Method registerLegacyListener

/**
 * Registers a legacy event listener, i.e. a method which accepts individual arguments instead of an AbstractEvent
 * in its arguments. This provides backwards compatibility to Joomla! 3.x-style plugins.
 *
 * This method will register lambda functions (closures) which try to unwrap the arguments of the dispatched Event
 * into old style method arguments and call your on<Something> method with them. The result will be passed back to
 * the Event, as an element into an array argument called 'result'.
 *
 * @param   string  $methodName  The method name to register
 *
 * @return  void
 *
 * @since   4.0.0
 */
protected final function registerLegacyListener(string $methodName)
{
    $this->getDispatcher()->addListener($methodName, function (AbstractEvent $event) use($methodName) {
        // Get the event arguments
        $arguments = $event->getArguments();
        // Extract any old results; they must not be part of the method call.
        $allResults = [];
        if (isset($arguments['result'])) {
            $allResults = $arguments['result'];
            unset($arguments['result']);
        }
        // Convert to indexed array for unpacking.
        $arguments = \array_values($arguments);
        $result = $this->{$methodName}(...$arguments);
        // Ignore null results
        if ($result === null) {
            return;
        }
        // Restore the old results and add the new result from our method call
        $allResults[] = $result;
        $event['result'] = $allResults;
    });
}