dedepress on "How to get $_GET var on customize.php?"

I'm trying to do something with $_GET vars on customize.php page, but It is quite difficult than I expected.

//======== Example

WP Version: 4.0-beta4
Also tested with 3.9.2, so I think it's not a wp beta version bug.

URL:

http://localhost/wp/wp-admin/customize.php?var=google

Code:

add_action('customize_register', 'testing');

function testing($wp_customize) {

    // this can't get vars and throw a undefined index error,
    // but if I check source code of the page, I can see it's here.
    print_r($_GET[var]); 

    // Notice: the stuff below inside will render properly,
    // in other words, I got the var once,
    // but it cause another fatal problem:
    // the setting which added inside here will not be saved.
    if(!empty($_GET[var]) {
        $wp_customize->add_setting(...);
        $wp_customize->add_control(...);
    }

}

if I hook the testing() function to 'customize_controls_init', I'm able to get $_GET vars and no errors, but it also cause the same problem: the settings will not be saved.

seems it's caused by iframe and refresh, but I tried a variety of ways and still can't make it working well.

Anyone have an point? Thanks in advance.