget_info() – the widget types to use for the field type.
- hook_field_formatter_info() – the display of field values.
The task in Drupal 8 is founded on the same principles although the implementation differs. The first thing to remember here is, “there is a class for that!”. A lot of the hard work has been done in Drupal core and all we need to do is extend some classes and override default methods to suit our implementation.
Creating a module
All contrib and custom modules should be placed inside the “modules” folder in your site root. Core-related code is now in “core”. However, it’s also best practice to have “contrib” and “custom” sub-folder in “modules” for clear separation of these types of modules. So we’ll create our “country” folder under modules\custom. What used to go inside *.info file is now in country.yml, so we create that file too and add the following:
name: Country
type: module
description: Defines a simple country field type.
package: Field types
version: VERSION
core: 8.x
dependencies:
- fieldInside your module directory, you need a “src” subdirectory to keep all your class files. To organise things further, you need a “Plugin” sub-folder in “src”. There are different types of plugins e.g. fields, actions, blocks and menus. So you need to create another folder called “Field” inside Plugin and you’ll end up with a directory structure like src\Plugin\Field
Next, we need to define our data type, widget and formatter. These will be in classes with each occupying its own folder again. Let’s take them one by one.
Data Type
The folder is called FieldType, so create it – src\Plugin\Field\FieldType. Create a class, which we shall call “CountryItem”. The file is called CountryItem.php and in it we should have:
class CountryItem {
}How do we define our field type? With the new plugin system, this requires an annotation1 – something like a special comment block to allow core classes know about our field type. Your code should now look like this:
/**
* Plugin implementation of the 'country' field type.
*
* @FieldType(
* id = "country",
* label = @Translation("Country"),
* description = @Translation("Stores the ISO-2 name of a country."),
* category = @Translation("Custom"),
* default_widget = "country_default",
* default_formatter = "country_default"
* )
*/
class CountryItem {
}
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