Issue #2829588 by damiankloip, Fabianx, markcarver, dawehner, catch: Provide a "url.path.is_front" cache context to allow contributed themes to move global "is_front" variable back

core/core.services.yml

@@ -93,6 +93,11 @@ services:
     arguments: ['@request_stack']
     tags:
       - { name: cache.context }
+  cache_context.url.path.is_front:
+    class: Drupal\Core\Cache\Context\IsFrontPathCacheContext
+    arguments: ['@path.matcher']
+    tags:
+      - { name: cache.context }
   cache_context.url.query_args:
     class: Drupal\Core\Cache\Context\QueryArgsCacheContext
     arguments: ['@request_stack']

core/lib/Drupal/Core/Cache/Context/IsFrontPathCacheContext.php

@@ -0,0 +1,52 @@
+<?php
+
+namespace Drupal\Core\Cache\Context;
+
+use Drupal\Core\Cache\CacheableMetadata;
+use Drupal\Core\Path\PathMatcherInterface;
+
+/**
+ * Defines a cache context for whether the URL is the front page of the site.
+ *
+ * Cache context ID: 'url.path.is_front'.
+ */
+class IsFrontPathCacheContext implements CacheContextInterface {
+
+  /**
+   * @var \Drupal\Core\Path\PathMatcherInterface
+   */
+  protected $pathMatcher;
+
+  /**
+   * Constructs an IsFrontPathCacheContext object.
+   *
+   * @param \Drupal\Core\Path\PathMatcherInterface $path_matcher
+   */
+  public function __construct(PathMatcherInterface $path_matcher) {
+    $this->pathMatcher = $path_matcher;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function getLabel() {
+    return t('Is front page');
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getContext() {
+    return 'is_front.' . (int) $this->pathMatcher->isFrontPage();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getCacheableMetadata() {
+    $metadata = new CacheableMetadata();
+    $metadata->addCacheTags(['config:system.site']);
+    return $metadata;
+  }
+
+}

core/tests/Drupal/Tests/Core/Cache/Context/IsFrontPathCacheContextTest.php

@@ -0,0 +1,46 @@
+<?php
+
+namespace Drupal\Tests\Core\Cache\Context;
+
+use Drupal\Core\Cache\Context\IsFrontPathCacheContext;
+use Drupal\Core\Path\PathMatcherInterface;
+use Drupal\Tests\UnitTestCase;
+
+/**
+ * @coversDefaultClass \Drupal\Core\Cache\Context\IsFrontPathCacheContext
+ * @group Cache
+ */
+class IsFrontPathCacheContextTest extends UnitTestCase {
+
+  /**
+   * @covers ::getContext
+   */
+  public function testGetContextFront() {
+    $cache_context = new IsFrontPathCacheContext($this->createPathMatcher(TRUE)->reveal());
+    $this->assertSame('is_front.1', $cache_context->getContext());
+  }
+
+  /**
+   * @covers ::getContext
+   */
+  public function testGetContextNotFront() {
+    $cache_context = new IsFrontPathCacheContext($this->createPathMatcher(FALSE)->reveal());
+    $this->assertSame('is_front.0', $cache_context->getContext());
+  }
+
+  /**
+   * Creates a PathMatcherInterface prophecy.
+   *
+   * @param bool $is_front
+   *
+   * @return \Prophecy\Prophecy\ObjectProphecy
+   */
+  protected function createPathMatcher($is_front) {
+    $path_matcher = $this->prophesize(PathMatcherInterface::class);
+    $path_matcher->isFrontPage()
+      ->willReturn($is_front);
+
+    return $path_matcher;
+  }
+
+}