CakePHP - JsonView渲染是控制器的子功能

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 !