johnReeve/contact_page_meta.php

## contact_page_meta.php
<?php


/**
 * Here is some functionality specific to the contact page.
 *
 */

class Prep_Contact_Page {

	private $current_screen;
	private $textdomain = 'foundationpress';
	private $metabox_id = 'contact_page_metabox';

	public function __construct($current_screen) {
		$this->current_screen = $current_screen;

		add_action( 'add_meta_boxes', array( $this, 'add_meta_box' ) );
		add_action('admin_head', [$this, 'hide_contact_page_meta' ] );
		add_action( 'save_post', array( $this, 'save' ) );
	}

	/**
	 * Adds the meta box container.
	 */
	public function add_meta_box( $post_type ) {
		$post_types = array('page');     //limit meta box to certain post types
		if ( in_array( $post_type, $post_types )) {
			add_meta_box(
				$this->metabox_id
				,__( 'Contact Page Information', $this->textdomain )
				,array( $this, 'render_meta_box_content' )
				,$post_type
				,'advanced'
				,'high'
			);
		}
	}

	/**
	 * Render Meta Box content.
	 *
	 * @param WP_Post $post The post object.
	 */
	public function render_meta_box_content( $post ) {

		// Add an nonce field so we can check for it later.
		wp_nonce_field( $this->metabox_id, $this->metabox_id . '_nonce' );

		// Use get_post_meta to retrieve an existing value from the database.
		$contact_page_address_box = get_post_meta( $post->ID, '_contact_page_address_box', true );

		// Display the form, using the current value.
		echo '<label for="_contact_page_address_box">';
			_e( 'Contact Page: Address', $this->textdomain );
		echo '</label><br>';
		echo '<textarea id="_contact_page_address_box" name="_contact_page_address_box" style="width:100%;min-height:200px">';
		echo esc_html( $contact_page_address_box );
		echo '</textarea>';
	}

	/**
	 * Save the meta when the post is saved.
	 *
	 * @param int $post_id The ID of the post being saved.
	 */
	public function save( $post_id ) {

		/*
		 * We need to verify this came from the our screen and with proper authorization,
		 * because save_post can be triggered at other times.
		 */

		// Check if our nonce is set.
		if ( ! isset( $_POST[$this->metabox_id . '_nonce' ] ) )
			return $post_id;

		$nonce = $_POST[$this->metabox_id . '_nonce' ];

		// Verify that the nonce is valid.
		if ( ! wp_verify_nonce( $nonce, $this->metabox_id ) )
			return $post_id;

		// If this is an autosave, our form has not been submitted,
		//     so we don't want to do anything.
		if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE )
			return $post_id;

		// Check the user's permissions.
		if ( 'page' == $_POST['post_type'] ) {

			if ( ! current_user_can( 'edit_page', $post_id ) )
				return $post_id;

		} else {

			if ( ! current_user_can( 'edit_post', $post_id ) )
				return $post_id;
		}

		/* OK, its safe for us to save the data now. */

		// Sanitize the user input.
		$contact_page_address_box = balanceTags( $_POST['_contact_page_address_box'] );

		// Update the meta field.
		update_post_meta( $post_id, '_contact_page_address_box', $contact_page_address_box );
	}

	/*
	 * Change Meta Box visibility according to Page Template
	 *
	 * Observation: this example swaps the Featured Image meta box visibility
	 *
	 * Usage:
	 * - adjust $('#postimagediv') to your meta box
	 * - change 'page-portfolio.php' to your template's filename
	 * - remove the console.log outputs
	 */

	public function hide_contact_page_meta() {

		if('page' != $this->current_screen->id) return;

		echo <<<HTML
        <script type="text/javascript">
        jQuery(document).ready( function($) {

            var metabox = $("#{$this->metabox_id}"),
            	trigger_value = "templates/page-contact.php";

            /**
             * Adjust visibility of the meta box at startup
            */
            if($('#page_template').val() == trigger_value) {
                // show the meta box
                metabox.show();
            } else {
                // hide your meta box
                metabox.hide();
            }

            /**
             * Live adjustment of the meta box visibility
            */
            $('#page_template').live('change', function(){
                    if($(this).val() == trigger_value) {
                    // show the meta box
                    metabox.show();
                } else {
                    // hide your meta box
                    metabox.hide();
                }

            });
        });
        </script>
HTML;
	}
}


add_action( 'current_screen', function () {
	global $current_screen;
	$contactPage = new Prep_Contact_Page($current_screen);
});
	<?php


	/**
	* Here is some functionality specific to the contact page.
	*
	*/

	class Prep_Contact_Page {

	private $current_screen;
	private $textdomain = 'foundationpress';
	private $metabox_id = 'contact_page_metabox';

	public function __construct($current_screen) {
	$this->current_screen = $current_screen;

	add_action( 'add_meta_boxes', array( $this, 'add_meta_box' ) );
	add_action('admin_head', [$this, 'hide_contact_page_meta' ] );
	add_action( 'save_post', array( $this, 'save' ) );
	}

	/**
	* Adds the meta box container.
	*/
	public function add_meta_box( $post_type ) {
	$post_types = array('page'); //limit meta box to certain post types
	if ( in_array( $post_type, $post_types )) {
	add_meta_box(
	$this->metabox_id
	,__( 'Contact Page Information', $this->textdomain )
	,array( $this, 'render_meta_box_content' )
	,$post_type
	,'advanced'
	,'high'
	);
	}
	}

	/**
	* Render Meta Box content.
	*
	* @param WP_Post $post The post object.
	*/
	public function render_meta_box_content( $post ) {

	// Add an nonce field so we can check for it later.
	wp_nonce_field( $this->metabox_id, $this->metabox_id . '_nonce' );

	// Use get_post_meta to retrieve an existing value from the database.
	$contact_page_address_box = get_post_meta( $post->ID, '_contact_page_address_box', true );

	// Display the form, using the current value.
	echo '<label for="_contact_page_address_box">';
	_e( 'Contact Page: Address', $this->textdomain );
	echo '</label><br>';
	echo '<textarea id="_contact_page_address_box" name="_contact_page_address_box" style="width:100%;min-height:200px">';
	echo esc_html( $contact_page_address_box );
	echo '</textarea>';
	}

	/**
	* Save the meta when the post is saved.
	*
	* @param int $post_id The ID of the post being saved.
	*/
	public function save( $post_id ) {

	/*
	* We need to verify this came from the our screen and with proper authorization,
	* because save_post can be triggered at other times.
	*/

	// Check if our nonce is set.
	if ( ! isset( $_POST[$this->metabox_id . '_nonce' ] ) )
	return $post_id;

	$nonce = $_POST[$this->metabox_id . '_nonce' ];

	// Verify that the nonce is valid.
	if ( ! wp_verify_nonce( $nonce, $this->metabox_id ) )
	return $post_id;

	// If this is an autosave, our form has not been submitted,
	// so we don't want to do anything.
	if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE )
	return $post_id;

	// Check the user's permissions.
	if ( 'page' == $_POST['post_type'] ) {

	if ( ! current_user_can( 'edit_page', $post_id ) )
	return $post_id;

	} else {

	if ( ! current_user_can( 'edit_post', $post_id ) )
	return $post_id;
	}

	/* OK, its safe for us to save the data now. */

	// Sanitize the user input.
	$contact_page_address_box = balanceTags( $_POST['_contact_page_address_box'] );

	// Update the meta field.
	update_post_meta( $post_id, '_contact_page_address_box', $contact_page_address_box );
	}

	/*
	* Change Meta Box visibility according to Page Template
	*
	* Observation: this example swaps the Featured Image meta box visibility
	*
	* Usage:
	* - adjust $('#postimagediv') to your meta box
	* - change 'page-portfolio.php' to your template's filename
	* - remove the console.log outputs
	*/

	public function hide_contact_page_meta() {

	if('page' != $this->current_screen->id) return;

	echo <<<HTML
	<script type="text/javascript">
	jQuery(document).ready( function($) {

	var metabox = $("#{$this->metabox_id}"),
	trigger_value = "templates/page-contact.php";

	/**
	* Adjust visibility of the meta box at startup
	*/
	if($('#page_template').val() == trigger_value) {
	// show the meta box
	metabox.show();
	} else {
	// hide your meta box
	metabox.hide();
	}

	/**
	* Live adjustment of the meta box visibility
	*/
	$('#page_template').live('change', function(){
	if($(this).val() == trigger_value) {
	// show the meta box
	metabox.show();
	} else {
	// hide your meta box
	metabox.hide();
	}

	});
	});
	</script>
	HTML;
	}
	}


	add_action( 'current_screen', function () {
	global $current_screen;
	$contactPage = new Prep_Contact_Page($current_screen);
	});