WordPress: Generate @2x image sizes automatically.

functions.php

<?php
/**
 * @2x image sizes
 */
function make_it_retina($file, $width, $height, $crop = false) {
    if ($width || $height) {
        $resized_file = wp_get_image_editor($file);

        if (!is_wp_error($resized_file)) {
            $resized_file->resize($width * 2, $height * 2, $crop);
            $filename = $resized_file->generate_filename($width . 'x' . $height . '@2x');
            $resized_file->save($filename);
        }

        if (!is_wp_error($resized_file) && $resized_file && $info = getimagesize($filename)) {
            $filename = apply_filters('scout_make_it_retina', $filename);
            return [
                'file'      => wp_basename($filename),
                'width'     => $info[0],
                'height'    => $info[1],
            ];
        }
    }

    return false;
}

add_filter('wp_generate_attachment_metadata', function ($metadata, $attachment_id) {
    $attachment = get_post($attachment_id);
    $file = get_attached_file($attachment_id);
    $old_metadata = $metadata;
    foreach ($metadata as $k => $v) {
        if (is_array($v)) {
            foreach ($v as $key => $val) {
                if (is_array($val)) {
                    \App\make_it_retina($file, $val['width'], $val['height'], true);
                }
            }
        }
    }
    return $old_metadata;
}, 10, 2);

add_filter('delete_attachment', function ($attachment_id) {
    $metas = wp_get_attachment_metadata($attachment_id);
    $updir = wp_upload_dir();
    $path = pathinfo($metas['file']);
    $path_name = $path['dirname'];
    $updir = wp_upload_dir();
    foreach ($metas as $meta => $meta_val) {
        if ($meta === 'sizes') {
            foreach ($meta_val as $sizes => $size) {
                $original_filename = $updir['basedir'] . '/' . $path_name . '/' . $size['file'];
                $x2_filename = substr_replace($original_filename, '@2x.', strrpos($original_filename, '.'), strlen('.'));
                if (file_exists($x2_filename)) {
                    unlink($x2_filename);
                }
            }
        }
    }
});