Basic wordpress customizer functions to change the content & see the changes as they happen

customiser.php

<?php

  add_action( 'customize_register', __CLASS__ . '::custom_content_area' );


  function custom_content_area( $wp_customize ) {

    $wp_customize->add_section('custom_content_area_section', array(
        'title'       => __( 'Content area', 'wordpress' ),
        'description' => __( 'Globally change the content area', 'wordpress' ),
        'priority'    => 60,
        'transport'   => 'postMessage',
    ));

    $wp_customize->add_setting( 'custom_content_area', array(
      'default'     => '',
      'capability'  => 'edit_theme_options',
      'transport'   => 'postMessage'
    ));

    $wp_customize->add_control( 'custom_content_area', array(
      'label'       => __( 'Content replacement tool', 'wordpress' ),
      'description' => __( 'Change the content of the area', 'wordpress' ),
      'type'        => 'textarea',
      'section'     => 'custom_content_area_section',
      'input_attrs'  => [
        'placeholder' => __( 'Change your content by typing here', 'wordpress' ),
      ],
      'settings'    => 'custom_content_area'
    ));

    $wp_customize->selective_refresh->add_partial( 'custom_content_area', array(
        'selector'            => '.content--area',
        'container_inclusive' => true,
        'render_callback'     => 'display_the_custom_content',
        'fallback_refresh'    => false, // Prevents refresh loop when document does not contain .cta-wrap selector. This should be fixed in WP 4.7.
    ) );
  }


  function display_the_custom_content() {
    $replacement_content = get_theme_mod( 'custom_content_area' );
    $default_content     = get_bloginfo( 'description' );

    echo '<div class="content--area">';
    echo ( $replacement_content ) ? $replacement_content : $default_content;
    echo '</div>';
  }