Drupal 8.0.0 beta 12. More info: https://www.drupal.org/node/2514176

core/lib/Drupal/Core/Breadcrumb/BreadcrumbBuilderInterface.php

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+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Core\Breadcrumb\BreadcrumbBuilderInterface.
+ */
+
+namespace Drupal\Core\Breadcrumb;
+
+use Drupal\Core\Routing\RouteMatchInterface;
+
+/**
+ * Defines an interface for classes that build breadcrumbs.
+ */
+interface BreadcrumbBuilderInterface {
+
+  /**
+   * Whether this breadcrumb builder should be used to build the breadcrumb.
+   *
+   * @param \Drupal\Core\Routing\RouteMatchInterface $route_match
+   *   The current route match.
+   *
+   * @return bool
+   *   TRUE if this builder should be used or FALSE to let other builders
+   *   decide.
+   */
+  public function applies(RouteMatchInterface $route_match);
+
+  /**
+   * Builds the breadcrumb.
+   *
+   * @param \Drupal\Core\Routing\RouteMatchInterface $route_match
+   *   The current route match.
+   *
+   * @return \Drupal\Core\Link[]
+   *   An array of links for the breadcrumb. Returning an empty array will
+   *   suppress all breadcrumbs.
+   */
+  public function build(RouteMatchInterface $route_match);
+
+}

core/lib/Drupal/Core/Breadcrumb/BreadcrumbManager.php

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+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Core\Breadcrumb\BreadcrumbManager.
+ */
+
+namespace Drupal\Core\Breadcrumb;
+
+use Drupal\Component\Utility\SafeMarkup;
+use Drupal\Core\Extension\ModuleHandlerInterface;
+use Drupal\Core\Routing\RouteMatchInterface;
+
+/**
+ * Provides a breadcrumb manager.
+ *
+ * Can be assigned any number of BreadcrumbBuilderInterface objects by calling
+ * the addBuilder() method. When build() is called it iterates over the objects
+ * in priority order and uses the first one that returns TRUE from
+ * BreadcrumbBuilderInterface::applies() to build the breadcrumbs.
+ *
+ * @see \Drupal\Core\DependencyInjection\Compiler\RegisterBreadcrumbBuilderPass
+ */
+class BreadcrumbManager implements ChainBreadcrumbBuilderInterface {
+
+  /**
+   * The module handler to invoke the alter hook.
+   *
+   * @var \Drupal\Core\Extension\ModuleHandlerInterface
+   */
+  protected $moduleHandler;
+
+  /**
+   * Holds arrays of breadcrumb builders, keyed by priority.
+   *
+   * @var array
+   */
+  protected $builders = array();
+
+  /**
+   * Holds the array of breadcrumb builders sorted by priority.
+   *
+   * Set to NULL if the array needs to be re-calculated.
+   *
+   * @var \Drupal\Core\Breadcrumb\BreadcrumbBuilderInterface[]|null
+   */
+  protected $sortedBuilders;
+
+  /**
+   * Constructs a \Drupal\Core\Breadcrumb\BreadcrumbManager object.
+   *
+   * @param \Drupal\Core\Extension\ModuleHandlerInterface $module_handler
+   *   The module handler.
+   */
+  public function __construct(ModuleHandlerInterface $module_handler) {
+    $this->moduleHandler = $module_handler;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function addBuilder(BreadcrumbBuilderInterface $builder, $priority) {
+    $this->builders[$priority][] = $builder;
+    // Force the builders to be re-sorted.
+    $this->sortedBuilders = NULL;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function applies(RouteMatchInterface $route_match) {
+    return TRUE;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function build(RouteMatchInterface $route_match) {
+    $breadcrumb = array();
+    $context = array('builder' => NULL);
+    // Call the build method of registered breadcrumb builders,
+    // until one of them returns an array.
+    foreach ($this->getSortedBuilders() as $builder) {
+      if (!$builder->applies($route_match)) {
+        // The builder does not apply, so we continue with the other builders.
+        continue;
+      }
+
+      $build = $builder->build($route_match);
+
+      if (is_array($build)) {
+        // The builder returned an array of breadcrumb links.
+        $breadcrumb = $build;
+        $context['builder'] = $builder;
+        break;
+      }
+      else {
+        throw new \UnexpectedValueException(SafeMarkup::format('Invalid breadcrumb returned by !class::build().', array('!class' => get_class($builder))));
+      }
+    }
+    // Allow modules to alter the breadcrumb.
+    $this->moduleHandler->alter('system_breadcrumb', $breadcrumb, $route_match, $context);
+    // Fall back to an empty breadcrumb.
+    return $breadcrumb;
+  }
+
+  /**
+   * Returns the sorted array of breadcrumb builders.
+   *
+   * @return \Drupal\Core\Breadcrumb\BreadcrumbBuilderInterface[]
+   *   An array of breadcrumb builder objects.
+   */
+  protected function getSortedBuilders() {
+    if (!isset($this->sortedBuilders)) {
+      // Sort the builders according to priority.
+      krsort($this->builders);
+      // Merge nested builders from $this->builders into $this->sortedBuilders.
+      $this->sortedBuilders = array();
+      foreach ($this->builders as $builders) {
+        $this->sortedBuilders = array_merge($this->sortedBuilders, $builders);
+      }
+    }
+    return $this->sortedBuilders;
+  }
+
+}

core/lib/Drupal/Core/Breadcrumb/ChainBreadcrumbBuilderInterface.php

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+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Core\Breadcrumb\ChainBreadcrumbBuilderInterface.
+ */
+
+namespace Drupal\Core\Breadcrumb;
+
+/**
+ * Defines an interface a chained service that builds the breadcrumb.
+ */
+interface ChainBreadcrumbBuilderInterface extends BreadcrumbBuilderInterface {
+
+  /**
+   * Adds another breadcrumb builder.
+   *
+   * @param \Drupal\Core\Breadcrumb\BreadcrumbBuilderInterface $builder
+   *   The breadcrumb builder to add.
+   * @param int $priority
+   *   Priority of the breadcrumb builder.
+   */
+  public function addBuilder(BreadcrumbBuilderInterface $builder, $priority);
+
+}