diff --git a/src/Task/IndexTaskManager.php b/src/Task/IndexTaskManager.php
index 806dc0c..89f5b8a 100644
--- a/src/Task/IndexTaskManager.php
+++ b/src/Task/IndexTaskManager.php
@@ -65,7 +65,7 @@ class IndexTaskManager implements IndexTaskManagerInterface {
         'status' => 1,
         'pages' => array_fill_keys($datasource_ids, 0),
       );
-      $this->state->set($this->getIndexState($index), $index_state);
+      $this->state->set($this->getIndexStateKey($index), $index_state);
     }
   }
 
@@ -109,7 +109,7 @@ class IndexTaskManager implements IndexTaskManagerInterface {
       return NULL;
     }
 
-    $this->state->set($this->getIndexState($index), $index_state);
+    $this->state->set($this->getIndexStateKey($index), $index_state);
     return $added;
   }
 
@@ -219,7 +219,7 @@ class IndexTaskManager implements IndexTaskManagerInterface {
         $this->state->delete($this->getIndexStateKey($index));
       }
       else {
-        $this->state->set($this->getIndexState($index), $index_state);
+        $this->state->set($this->getIndexStateKey($index), $index_state);
       }
     }
   }
@@ -229,7 +229,7 @@ class IndexTaskManager implements IndexTaskManagerInterface {
    */
   public function isTrackingComplete(IndexInterface $index) {
     $index_state = $this->getIndexState($index);
-    return (bool) $index_state['status'];
+    return !$index_state['status'];
   }
 
   /**
diff --git a/src/Task/IndexTaskManagerInterface.php b/src/Task/IndexTaskManagerInterface.php
index e9c1df6..5754bd8 100644
--- a/src/Task/IndexTaskManagerInterface.php
+++ b/src/Task/IndexTaskManagerInterface.php
@@ -14,16 +14,74 @@ use Drupal\search_api\IndexInterface;
  */
 interface IndexTaskManagerInterface {
 
+  /**
+   * Creates a task to start tracking for the given index, or some datasources.
+   *
+   * @param \Drupal\search_api\IndexInterface $index
+   *   The search index.
+   * @param string[]|null $datasource_ids
+   *   (optional) The IDs of specific datasources for which tracking should
+   *   start. Or NULL to start tracking for all datasources.
+   */
   public function startTracking(IndexInterface $index, array $datasource_ids = NULL);
 
+  /**
+   * Adds a single page of items to the tracker.
+   *
+   * @param \Drupal\search_api\IndexInterface $index
+   *   The search index.
+   *
+   * @return int|null
+   *   The number of items tracked. Or NULL if no items were added and tracking
+   *   for this index has been completed. (Note that 0 can also be returned,
+   *   which does not mean that tracking has been completed for the index.)
+   */
   public function addItemsOnce(IndexInterface $index);
 
+  /**
+   * Sets a batch to track all remaining items for the given index.
+   *
+   * @param \Drupal\search_api\IndexInterface $index
+   *   The search index.
+   */
   public function addItemsBatch(IndexInterface $index);
 
+  /**
+   * Tracks all remaining items for the given index.
+   *
+   * Since no kind of batch processing is used, this might run out of memory or
+   * execution time on larger sites.
+   *
+   * @param \Drupal\search_api\IndexInterface $index
+   *   The search index.
+   *
+   * @return mixed
+   */
   public function addItemsAll(IndexInterface $index);
 
+  /**
+   * Stops tracking for the given index.
+   *
+   * Will delete any remaining tracking tasks and also remove all items from
+   * tracking for this index.
+   *
+   * @param \Drupal\search_api\IndexInterface $index
+   *   The search index.
+   * @param string[]|null $datasource_ids
+   *   (optional) The IDs of the datasources for which to stop tracking. Or NULL
+   *   to stop tracking for all datasources.
+   */
   public function stopTracking(IndexInterface $index, array $datasource_ids = NULL);
 
+  /**
+   * Checks whether tracking has already been completed for the given index.
+   *
+   * @param \Drupal\search_api\IndexInterface $index
+   *   The search index.
+   *
+   * @return bool
+   *   TRUE if tracking has been completed for the given index, FALSE otherwise.
+   */
   public function isTrackingComplete(IndexInterface $index);
 
 }