class-customize-single-postmeta-setting.php

<?php
/**
 * Class Customize_Postmeta_Setting.
 *
 * @package WPSE_257322
 */

namespace WPSE_257322;

/**
 * Class Customize_Postmeta_Setting
 *
 * This is adapted from `WP_Customize_Postmeta_Setting` in the Customize Posts plugin.
 *
 * @link https://github.com/xwp/wp-customize-posts/blob/0.8.5/php/class-wp-customize-postmeta-setting.php
 */
class Customize_Single_Postmeta_Setting extends \WP_Customize_Setting {

	const ID_PATTERN = '#^wpse_257322_postmeta\[(?P<post_id>\d+)]\[(?P<meta_key>.+?)]$#';

	const TYPE = 'wpse_257322_postmeta';

	/**
	 * Type of setting.
	 *
	 * @access public
	 * @var string
	 */
	public $type = self::TYPE;

	/**
	 * Post type.
	 *
	 * @access public
	 * @var string
	 */
	public $post_type;

	/**
	 * Post ID.
	 *
	 * @access public
	 * @var string
	 */
	public $post_id;

	/**
	 * Meta key.
	 *
	 * @access public
	 * @var string
	 */
	public $meta_key;

	/**
	 * Parse setting ID.
	 *
	 * @param string $setting_id Setting ID.
	 * @return array|bool Parsed setting ID or false if parse error.
	 */
	static function parse_setting_id( $setting_id ) {
		if ( ! preg_match( self::ID_PATTERN, $setting_id, $matches ) ) {
			return false;
		}
		$matches['post_id'] = intval( $matches['post_id'] );
		if ( $matches['post_id'] <= 0 ) {
			return false;
		}
		return wp_array_slice_assoc( $matches, array( 'post_id', 'meta_key' ) );
	}

	/**
	 * Create setting ID.
	 *
	 * @param int    $post_id  Post ID.
	 * @param string $meta_key Meta key.
	 * @return string Setting ID.
	 */
	static function create_setting_id( $post_id, $meta_key ) {
		return sprintf( '%s[%d][%s]', self::TYPE, $post_id, $meta_key );
	}

	/**
	 * WP_Customize_Post_Setting constructor.
	 *
	 * @access public
	 *
	 * @param \WP_Customize_Manager $manager Manager.
	 * @param string                $id      Setting ID.
	 * @param array                 $args    Setting args.
	 * @throws \Exception If the ID is in an invalid format.
	 * @global array $wp_meta_keys
	 */
	public function __construct( \WP_Customize_Manager $manager, $id, $args = array() ) {
		global $wp_meta_keys;

		$parsed_setting_id = self::parse_setting_id( $id );
		if ( ! $parsed_setting_id ) {
			throw new \Exception( 'Illegal setting ID format.' );
		}
		$args = array_merge( $args, $parsed_setting_id );
		$post = get_post( $args['post_id'] );
		if ( ! $post ) {
			throw new \Exception( 'Unknown post' );
		}
		$post_type_obj = get_post_type_object( $post->post_type );
		if ( ! $post_type_obj ) {
			throw new \Exception( 'Unrecognized post type: ' . $post->post_type );
		}
		if ( ! registered_meta_key_exists( 'post', $args['meta_key'] ) ) {
			throw new \Exception( 'Unregistered meta key: ' . $args['meta_key'] );
		}
		$registered_meta = $wp_meta_keys['post'][ $args['meta_key'] ];
		if ( empty( $registered_meta['single'] ) ) {
			throw new \Exception( 'Only single postmeta are currently supported.' );
		}
		if ( empty( $args['capability'] ) ) {

			// See \WPSE_257322\filter_map_meta_cap().
			$args['capability'] = sprintf( 'edit_post_meta[%d][%s]', $args['post_id'], $args['meta_key'] );
		}
		parent::__construct( $manager, $id, $args );
	}

	/**
	 * Return a post's setting value.
	 *
	 * @return mixed Meta value.
	 */
	public function value() {
		$single = false; // For the sake of disambiguating empty values in filtering.
		$values = get_post_meta( $this->post_id, $this->meta_key, $single );

		$value = array_shift( $values );
		if ( ! isset( $value ) ) {
			$value = $this->default;
		}
		return $value;
	}

	/**
	 * Sanitize (and validate) an input.
	 *
	 * Note for non-single postmeta, the validation and sanitization callbacks will be applied on each item in the array.
	 *
	 * @see update_metadata()
	 * @access public
	 *
	 * @param string $value The value to sanitize.
	 * @return mixed|\WP_Error|null Sanitized post array or WP_Error if invalid (or null if not WP 4.6-alpha).
	 */
	public function sanitize( $value ) {
		$has_setting_validation = method_exists( 'WP_Customize_Setting', 'validate' );

		$meta_type = 'post';
		$object_id = $this->post_id;
		$meta_key = $this->meta_key;
		$prev_value = ''; // Updating plural meta is not supported.

		/**
		 * Filter a Customize setting value in form.
		 *
		 * @param mixed                 $value  Value of the setting.
		 * @param \WP_Customize_Setting $this   WP_Customize_Setting instance.
		 */
		$value = apply_filters( "customize_sanitize_{$this->id}", $value, $this );

		// Apply sanitization if value didn't fail validation.
		if ( ! is_wp_error( $value ) && ! is_null( $value ) ) {
			$value = sanitize_meta( $meta_key, $value, $meta_type );
		}
		if ( is_wp_error( $value ) ) {
			return $has_setting_validation ? $value : null;
		}

		/** This filter is documented in wp-includes/meta.php */
		$check = apply_filters( "update_{$meta_type}_metadata", null, $object_id, $meta_key, $value, $prev_value );
		if ( null !== $check ) {
			/* translators: placeholder is meta key */
			return $has_setting_validation ? new \WP_Error( 'not_allowed', sprintf( __( 'Update to post meta "%s" blocked.', 'wpse-257322' ), $meta_key ) ) : null;
		}

		return $value;
	}

	/**
	 * Add filter to preview customized value.
	 *
	 * @return bool
	 */
	public function preview() {
		if ( ! $this->is_previewed ) {
			add_filter( 'get_post_metadata', array( $this, 'filter_get_post_meta_to_preview' ), 1000, 4 );
			$this->is_previewed = true;
		}
		return true;
	}

	/**
	 * Filter postmeta to inject customized post meta values.
	 *
	 * @param null|array|string $value     The value get_metadata() should return - a single metadata value, or an array of values.
	 * @param int               $object_id Object ID.
	 * @param string            $meta_key  Meta key.
	 * @param bool              $single    Whether to return only the first value of the specified $meta_key.
	 * @return mixed Value.
	 */
	public function filter_get_post_meta_to_preview( $value, $object_id, $meta_key, $single ) {

		// Short-circuit if a plugin already modified the value.
		if ( null !== $value ) {
			return $value;
		}

		// Short circuit if object is not the post associated with this setting.
		if ( intval( $object_id ) !== $this->post_id ) {
			return $value;
		}

		// Abort if not filtering this key. Note that get_post_meta( $id ) to get all values is not implemented here.
		if ( $this->meta_key !== $meta_key ) {
			return $value;
		}

		// Abort if there is no customized value in the changeset.
		$customized_value = $this->post_value( null );
		if ( null === $customized_value ) {
			return $value;
		}

		$value = $customized_value;
		return $single ? $value : array( $value );
	}

	/**
	 * Update the post.
	 *
	 * Please note that the capability check will have already been done.
	 *
	 * @see \WP_Customize_Setting::save()
	 *
	 * @param string $meta_value The value to update.
	 * @return bool The result of saving the value.
	 */
	protected function update( $meta_value ) {
		$result = update_post_meta( $this->post_id, $this->meta_key, $meta_value );
		return ( false !== $result );
	}
}

wpse-257322.php

<?php
/**
 * Plugin Name: Post/Page Content SHOUTING!
 * Description: Demonstration of a PHP-centric approach to registering page/post-specific sections and controls in the customizer.
 * Plugin URI: http://wordpress.stackexchange.com/questions/257322/customizer-loading-settings-controls-sections-panels-based-on-a-id-page-id
 * Author: Weston Ruter, XWP
 * Author URI: https://make.xwp.co/
 * License: GPLv2+
 */

/*
 * Copyright (c) 2017 XWP (https://make.xwp.co/)
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License, version 2 or, at
 * your discretion, any later version, as published by the Free
 * Software Foundation.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
 */

namespace WPSE_257322;

/**
 * Register meta key.
 */
function register_meta_key() {
	register_meta( 'post', 'should_shout', array(
		'single' => true,
	) );
}
add_action( 'init', __NAMESPACE__ . '\register_meta_key' );

/**
 * Apply filters on the_content to SHOUT!
 *
 * @param string $content Content.
 * @return string Content.
 */
function filter_post_content_to_shout( $content ) {
	if ( get_post_meta( get_the_ID(), 'should_shout', true ) ) {
		$content = strtoupper( $content );
	}
	return $content;
}
add_filter( 'the_content', __NAMESPACE__ . '\filter_post_content_to_shout' );

/**
 * Autoload class.
 *
 * @param string $class Class to autoload.
 */
function load_classes( $class ) {
	if ( __NAMESPACE__ . '\Customize_Single_Postmeta_Setting' === $class ) {
		require_once dirname( __FILE__ ) . '/class-customize-single-postmeta-setting.php';
	}
}
spl_autoload_register( __NAMESPACE__ . '\load_classes' );

/**
 * Filters a dynamic setting's constructor args to ensure they are recognized when updating or publishing a changeset.
 *
 * @param false|array $setting_args The arguments to the WP_Customize_Setting constructor.
 * @param string      $setting_id   ID for dynamic setting, usually coming from `$_POST['customized']`.
 * @return false|array Setting args.
 */
function filter_dynamic_setting_args( $setting_args, $setting_id ) {
	$parsed_setting_id = Customize_Single_Postmeta_Setting::parse_setting_id( $setting_id );
	if ( false === $parsed_setting_id ) {
		return $setting_args;
	}
	$post = get_post( $parsed_setting_id['post_id'] );
	if ( ! $post || ! post_type_exists( $post->post_type ) ) {
		return $setting_args;
	}
	if ( false === $setting_args ) {
		$setting_args = array();
	}

	$setting_args = array_merge( $setting_args, array(
		'type' => 'wpse_257322_postmeta', // See wpse_257322_filter_dynamic_setting_class().
		'transport' => 'refresh',
		'default' => false,
		'sanitize_callback' => function( $value ) {
			return (bool) $value;
		},
	) );
	return $setting_args;
}
add_filter( 'customize_dynamic_setting_args', __NAMESPACE__ . '\filter_dynamic_setting_args', 10, 2 );

/**
 * Filter dynamic setting class.
 *
 * @param string $setting_class WP_Customize_Setting or a subclass.
 * @param string $setting_id    ID for dynamic setting, usually coming from `$_POST['customized']`.
 * @param array  $setting_args  WP_Customize_Setting or a subclass.
 * @return string Setting class.
 */
function filter_dynamic_setting_class( $setting_class, $setting_id, $setting_args ) {
	unset( $setting_id ); // Unused.
	if ( isset( $setting_args['type'] ) && 'wpse_257322_postmeta' === $setting_args['type'] ) {
		$setting_class = __NAMESPACE__ . '\Customize_Single_Postmeta_Setting';
	}
	return $setting_class;
}
add_filter( 'customize_dynamic_setting_class', __NAMESPACE__ . '\filter_dynamic_setting_class', 10, 3 );

/**
 * Map dynamic postmeta capabilities to static capabilities.
 *
 * This is used so that a static capability string can be used in the setting but
 * have it map dynamically at runtime.
 *
 * @param array  $caps    Returns the user's actual capabilities.
 * @param string $cap     Capability name.
 * @param int    $user_id The user ID.
 * @return array Caps.
 */
function filter_map_meta_cap( $caps, $cap, $user_id ) {
	if ( preg_match( '/^edit_post_meta\[\d+/', $cap ) ) {
		$keys = explode( '[', str_replace( ']', '', $cap ) );
		$map_meta_cap_args = array(
			array_shift( $keys ),
			$user_id,
			intval( array_shift( $keys ) ),
			array_shift( $keys ),
		);
		$caps = call_user_func_array( 'map_meta_cap', $map_meta_cap_args );
	}
	return $caps;
}
add_filter( 'map_meta_cap', __NAMESPACE__ . '\filter_map_meta_cap', 10, 3 );

/**
 * Register section, control, and setting for given post.
 *
 * @param int $post_id Post/page ID.
 * @global \WP_Customize_Manager $wp_customize
 */
function register_post_constructs( $post_id ) {
	global $wp_customize;

	$post = get_post( $post_id );
	if ( empty( $post ) ) {
		return;
	}

	$section = $wp_customize->add_section( "wpse-257322-post-meta-{$post->ID}", array(
		/* translators: post title */
		'title' => sprintf( __( 'Meta: %s', 'wpse-257322' ), $post->post_title ),
	) );

	// Used for both the setting and the control.
	$customize_id = Customize_Single_Postmeta_Setting::create_setting_id( $post_id, 'should_shout' );

	$dynamic_settings = $wp_customize->add_dynamic_settings( array( $customize_id ) ); // See wpse_257322_filter_dynamic_setting_args().

	$wp_customize->add_control( $customize_id, array(
		'label' => __( 'SHOUT TEXT?', 'wpse-257322' ),
		'type' => 'checkbox',
		'settings' => $customize_id,
		'section' => $section->id,
	) );

	/**
	 * Dynamically-added settings.
	 *
	 * @var \WP_Customize_Setting[] $dynamic_settings
	 */
	foreach ( $dynamic_settings as $dynamic_setting ) {
		$dynamic_setting->preview();
	}
}

/**
 * Register for controls.
 *
 * @global \WP_Customize_Manager $wp_customize
 */
function customize_register_for_controls() {
	global $wp_customize;
	$queried_object_id = url_to_postid( $wp_customize->get_preview_url() );
	if ( $queried_object_id ) {
		register_post_constructs( $queried_object_id );
	}
}
add_action( 'customize_controls_init', __NAMESPACE__ . '\customize_register_for_controls', 9 ); // Priority 9 so before prepare_controls.

/**
 * Register for preview.
 */
function customize_register_for_preview() {
	if ( ! is_customize_preview() ) {
		return;
	}
	$post_id = get_queried_object_id();
	if ( $post_id ) {
		register_post_constructs( $post_id );
	}
}
add_action( 'wp', __NAMESPACE__ . '\customize_register_for_preview' );

I have problem same @ParhamG.