Why do we need to override Config Entity Types?
In short, to customise and meet our dynamic requirements which may not be supported by config entity type definition as a part of @ConfigEntityType annotations in core or contributed modules, we need to override existing config entity types and write some custom code :).
How can we override Config Entity Types?
Entity types use object based annotation unlike array based annotation which is commonly used. Also, Unlike Content Entity Types where every thing is a field, NOTHING is a field for Configuration Entity type.
Every Drupal config entity type is defined as a particular ConfigEntityType Annotation. Entity controller is completely different from the Controller of MVC pattern. To avoid this confusion in terminology Entity Controllers are termed as handlers, each form related to a particular entity type say taxonomy_vocabulary is declared inside handlers with form key.
In this article, will take an example of adding custom form elements to config entity type forms to explain this.
In case we need to add a custom element to any of these forms, we need to follow these 2 steps:
I) Set a new handler class specific to that form.
where, id = configEntityType id, form_type eg: default, reset, delete etc is whichever form we want to override and MyModuleForm is the Class name of new form we'll define in Step II.
Here is the sample code of overriding default form of taxonomy vocabulary.
II) Define the class set in Step I.
{this is optional, we need to do this to keep the new class name same as base class i.e VocabularyForm}.
OR simply,
This override is important because we need to inherit functions and form elements defined in the parent class i.e VocabularyForm and also add additional feature i.e form element without disturbing the core code. This is purely OOPs concept of inheritance.
Config Entities have by default "getThirdPartySetting()" function { Config entities inherit this function if these extend ConfigEntityBase class which implements ConfigEntityInterface interface which in turn extends ThirdPartySettingsInterface interface}. This thirdParty function allows to set and retrieve a value particularly for a module.
else :
and of course inherit the parent save function.
Any Configuration Entity Type (Date format etc) can be overridden similarly, this can be extended to list_builder, access etc. Apart from overriding, we can also add new flavours of entity controller using the above steps.