MC-16005: Deferred loading / parsing of JS p2

Author: vzabaznov

app/code/Magento/Backend/etc/adminhtml/system.xml

@@ -182,6 +182,10 @@
                     <source_model>Magento\Config\Model\Config\Source\Yesno</source_model>
                     <comment>Minification is not applied in developer mode.</comment>
                 </field>
+                <field id="move_inline_to_bottom" translate="label" type="select" sortOrder="25" showInDefault="1" showInWebsite="1" showInStore="1" canRestore="1">
+                    <label>Move JS code to the bottom of the page</label>
+                    <source_model>Magento\Config\Model\Config\Source\Yesno</source_model>
+                </field>
             </group>
             <group id="css" translate="label" type="text" sortOrder="110" showInDefault="1" showInWebsite="1" showInStore="1">
                 <label>CSS Settings</label>

app/code/Magento/Theme/Controller/Result/JsFooterPlugin.php

@@ -5,34 +5,60 @@
  */
 namespace Magento\Theme\Controller\Result;
 
+use Magento\Framework\App\Config\ScopeConfigInterface;
+use Magento\Store\Model\ScopeInterface;
+use Magento\Framework\App\Response\Http;
+
 /**
- * Plugin for putting messages to cookies
+ * Plugin for putting all js to footer.
  */
 class JsFooterPlugin
 {
+    const XML_PATH_DEV_MOVE_JS_TO_BOTTOM = 'dev/js/move_inline_to_bottom';
+
+    /**
+     * @var ScopeConfigInterface
+     */
+    private $scopeConfig;
+
+    /**
+     * @param ScopeConfigInterface $scopeConfig
+     */
+    public function __construct(ScopeConfigInterface $scopeConfig)
+    {
+        $this->scopeConfig = $scopeConfig;
+    }
+
     /**
-     * Put all javascript to footer before sending the response
+     * Put all javascript to footer before sending the response.
      *
-     * @param \Magento\Framework\App\Response\Http $subject
+     * @param Http $subject
      * @return void
      */
-    public function beforeSendResponse(\Magento\Framework\App\Response\Http $subject)
+    public function beforeSendResponse(Http $subject)
     {
-        $content = $subject->getContent();
-        $script = [];
-        if (strpos($content, '</body') !== false) {
-            $pattern = '#<script[^>]*+(?<!text/x-magento-template.)>.*?</script>#is';
-            $content = preg_replace_callback(
-                $pattern,
-                function ($matchPart) use (&$script) {
-                    $script[] = $matchPart[0];
-                    return '';
-                },
-                $content
-            );
-            $subject->setContent(
-                str_replace('</body', implode("\n", $script) . "\n</body", $content)
-            );
+        if (
+            $this->scopeConfig->isSetFlag(
+            self::XML_PATH_DEV_MOVE_JS_TO_BOTTOM,
+            ScopeInterface::SCOPE_STORE
+            )
+        ) {
+            $content = $subject->getContent();
+            $script = [];
+            if (strpos($content, '</body') !== false) {
+                $pattern = '#<script[^>]*+(?<!text/x-magento-template.)>.*?</script>#is';
+                $content = preg_replace_callback(
+                    $pattern,
+                    function ($matchPart) use (&$script) {
+                        $script[] = $matchPart[0];
+                        return '';
+                    },
+                    $content
+                );
+                $subject->setContent(
+                    str_replace('</body', implode("\n", $script) . "\n</body", $content)
+                );
+            }
         }
     }
 }

app/code/Magento/Theme/etc/config.xml

@@ -66,6 +66,11 @@ Disallow: /*SID=
             <static>
                 <sign>1</sign>
             </static>
+            <dev>
+                <js>
+                    <move_inline_to_bottom>0</move_inline_to_bottom>
+                </js>
+            </dev>
         </dev>
     </default>
 </config>

app/code/Magento/Theme/etc/system.xml

@@ -0,0 +1,19 @@
+<?xml version="1.0"?>
+<!--
+/**
+ * Copyright © Magento, Inc. All rights reserved.
+ * See COPYING.txt for license details.
+ */
+-->
+<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Config:etc/system_file.xsd">
+    <system>
+        <section id="dev" translate="label" type="text" sortOrder="920" showInDefault="1" showInWebsite="1" showInStore="1">
+            <group id="js" translate="label" type="text" sortOrder="100" showInDefault="1" showInWebsite="1" showInStore="1">
+                <field id="move_inline_to_bottom" translate="label" type="select" sortOrder="25" showInDefault="1" showInWebsite="1" showInStore="1" canRestore="1">
+                    <label>Move JS code to the bottom of the page</label>
+                    <source_model>Magento\Config\Model\Config\Source\Yesno</source_model>
+                </field>
+            </group>
+        </section>
+    </system>
+</config>