Pull Request for Issue # .

Summary of Changes

This improvement adds ability to get extension for categories from another field by filling attribute 'extension' of the categories field with the name of the extension type field in format 'field:fields_group_name.field_name', i.e.:

<fields name="params">
    <fieldset name="basic">
        <field
            name="extension_type"
            type="list"
            label="CATEGORIES_EXTENSION_TYPE_LABEL"
            description="CATEGORIES_EXTENSION_TYPE_DESC"
            default="com_content"
            required="true"
        >
            <option value="com_content">CONTENT_CATEGORIES</option>
            <option value="com_contact">CONTACT_CATEGORIES</option>
        </field>
        <field
            name="categories"
            type="category"
            label="CATEGORIES_LABEL"
            description="CATEGORIES_DESC"
            extension="field:params.extension_type"
            default=""
            multiple="true"
        />
    </fieldset>
</fields>

This improvement is backward compatible.

Testing Instructions

1. Make backup of the /modules/mod_breadcrumbs/mod_breadcrumbs.xml file.
2. Replace the /modules/mod_breadcrumbs/mod_breadcrumbs.xml file with this file (removing '.txt')
   mod_breadcrumbs.xml.txt

In the administrator panel:

1. Create some test content categories.
2. Create some test contact categories.
3. Create module of type 'breadcrumbs'.
4. Check the lists of categories for the categories fields. There must be:
   Standard content categories - content categories
   Modal content categories - content categories
   Standard selectable categories - contact categories
   Modal selectable categories - contact categories
5. Change the value of the 'Extension type' field to 'Content categories' and save the module.
6. Check the lists of categories for the categories fields. There must be:
   Standard content categories - content categories
   Modal content categories - content categories
   Standard selectable categories - content categories
   Modal selectable categories - content categories

P.S. You can use any module for testing instead of mod_breadcrumbs. The main is to add the fields for selecting component type and for selecting categories to its xml-file as in the given file mod_breadcrumbs.xml.txt.

Documentation Changes Required

There need to add the description of this feature to the article 'Category form field type' (https://docs.joomla.org/Category_form_field_type):
extension (mandatory) is the name of the extension for which the categories will be retrieved or the reference to the field for selecting extension. For example, to list content categories, use the value 'com_content', to get extension from the value of the field 'extension_type' of the field group 'params' use 'field:params.extension_type'.