How WordPress autosave can save plugin fields?

I have just made a plugin that needed to be autosaved, so i started to dig into the code. I did now that it have to be trigged by some javascript so started to look into the console in Chrome when autosave fires. I then saw that autosave.js looks for a class tags-input in admin-filters.php you can see the comment:

// NOTE: the class "tags-input" allows to include the field in the autosave $_POST (see autosave.js)

So i just added the class tags-input to my hidden fields and it now saves when autosave fires!

Here is my example: <input type="hidden" class="tags-input" name="key" value="value" />

Update:

Now you have to add your data by your own ajax function. (add a new js-file and include in footer. admin_enqueue_scripts)

jQuery(document).ajaxSend(function(e, x, a) {
    var data = 1;
    a.data += '&' + jQuery.param( {test_data: data} );
});

And the saving:

add_action('save_post', 'autosave_save_custom');

function autosave_save_custom( $post ) {
    if( wp_is_post_autosave($post) && isset( $_POST['test_data'] ) ) {
        $test_data = intval( $_POST['test_data'] );
        $post_id = $post->ID;
        update_metadata( $post->post_type, $post_id, 'test_data', $test_data );
    }
}