WP Trac #42573: Fix for theme template caching. https://core.trac.wordpress.org/ticket/42573

wp-42573.php

<?php

/**
 * Plugin name: WP Trac #42573: Fix for theme template file caching.
 * Description: Flush the theme file cache each time the admin screens are loaded or with admin bar button option
 * Plugin URI: https://core.trac.wordpress.org/ticket/42573
 * Author: Weston Ruter, XWP.
 * Author URI: https://weston.ruter.net
 */

/* choose to flush cache when admin screens are loaded or with admin bar button click, the option is set with WP_42573_USE_ON_LOAD define configuration.
if the button option is on, success notice will show up after cache flush.
the plugin is running only in the admin screen for the website's client side performance. */


/********* return if it's not admin screen *********/

if ( !is_admin() ) return;



/********* config *********/

define( 'WP_42573_USE_ON_LOAD', false );
define( 'WP_42573_FLUSH_TRANSIENT_QSTRING' , 'transient=flush' );



/********* functions *********/

// add notice
function wp_42573_notice( $text = 'notice!', $status = 'success', $is_dismissible = true ){

	// dismiss button option
	$dismissible = ( $is_dismissible ) ? ' is-dismissible' : '';

	$output = '<div class="notice notice-' . $status . $dismissible . '">
		<p>' . $text . '</p>
	</div>';

	echo $output;
	
}

// success notice action
function wp_42573_action_success_notice(){

	// add transient cache flush success notice
	add_action( 'admin_notices', function(){
		$text = __( 'Succeed to flush transient cache.', 'default' );
		$status = 'success';
		wp_42573_notice( $text, $status );
	});
	
}

// check admin screen
function wp_42573_screen_check( $current_screen = null ){

	// get admin screen object if the parameter is null 
	if ( $current_screen === null ) $current_screen = get_current_screen();

	// match admin screen and return true or false
	return ( in_array( $current_screen->base, array( 'post', 'edit', 'theme-editor' ), true ) );	

}

// add flush transient cache button to the admin bar
function wp_42573_add_items_to_admin_bar( $admin_bar ){

	// return if wrong screen
	if ( ! wp_42573_screen_check() ) return;

	// get current page
	global $pagenow;

	// add flush transient cache button
    $admin_bar->add_menu( array(
        'id'    => 'flush-transient',
        'parent' => 'top-secondary',
        'title' => 'Flush Transient Cache',
        'href'  => wp_nonce_url( network_admin_url( $pagenow . '?' . $_SERVER['QUERY_STRING'] . '&' . WP_42573_FLUSH_TRANSIENT_QSTRING ) ),
        'meta'  => array(
            'title' => __('Flush Transient Cache'),
        ),
    ) );

}

// flush transient cache
function wp_42573_fix_template_caching() { 

	// Only if theme object exists
	$theme = wp_get_theme();
	if ( ! $theme->exists() ) return;

	// flush transient cache
	$cache_hash = md5( $theme->get_theme_root() . '/' . $theme->get_stylesheet() );
	$label = sanitize_key( 'files_' . $cache_hash . '-' . $theme->get( 'Version' ) );
	$transient_key = substr( $label, 0, 29 ) . md5( $label );
	$res = delete_transient( $transient_key );

	// return flush result
	return $res;

}

function wp_42573_current_screen( WP_Screen $current_screen ) { 

	// flush transient cache on load
	if ( WP_42573_USE_ON_LOAD ){

		// if right screen then flush transient cache
		if ( wp_42573_screen_check( $current_screen ) ) wp_42573_fix_template_caching();

	}
	else { // flush transient cache with admin bar button

		// add flush transient cache button to the admin bar if match the correct screens
		add_action('admin_bar_menu', 'wp_42573_add_items_to_admin_bar');

		// return if the url query string not contains the transient cache flush key & value or the cache flush didn't succeed
		if ( strpos( $_SERVER['QUERY_STRING'], WP_42573_FLUSH_TRANSIENT_QSTRING ) === false || ! wp_42573_fix_template_caching() ) return;

		// add success notice action
		wp_42573_action_success_notice();

	}

}



/********* actions *********/

// wp_42573_current_screen function as initial action 
add_action('current_screen', 'wp_42573_current_screen' );

?>