Provide a "url.path.is_front" cache context to allow contributed themes to move global "is_front" variable back

Here is a first patch to get things going.

+++ b/core/core.services.yml
@@ -93,6 +93,11 @@ services:
+  cache_context.url.path.front_page:
+    class: Drupal\Core\Cache\Context\PathIsFrontPageCacheContext

Even though the name is misleading the path matcher depends on the current_route context and hence this is a route cache context, not a path one.

So I think a good name would be:

route.is_front_page

And a good name for the class would be:

IsFrontPageRouteCacheContext

Besides that looks great!

Okay, as this depends on the path matcher I think the original is right, lets use:

- cache_context.url.path.is_front

And therefore IsFrontPathCacheContext as class name.

That works for me! Thanks Fabian.

To summarise for others: Route is not technically correct for this as PathMatcherInterface does not rely directly on the (current) route. The default PathMatcher relies on the route match service, but this could be switched out for anything implementing the interface. So we decided keeping it as 'path' makes sense.

EDIT: yes, I messed up the naming of the interdiff file.

+++ b/core/lib/Drupal/Core/Cache/Context/IsFrontPathCacheContext.php
@@ -19,7 +19,7 @@ class PathIsFrontPageCacheContext implements CacheContextInterface  {
+   * Constructs a IsFrontPathCacheContext object.

an

1. +++ b/core/lib/Drupal/Core/Cache/Context/IsFrontPathCacheContext.php
   @@ -0,0 +1,51 @@
   +
   +
   +use Drupal\Core\Cache\CacheableMetadata;
   

   Nitpick alarm: two empty lines :P

2. +++ b/core/lib/Drupal/Core/Cache/Context/IsFrontPathCacheContext.php
   @@ -0,0 +1,51 @@
   +    return 'is_front.' . (int) $this->pathMatcher->isFrontPage();
   

   It is pretty sad that this actually triggers a url generation, see core/lib/Drupal/Core/Path/PathMatcher.php:99, but sure that's not a problem from this particular patch.

3. +++ b/core/lib/Drupal/Core/Cache/Context/IsFrontPathCacheContext.php
   @@ -0,0 +1,51 @@
   +  /**
   +   * {@inheritdoc}
   +   */
   +  public function getCacheableMetadata() {
   +    return new CacheableMetadata();
   +  }
   +
   

   This could have a cache tag for the frontpage configuration.

Thanks Daniel. Always appreciated muchly.

1. Fixed!
2. Mhh, yeah. Not much we can do about that here. PathMatcher should sort itself out a bit if it's a problem
3. Added. However, it's kind of crappy as it ties this class to the default PathMatcher implementation but apparently we do that in tons of other places already. So I guess that ship has sailed..

Thanks Daniel. Always appreciated muchly.

I also appreciate your music recommendations!

Thank you @damiankloip

Re-titling. This needs a change record for the API addition but otherwise looks good.

+++ b/core/lib/Drupal/Core/Cache/Context/IsFrontPathCacheContext.php
@@ -0,0 +1,52 @@
+ * Defines an 'is front' path cache context.

Can we say something like. Defines a cache context for whether the URL is the front page of the site or not? If I didn't already know the history if is_front this would confuse me.

Here is that change. Thanks @catch.

RTBC - we still need a change record though

Thanks Fabian! Small CR here: https://www.drupal.org/node/2830442

#16: Looks great, I adjusted the CR a little bit and added another complete example.

Looks good Fabian!

Committed 46ff3bb and pushed to 8.3.x. Thanks!

diff --git a/core/lib/Drupal/Core/Cache/Context/IsFrontPathCacheContext.php b/core/lib/Drupal/Core/Cache/Context/IsFrontPathCacheContext.php
index 7d091a4..141b13a 100644
--- a/core/lib/Drupal/Core/Cache/Context/IsFrontPathCacheContext.php
+++ b/core/lib/Drupal/Core/Cache/Context/IsFrontPathCacheContext.php
@@ -10,7 +10,7 @@
  *
  * Cache context ID: 'url.path.is_front'.
  */
-class IsFrontPathCacheContext implements CacheContextInterface  {
+class IsFrontPathCacheContext implements CacheContextInterface {
 
   /**
    * @var \Drupal\Core\Path\PathMatcherInterface

Fixed on commit.

Thanks for also updating https://www.drupal.org/docs/8/api/cache-api/cache-contexts!

This now has both a CR and updated docs.

@markcarver the current patch release only receives bug fixes so this isn't eligible for backport.