Drupal 8 - Add custom cache context - caching

Question

I have the following situation: I want to hide or show some local Tasks (Tabs) based on a field on the current user. Therefore I have implemented a hook_menu_local_tasks_alter() in my_module/my_module.module:
function my_module_menu_local_tasks_alter(&$data, $route_name, \Drupal\Core\Cache\RefinableCacheableDependencyInterface &$cacheability) {
... some logic ...
if ($user->get('field_my_field')->getValue() === 'some value')
unset($data['tabs'][0]['unwanted_tab_0']);
unset($data['tabs'][0]['unwanted_tab_1']);
... some logic ...
}
This works fine but I need to clear the caches if the value of field_my_field changes.
So I found that I need to implement a Cache Context like this in my my_module_menu_local_tasks_alter:
$cacheability
->addCacheTags([
'user.available_regions',
]);
I have defined my Cache Context like this:
my_module/my_module.services.yml:
services:
cache_context.user.available_regions:
class: Drupal\my_module\CacheContext\AvailableRegions
arguments: ['#current_user']
tags:
- { name: cache.context }
my_module/src/CacheCotext/AvailableRegions.php:
<?php
namespace Drupal\content_sharing\CacheContext;
use Drupal\Core\Cache\CacheableMetadata;
use Drupal\Core\Cache\Context\CacheContextInterface;
use Drupal\Core\Session\AccountProxyInterface;
/**
* Class AvailableRegions.
*/
class AvailableRegions implements CacheContextInterface {
protected $currentUser;
/**
* Constructs a new DefaultCacheContext object.
*/
public function __construct(AccountProxyInterface $current_user) {
$this->currentUser = $current_user;
}
/**
* {#inheritdoc}
*/
public static function getLabel() {
return t('Available sub pages.');
}
/**
* {#inheritdoc}
*/
public function getContext() {
// Actual logic of context variation will lie here.
$field_published_sites = $this->get('field_published_sites')->getValue();
$sites = [];
foreach ($field_published_sites as $site) {
$sites[] = $site['target_id'];
}
return implode('|', $sites);
}
/**
* {#inheritdoc}
*/
public function getCacheableMetadata() {
return new CacheableMetadata();
}
}
But every time I change the value of my field field_my_field I still need to clear the caches, so the Context is not working. Could anybody point me in the right direction how to get this solved or how to debug such kind of thigs?

Answer 1

Instead of providing a custom cache context, you should be able to use the default cacheability provided by core. I believe the issue is not so much the creation of the cacheable metadata, its seems that your hook_menu_local_tasks_alter is altering content that doesn't know it now relies on the user. So I believe you need 2 things:
General cache contexts that says 'this menu content now relies on the user', eg. user cache context.
Additional use of cache tag for a specific user to say: 'once this is cached, when this user entity changes, go regenerate the local tasks for this user'.
Note that HOOK_menu_local_tasks_alter provides a helper for cacheability, the third param of $cacheability. Drupal core also provides a mechanism here that allows us to say 'this piece of cache data relies on this other piece of cache data'.
Thus you should be able to do something like:
function my_module_menu_local_tasks_alter(&$data, $route_name, RefinableCacheableDependencyInterface &$cacheability) {
... some logic ...
// We are going to alter content by user.
$cacheability->addCacheableDependency($user);
// Note if you still really need your custom context, you could add it.
// Also note that any user.* contexts should already be covered above.
$cacheability->addCacheContexts(['some_custom_contexts']);
if ($user->get('field_my_field')->getValue() === 'some value')
unset($data['tabs'][0]['unwanted_tab_0']);
unset($data['tabs'][0]['unwanted_tab_1']);
... some logic ...
}