Can the Theme Customizer be used to generate static/cachable CSS, and how?

I am currently trying to make a customizable Bootstrap theme for my personal WordPress site, however, I am running into some issues.

Bootstrap can be customized by modifying the SCSS variables, and I am trying to bring that customization into the Theme Customizer by utilizing Leafo’s scssphp to recompile Bootstrap on the fly during the use of the Theme Customizer, and then using Minify to downsize it. My current code for that is as follows:

// /wp-content/themes/bootstrap/functions.php
function bootstrap_generate_css() {
  require_once(__DIR__ . '/vendor/autoload.php');
  $compiler = new Leafo\ScssPhp\Compiler();
  $bootstrap_scss = file_get_contents(__DIR__ . '/vendor/twbs/bootstrap/scss/bootstrap.scss');
  $compiler->setVariables(array(
    'primary' => get_theme_mod('bootstrap_colors_primary', '#007bff')
  ));
  $compiler->setImportPaths(__DIR__ . '/vendor/twbs/bootstrap/scss/');
  $css = $compiler->compile($bootstrap_scss);
  $minifier = new MatthiasMullie\Minify\CSS($css);
  $mincss = $minifier->minify();
  file_put_contents(__DIR__ . '/bootstrap.css', $mincss);
}

There doesn’t seem to be a “correct” way to call a function when a setting is changed, either, so I’ve gotten around that by using a sanitizer_callback:

function bootstrap_sanitize_color($value) {
  bootstrap_generate_css();
  return($value);
}

This has gotten me to the point that I can customize the colors using the Theme Customizer, however, the changes are not shown in the preview frame, and the changes do not take effect until I have published the theme changes, and then manually re-executed the bootstrap_generate_css() function.

Because of the size of bootstrap.css, it is preferable that it is not included into <head>, and it is furthermore preferable that it is not dynamically generated for every front-end pageview due to the fact that it takes 1.8 seconds to compile for me. I do not mind this delay when making changes in the Theme Customizer.

What would be the “proper” way to store the compiled CSS in WordPress, and also have the changes visible inside of the Theme Customizer preview?

1 Answer
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The sanitize_callback method is not a place where the compile process should be happening, this is for sanitizing a control’s setting value being passed.

Initially to accomplish this in BoldGrid Theme Framework, we used the leafo/scssphp library as well, but the lack of live previewing changes, and long wait times for simple changes like changing colors in a color picker, made for a poor user experience in testing. We implemented sass.js shortly after so the scss results could be compiled in JS as users made changes. We also save the compiled result as a theme_mod, and read that if the user is in a customizer preview or viewing a changeset. On the customize_save_after hook we compile and write out the contents using leafo scssphp compiler still, so that the final result is a CSS file with the compiled results that is loaded for the frontend site.

Overall this produced the best results with a minimal amount of work to implement.

Other options that worked were:

For each setting you provide the user access to, manually write some JS so the preview is updated to look as it should by modifying whatever properties need to be modified for each control.
Compile using the same method, and trigger an AJAX call when settings are changed and update the CSS inline.
Trigger a full refresh, and track changes for the temporary states with the changeset UUIDs as part of the filename and load corresponding CSS files as needed.

Tags: CSS theme-customizer

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