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Although hooks are automatically created the first time they are subscribed to, new hooks added to the Core itself need to be properly registered and documented.
Here are the steps you need to follow.
The very first step is just to dispatch the new hook wherever you need it, as explained in Dispatching a Hook.
All known hooks are registered in the ps_hook database table. This table is populated with Core hooks during the installation, by reading the install-dev/data/xml/hook.xml file, then during runtime every time a new hook is used.
Having Core hooks registered is important for two main reasons. First, every hook registered in database will be displayed in the Hook debugger, so it will help developers figure out which hooks are available on a given page. Second, known hooks are displayed in the Positions page, allowing to transplant modules from one hook into another (this is especially useful for display hooks).
Each hook has a name, a title and a definition. They are identified by an additional id attribute in XML, which is the same as its name.
<?xml version="1.0" encoding="UTF-8"?>
<entity_hook>
<fields id="name">
<field name="name"/>
<field name="title"/>
<field name="description"/>
</fields>
<entities>
<hook id="actionMaintenancePageFormSave">
<name>actionMaintenancePageFormSave</name>
<title>Processing Maintenance page form</title>
<description>This hook is called when the Maintenance Page form is processed</description>
</hook>
<hook id="...">
...
</hook>
</entities>
</entity_hook>
</xml>
Add new hooks at the bottom of the list, as hooks are registered sequentially.
The previous step only adds the hook to new shops. We also need to register the hook to shops that upgrade from a previous version.
The last step is to insert the new hooks in the ps_hooks table using the upgrade system. Locate the X.Y.Z.sql file that refers to the PrestaShop version that will include your change: for instance, if the release expected to include this change is 1.7.5.0, locate that file in the upgrade/sql folder from the autoupgrade module.
Once you have located the file, add the corresponding SQL commands to add new hooks:
INSERT IGNORE INTO `PREFIX_hook` (`name`, `title`, `description`) VALUES
('displayAdministrationPageForm', 'Manage Administration Page form fields', 'This hook adds, update or remove fields of the Administration Page form'),
('actionMaintenancePageFormSave', 'Processing Maintenance page form', 'This hook is called when the Maintenance Page form is processed');