WordPress Enqueues Class using prefetch

wordpress-prefetch-enqueues-class.php

<?php

/**
 * Class Enqueues
 */
class Enqueues {

	/**
	 * Used for version-ing of enqueued assets
	 *
	 * @var string
	 */
	private $version;

	/**
	 * Enqueues constructor.
	 */
	public function __construct() {

		$this->version = NAMESPACED_THEME_VERSION;
		add_action( 'init', array( $this, 'conditional_hooks' ) );
		add_action('wp_enqueue_scripts', array($this, 'register_theme_assets'), 20);
		add_action('wp_enqueue_scripts', array($this, 'enqueue_theme_assets'), 50);
	}

	/**
	 * WordPress conditional hooks
	 */
	public function conditional_hooks() {

		// This conditional defines the views that should use the prefetch workflow. Other views load scripts and styles as normal.
		if ( is_front_page() ) {
			add_filter( 'script_loader_tag', array( $this, 'filter_script_tag' ), 10, 3 );
			add_filter( 'style_loader_tag', array( $this, 'filter_style_tag' ), 10, 4 );
		}
	}
	
	/**
	 * Register theme stylesheets and scripts
	 */
	public function register_theme_assets() {

		$min = '';
		if (!defined('SCRIPT_DEBUG') || false === SCRIPT_DEBUG) {
			$min = '.min';
		}

		wp_register_script('site-script', get_template_directory_uri() . '/dist/js/site' . $min . '.js', array(), $this->version, 'true');

		wp_register_style('site-style', get_template_directory_uri() . '/dist/css/style.css', array('site-typekit'), $this->version, 'all');

		// View-specific stylesheets (pair up with enqueues below)
		wp_register_style('site-style-home-no-defer', get_template_directory_uri() . '/dist/css/style.home.css', array(), $this->version, 'screen');

	}

	/**
	 * Enqueue registered assets
	 */
	public function enqueue_theme_assets() {

		if ( is_front_page() ) {
			wp_enqueue_style('site-style-home-no-prefetch');
		}

		wp_enqueue_style('site-style');
		wp_enqueue_script('site-script');
	}

	/**
	 * filter_script_tag
	 *
	 * Defer all scripts
	 *
	 * @param string $tag The `<script>` tag for the enqueued script.
	 * @param string $handle The script's registered handle.
	 * @param string $src The script's source URL.
	 *
	 * @return string The script tag.
	 */
	public function filter_script_tag($tag, $handle, $src) {

		return sprintf('<%2$s src="%1$s" defer></%2$s>' . "\n", $src, 'script');
	}

	/**
	 * filter_style_tag
	 *
	 * Prefetch any stylesheet not set to "no-prefetch" in the handle.
	 *
	 * @param string $tag The `<link>` tag for the enqueued script.
	 * @param string $handle The script's registered handle.
	 * @param string $href The style's source URL.
	 * @param string $media The style's media.
	 *
	 * @return string The script tag.
	 */
	public function filter_style_tag($tag, $handle, $href, $media) {
		
		if (false === strpos($handle, 'no-prefetch')) {
			return '<link id="' . $handle . '-css" rel="prefetch" href="' . $href . '" as="style" onload="this.onload=null;this.rel=\'stylesheet\'" media="' . $media . '"><noscript><link rel="stylesheet" href="' . $href . '"></noscript>';
		}

		return $tag;
	}
}

new Enqueues();