Customizer live preview not working, refreshes but nothing change

Question

Here is my code:

// Hooked to "after_setup_theme"
if(get_theme_mod('enable_vertical_header')) {
    // init vertical header, register scripts, style, settings and output html in template
    $vertical_header = new Vertical_Header;
}

I created a checkbox option "enable_vertical_header" (default value 0) in Customizer to turn on/off the vertical header, when I toggle the option, the live preview area refreshes, but nothing change.

By echo the value of "enable_vertical_header", I see it always null, which make sense if on frontend. But in Customizer, should get_theme_mod not return the real-time value?

---

Update: the Control code

function my_custom_text_settings( $wp_customize ) {
    $wp_customize->add_section( 'my_section' , array(
        'title'      => __('My Section','mytheme'),
        'priority'   => 30,
    ) );

    // Register the settings
    $wp_customize->add_setting( 'enable_vertical_header', array(
        'default'        => 0,
        'type'           => 'theme_mod',
        'capability'     => 'edit_theme_options',
    ) );
    // Add the controls
    $wp_customize->add_control( 'enable_vertical_header', array(
        'label'       => __( 'My custom control', 'translation_domain' ),
        'section'     => 'my_section',
        'settings'    => 'enable_vertical_header',
        'type'        => 'checkbox',
        'priority'    => 10
    ) );

}
add_action( 'customize_register', 'my_custom_text_settings' );

Answer 1

Well, by some experiments, I found the answer.

get_theme_mod won't get you the real-time value when called in 'after_setup_theme' or 'init', it does work in 'customize_preview_init' and 'wp_loaded', so the working code:

add_action('customize_preview_init', 'my_header' );
function my_header() {
        if(get_theme_mod('enable_vertical_header')) {
            $vertical_header = new Vertical_Header;
        }   
}