Statamic srcset modifier for content strings and general src manipulation

SrcsetsModifier.php

<?php

/**
 * 
 * Srcset Modifier
 * This modifier is meant to be used on objects with inaccessible srcsets.
 * 
 * How to use:
 * {{ content | srcsets:1440,1080,960,768,480,150 }}
 * or <img src="{{ the_image | srcsets:1440,1080,960,768,480,150 }}">
 * 
 */

namespace Statamic\Addons\Srcsets;

use Statamic\Extend\Modifier;
use Statamic\API\Asset;

class SrcsetsModifier extends Modifier
{

    /**
     * Stores the content
     * @var  String
     */
    private $content;

    /**
     * Default sizes to generate
     * @var  Array
     */
    private $sizes = [
        960,
        768,
        150
    ];

    /**
     * Modify a value
     *
     * @param mixed  $value    The value to be modified
     * @param array  $params   Any parameters used in the modifier
     * @param array  $context  Contextual values
     * @return mixed
     */
    public function index($content, $params, $context)
    {

        // Store the params as sizes
        if(!empty($params)) {
            $this->sizes = explode(',',$params[0]);
            arsort($this->sizes);
        };

        // generate the srcsets
        return $this->setContent($content)
            ->addSrcSets()
            ->getContent();

    }

    /**
     * Stores the incoming content
     * @return  SrcsetsModifier
     */
    private function setContent ($content) {
        
        // Store the content
        $this->content = $content;

        // Make pipable
        return $this;

    }

    /**
     * Retreives the modified content
     * @return  String
     */
    private function getContent () {
        return $this->content;
    }

    /**
     * Generates the srcsets
     * @return  SrcsetsModifier
     */
    private function addSrcSets () {

        // Find all the data srcs
        if(preg_match('/^(\/|https?:\/\/)/', $this->content))
            $img_urls = [0,[$this->content]];
        else preg_match_all(
                '/<img[^>]+?src="([^"]+)/',
                $this->content, 
                $img_urls
            );

        // if there were matched images, loop through them
        if(!empty($img_urls[1])) foreach ($img_urls[1] as $url) {
            
            // Get the image
            $asset = Asset::wherePath(trim($url,'/'));

            // Start the srcset out empty
            $srcset = "";

            // Loop through the sizes and add them
            // to an srcset string
            foreach ($this->sizes as $size_index => $size)
                $srcset .= ($srcset ? ', ' : '')
                    . $asset->manipulate(['w' => $size]) 
                    . " {$size}w" ;
            
            // generate the correct image
            $this->content = str_replace(
                "{$url}",
                "{$url}\" srcset=\"{$srcset}",
                $this->content
            );

        }

        // Make the method pipable
        return $this;

    }

}