PHP Class

It is possible to implement a new API event (or plugin) by creating an apposite PHP file, containing a new class that inherits the methods defined by VAPApiEvent.

The class name of a plugin/event varies according to the name used for the PHP file, which has to be concatenated to VikApiEvent.

Therefore, if we have a file named as "foo.php", the related class name will be built as: VikApiEventFoo. Each underscore that may be contained in the name must be removed and the subsequent letter should be capitalized. In example, "foo_bar_baz.php" will contain a class named as: VikApiEventFooBarBaz.

Here's the shell of the class to develop, by assuming a file named as "my_api_plugin.php".

<?php
// No direct access to this file
defined('_JEXEC') or die('Restricted access');

class VAPApiEventMyApiPlugin extends VAPApiEvent
{
    /**
     * The custom action that the event have to perform.
     * This method should not contain any exit or die function, 
     * otherwise the event won't be properly terminated.
     *
     * @param   array           $args      The provided arguments for the event.
     * @param   VAPApiResponse  $response  The response object for admin.
     *
     * @return  mixed           The response to output or the error message (VAPApiError).
     */
    protected function doAction(array $args, VAPApiResponse $response)
    {
        /**
         * @todo mandatory
         */
    }

    /**
     * Returns true if the plugin is always authorised, otherwise false.
     * When this value is false, the system will need to authorise the plugin 
     * through the ACL of the user.
     *
     * @return  boolean
     */
    public function alwaysAllowed()
    {
        /**
         * @todo optional
         */
    }

    /**
     * Returns the description of the plugin.
     *
     * @return  string
     */
    public function getDescription()
    {
        /**
         * @todo optional
         */
    }
}

Where should I place that file?

You should refer to the vikappointments_init_application_framework hook to understand how to support third-party API plugins/events.