CakePHP - JsonView rendering is sub function of the controller - php

Question

I'm using Cakephp with json parse extension and the RequestHandler component in order to create Web services using json.
I created a controller named Ws
In this controller I have a named userSubscribe
In order to avoid a lot of If else statements in the next methods, I thought about using a private function inside this controller that will check somes conditions and stop the script normaly BUT ALSO render the json normaly. I just want to do a DRY way !
My question is :
How could I render the json view in a sub function (called by the userSubscribe) ?
To make it clear, here is the style code that would like
public function userSubscribe() {
$this->check();
// Following code only executed if check didn't render the json view
// $data = ...
$code = 1;
$i = 2;
}
private function check() {
$input = &$this->request->data;
if ($_SERVER["CONTENT_TYPE"] != "application/json") { // For example
$result = "KO";
$this->set(compact("result"));
$this->set('_serialize', 'result');
$this->render(); // HERE, it will stop the 'normal behaviour' and render the json with _serialize
}
if (!isset($input["input"])) {
$result = "KO";
$this->set(compact("result"));
$this->set('_serialize', 'result');
$this->render(); // HERE, it will stop the 'normal behaviour' and render the json with _serialize
}
}
It's seems to be quite simple to do, but why can't I find the answer ?!
Thanks in advance for clue/advise/anything !