Provide a service for handling context-aware plugins

1. +++ b/core/lib/Drupal/Core/Plugin/Context/ContextHandler.php
   @@ -0,0 +1,179 @@
   +   * @param \Drupal\Core\TypedData\DataDefinitionInterface[] $requirements
   

   I'm not sure requirement is the right term here as it might contain optional context as well.
   What about $contexts (described as available contexts) plus $needed_contexts ? I.e., probably the requirements move to ContextDefinitionInterface in the follow up as well.

2. +++ b/core/lib/Drupal/Core/Plugin/Context/ContextHandler.php
   @@ -0,0 +1,179 @@
   +  public function checkRequirements(array $contexts, array $requirements) {
   

   Minor, but I'd have expected this helper to be below getPluginDefinitionsForContexts().

3. +++ b/core/lib/Drupal/Core/Plugin/Context/ContextHandler.php
   @@ -0,0 +1,179 @@
   +  public function getPluginDefinitionsForContexts(array $contexts, array $definitions) {
   

   It was not clear to me what $definitions need be passed here exactly before reading the code. From the method name I'd have expected it to use $this->getDefinitions() internally. If they are passed trough it's actually not getting you anything, it's filtering.

4. +++ b/core/lib/Drupal/Core/Plugin/Context/ContextHandler.php
   @@ -0,0 +1,179 @@
   +        $requirements[$context_id] = new DataDefinition($definition);
   

   That wrongly hardcodes a class; i.e. needs at least a todo to use the right class or migrate to context definition.

5. +++ b/core/lib/Drupal/Core/Plugin/Context/ContextHandler.php
   @@ -0,0 +1,179 @@
   +  public function getValidContexts(array $contexts, DataDefinitionInterface $definition) {
   

   "Valid context" incorrectly makes me think of context that passes validation. Can we call this "getMatchingContext()"

6. +++ b/core/lib/Drupal/Core/Plugin/Context/ContextHandler.php
   @@ -0,0 +1,179 @@
   +   * @param \Drupal\Component\Plugin\Context\ContextInterface[] $contexts
   +   *   An array of contexts to set on the plugin.
   

   Does it have to match the plugin context or not? We need to be specific of what kind of context is expected where, i.e. available or needed context or whatever we end up calling them.

7. +++ b/core/lib/Drupal/Core/Plugin/Context/ContextHandler.php
   @@ -0,0 +1,179 @@
   +    if ($plugin instanceof ConfigurablePluginInterface) {
   +      $configuration = $plugin->getConfiguration();
   +      if (isset($configuration['context_assignments'])) {
   +        $assignments = array_flip($configuration['context_assignments']);
   +      }
   

   Instead of checking for the interface here, can we just pass in the context assignments and let the method apply them?

   -> applyContextMapping() would make sense to me.

8. +++ b/core/lib/Drupal/Core/Plugin/Context/ContextHandler.php
   @@ -0,0 +1,179 @@
   +      $name = isset($assignments[$name]) ? $assignments[$name] : $name;
   +      if (isset($plugin_contexts[$name])) {
   +        $plugin->setContextValue($name, $context->getContextValue());
   

   This should throw an exception if something is in the assignment but it cannot be applied. Silently skipping it even might have security implications, e.g. silently skipping an optional account for which to check access...

9. +++ b/core/tests/Drupal/Tests/Core/Plugin/ContextHandlerTest.php
   @@ -0,0 +1,379 @@
   +  public function providerTestGetAvailablePluginDefinitions() {
   

   Some methods test methods miss a doc block.

10. +++ b/core/tests/Drupal/Tests/Core/Plugin/ContextHandlerTest.php
    @@ -0,0 +1,379 @@
    +interface TestConfigurableContextAwarePluginInterface extends ContextAwarePluginInterface, ConfigurablePluginInterface {
    

    ouch

Thanks, mostly some more minor stuff left:

1. +++ b/core/lib/Drupal/Core/Plugin/Context/ContextHandler.php
   @@ -152,27 +156,33 @@ public function getValidContexts(array $contexts, DataDefinitionInterface $defin
   +   *   An array of available contexts to set on the plugin. They will only be
   

   It's not the array of context to set, it's just the available context - whatever that is :)

2. +++ b/core/lib/Drupal/Core/Plugin/Context/ContextHandler.php
   @@ -152,27 +156,33 @@ public function getValidContexts(array $contexts, DataDefinitionInterface $defin
   +   * @param array $assignments
   

   $mapping ?

3. +++ b/core/lib/Drupal/Core/Plugin/Context/ContextHandler.php
   @@ -152,27 +156,33 @@ public function getValidContexts(array $contexts, DataDefinitionInterface $defin
   +   *   plugin expects 'node', then this map would contain 'entity' => 'node'.
   

   Maybe, improved it to

   expects a context named 'node' just to avoid confusing people that this is not entity:node

+++ b/core/lib/Drupal/Core/Plugin/Context/ContextHandler.php
@@ -0,0 +1,189 @@
+class ContextHandler {

There should be an interface for that - sry for missing that earlier.

Thanks, changes look good. -> RTBC.

Fond one minor remark, but that should not hold us back from committing this.

+++ b/core/lib/Drupal/Core/Plugin/Context/ContextHandlerInterface.php
@@ -0,0 +1,83 @@
+   *   An array of valid contexts.

array of matching contexts

Committed 47e7496 and pushed to 8.x. Thanks!

+++ b/core/modules/block/src/Tests/BlockUiTest.php
@@ -158,6 +158,24 @@ public function testCandidateBlockList() {
+  public function testContextAwareBlocks() {
+    $arguments = array(
+      ':ul_class' => 'block-list',
+      ':li_class' => 'test-context-aware',
+      ':href' => 'admin/structure/block/add/test_context_aware/stark',
+      ':text' => 'Test context-aware block',
+    );
+
+    $this->drupalGet('admin/structure/block');
+    $elements = $this->xpath('//details[@id="edit-category-block-test"]//ul[contains(@class, :ul_class)]/li[contains(@class, :li_class)]/a[contains(@href, :href) and text()=:text]', $arguments);
+    $this->assertTrue(empty($elements), 'The context-aware test block does not appear.');
+    $definition = \Drupal::service('plugin.manager.block')->getDefinition('test_context_aware');
+    $this->assertTrue(!empty($definition), 'The context-aware test block exists.');
+  }

I think the $this->assertTrue(empty($elements), 'The context-aware test block does not appear.'); is more than a little bit brittle but we can fix that when it bites us back.