Add transformers sequence validator

src/Optimizer/Exception/InvalidTransformersSequence.php

@@ -0,0 +1,28 @@
+<?php
+
+namespace AmpProject\Optimizer\Exception;
+
+use DomainException;
+
+/**
+ * Exception thrown when an invalid transformer sequence is provided
+ * or missed transformer which required by another transformer.
+ *
+ * @package ampproject/amp-toolbox
+ */
+final class InvalidTransformersSequence extends DomainException implements AmpOptimizerException
+{
+
+    /**
+     * @param string $transformerClass
+     * @param array  $missingTags
+     * @return \AmpProject\Optimizer\Exception\InvalidTransformersSequence
+     */
+    public static function forTransformer($transformerClass, array $missingTags)
+    {
+        $tagList = implode(',', $missingTags);
+        $message = "Transformer '{$transformerClass}' must be after transformers that provide '{$tagList}' tags.";
+
+        return new self($message);
+    }
+}

src/Optimizer/TransformationEngine.php

@@ -88,6 +88,8 @@ private function initializeTransformers()
     {
         $this->transformers = [];
 
+        TransformersSequenceValidator::validate($this->configuration->get(Configuration::KEY_TRANSFORMERS));
+
         foreach ($this->configuration->get(Configuration::KEY_TRANSFORMERS) as $transformerClass) {
             $this->transformers[$transformerClass] = new $transformerClass(
                 ...$this->getTransformerDependencies($transformerClass)

src/Optimizer/Transformer/PreloadHeroImage.php

@@ -17,6 +17,7 @@
 use AmpProject\Optimizer\ImageDimensions;
 use AmpProject\Optimizer\Transformer;
 use AmpProject\Optimizer\TransformerConfiguration;
+use AmpProject\Optimizer\TransformerProvides;
 use AmpProject\RequestDestination;
 use AmpProject\Tag;
 use AmpProject\Url;
@@ -42,9 +43,11 @@
  *
  * @package ampproject/amp-toolbox
  */
-final class PreloadHeroImage implements Transformer
+final class PreloadHeroImage implements Transformer, TransformerProvides
 {
 
+    const TAG = 'preload_hero_image';
+
     /**
      * Class(es) to apply to a serverside-rendered image element.
      *
@@ -158,6 +161,11 @@
         $this->configuration = $configuration;
     }
 
+    public static function provides()
+    {
+        return [self::TAG];
+    }
+
     /**
      * Apply transformations to the provided DOM document.
      *

src/Optimizer/Transformer/ServerSideRendering.php

@@ -17,6 +17,7 @@
 use AmpProject\Optimizer\Exception\InvalidArgument;
 use AmpProject\Optimizer\Exception\InvalidHtmlAttribute;
 use AmpProject\Optimizer\Transformer;
+use AmpProject\Optimizer\TransformerRequires;
 use AmpProject\Role;
 use AmpProject\Tag;
 use DOMAttr;
@@ -37,9 +38,11 @@
  *
  * @package ampproject/amp-toolbox
  */
-final class ServerSideRendering implements Transformer
+final class ServerSideRendering implements Transformer, TransformerRequires
 {
 
+    const TAG = 'server_side_rendering';
+
     /**
      * List of layouts that support server-side rendering.
      *
@@ -95,6 +98,11 @@
      */
     private $customCss;
 
+    public static function requires()
+    {
+        return [PreloadHeroImage::TAG];
+    }
+
     /**
      * Apply transformations to the provided DOM document.
      *

src/Optimizer/TransformerProvides.php

@@ -0,0 +1,14 @@
+<?php
+
+namespace AmpProject\Optimizer;
+
+interface TransformerProvides
+{
+
+    /**
+     * Return the collection of dependencies that this transformer provides.
+     *
+     * @return string[]
+     */
+    public static function provides();
+}

src/Optimizer/TransformerRequires.php

@@ -0,0 +1,14 @@
+<?php
+
+namespace AmpProject\Optimizer;
+
+interface TransformerRequires
+{
+
+    /**
+     * Return the collection of dependencies that this transformer requires.
+     *
+     * @return string[]
+     */
+    public static function requires();
+}

src/Optimizer/TransformersSequenceValidator.php

@@ -0,0 +1,29 @@
+<?php
+
+namespace AmpProject\Optimizer;
+
+use AmpProject\Optimizer\Exception\InvalidTransformersSequence;
+
+class TransformersSequenceValidator
+{
+
+    /**
+     * @param string[] $transformers
+     */
+    public static function validate(array $transformers)
+    {
+        $providedTags = [];
+        foreach (array_values($transformers) as $transformerClass) {
+            if ($transformerClass instanceof TransformerRequires) {
+                $missingTags = array_diff($transformerClass::requires(), $providedTags);
+                if ($missingTags) {
+                    throw InvalidTransformersSequence::forTransformer($transformerClass, $missingTags);
+                }
+            }
+
+            if ($transformerClass instanceof TransformerProvides) {
+                $providedTags = array_merge($providedTags, $transformerClass::provides());
+            }
+        }
+    }
+}