How to cache bust a child theme style.css

Question

There may be several aspects to my question but I feel in essence it's easy: how do you make sure changes to a child theme style.css are properly propagated across caches?

I read in a few places that WP should/would be placing the WP version in nnn when the resource is fetched as http://host/wp-content/themes/theme-child/style.css?ver=nnn. In my installation at http://frightanic.com/ I see that the parent theme version is used instead. I have W3 Total Cache and a CDN in place but even if they're disabled a resource like wp-content/themes/frightanic/style.css?ver=3.0.7 is requested. 3.0.7 is the version of the Decode parent theme.

But be that as it may, if I update my child theme CSS without updating either WP or the parent theme at the same time how can I bust it from caches?

Answer 1

@dalbaeb's comment eventually lead to insightful discussions and a feasible solution. Thanks a lot!

I believe the reason my child theme CSS was loaded using 'ver=<parent-theme-version> was because I had followed the WP Codex on child themes 1:1. My functions.php contained this:

add_action('wp_enqueue_scripts', 'theme_enqueue_styles');
function theme_enqueue_styles() {
  wp_enqueue_style('parent-style', get_template_directory_uri() . '/style.css');
}

The code I ended up using was first mentioned in https://wordpress.stackexchange.com/a/182023/30783 but numerous sites on the Internet copy-pasted it (without giving proper credit).

// Making sure your child theme has an independent version and can bust caches: https://wordpress.stackexchange.com/a/182023/30783
// Filter get_stylesheet_uri() to return the parent theme's stylesheet
add_filter('stylesheet_uri', 'use_parent_theme_stylesheet');
// Enqueue this theme's scripts and styles (after parent theme)
add_action('wp_enqueue_scripts', 'my_theme_styles', 20);

function use_parent_theme_stylesheet()
{
    // Use the parent theme's stylesheet
    return get_template_directory_uri() . '/style.css';
}

function my_theme_styles()
{
    $themeVersion = wp_get_theme()->get('Version');

    // Enqueue our style.css with our own version
    wp_enqueue_style('child-theme-style', get_stylesheet_directory_uri() . '/style.css',
        array(), $themeVersion);
}

Update 2017-01-26

The current WP Theme handbook now contains a proper fix:: https://developer.wordpress.org/themes/advanced-topics/child-themes/#3-enqueue-stylesheet

Answer 2

This works well when you add directly in your header.php and refresh the cache everytime you update your css file:

<link rel="stylesheet" href="<?php bloginfo('stylesheet_url'); echo '?' . filemtime( get_stylesheet_directory() . '/style.css'); ?>" type="text/css" media="screen" />

It displays: style.css?324932684 where the number is the time when the file was edited

Answer 3

This may work as well. Using the php rand function:

function theme_enqueue_styles() {

    $parent_style = 'parent-style';

    wp_enqueue_style( $parent_style, get_template_directory_uri() . '/style.css' );
    wp_enqueue_style( 'child-style',
        get_stylesheet_directory_uri() . '/style.css?'.rand(),
        array( $parent_style )
    );
}
add_action( 'wp_enqueue_scripts', 'theme_enqueue_styles' );

Answer 4

WordPress now covers this with a function.

add_action( 'wp_enqueue_scripts', 'my_theme_enqueue_styles' );
function my_theme_enqueue_styles() {
    wp_enqueue_style( 'child-style', get_stylesheet_uri(),
        array( 'parenthandle' ), 
        wp_get_theme()->get('Version') // this only works if you have Version in the style header
    );
}

Uses the Version: 1.0.0 in your actual stylesheet Enque Stylesheet with revision