How to write Controller in GRAV CMS

Question

I created a form and want to save submitted data in back-end

//front-end

{% extends 'partials/base.html.twig' %}

{% block content %}
    <form method="post" action="savedata">
        <input type=text name="data">
        <input type="submit">
    </form>
{% endblock %}

//back-end

function savedata()
{
    echo 'saved data:'.$_POST['data'];
    exit;
}

I expect the code like this

$grav->post('/savedata', DataController->savedata);

But I cannot found any tutorial about custom Controller in documentation

How can I achieve this purpose?