diff --git a/core/modules/workflows/src/Entity/Workflow.php b/core/modules/workflows/src/Entity/Workflow.php
index d652814..5caf038 100644
--- a/core/modules/workflows/src/Entity/Workflow.php
+++ b/core/modules/workflows/src/Entity/Workflow.php
@@ -112,14 +112,14 @@ public function preSave(EntityStorageInterface $storage) {
   }
 
   /**
-   * {@inheritDoc}
+   * {@inheritdoc}
    */
   public function getTypePlugin() {
     return $this->getPluginCollection()->get($this->type);
   }
 
   /**
-   * {@inheritDoc}
+   * {@inheritdoc}
    */
   public function getPluginCollections() {
     return ['type_settings' => $this->getPluginCollection()];
diff --git a/core/modules/workflows/src/Form/WorkflowStateAddForm.php b/core/modules/workflows/src/Form/WorkflowStateAddForm.php
index 8001d44..faacba6 100644
--- a/core/modules/workflows/src/Form/WorkflowStateAddForm.php
+++ b/core/modules/workflows/src/Form/WorkflowStateAddForm.php
@@ -88,6 +88,7 @@ protected function copyFormValuesToEntity(EntityInterface $entity, array $form,
       $entity->addState($values['id'], $values['label']);
       if (isset($values['type_settings'])) {
         $configuration = $entity->getTypePlugin()->getConfiguration();
+        // @todo move to submitStateConfigurationForm. #2849827.
         $configuration['states'][$values['id']] += $values['type_settings'][$entity->getTypePlugin()->getPluginId()];
         $entity->set('type_settings', $configuration);
       }
diff --git a/core/modules/workflows/src/Plugin/WorkflowTypeBase.php b/core/modules/workflows/src/Plugin/WorkflowTypeBase.php
index 7e8115f..b326e96 100644
--- a/core/modules/workflows/src/Plugin/WorkflowTypeBase.php
+++ b/core/modules/workflows/src/Plugin/WorkflowTypeBase.php
@@ -25,6 +25,11 @@
 abstract class WorkflowTypeBase extends PluginBase implements WorkflowTypeInterface {
 
   /**
+   * A regex for matching a valid state/transition machine name.
+   */
+  const VALID_ID_REGEX = '/[^a-z0-9_]+/';
+
+  /**
    * {@inheritdoc}
    */
   public function __construct(array $configuration, $plugin_id, $plugin_definition) {
@@ -141,10 +146,10 @@ public function getInitialState(WorkflowInterface $workflow) {
    * {@inheritdoc}
    */
   public function addState($state_id, $label) {
-    if (isset($this->configuration['states'][$state_id])) {
+    if ($this->hasState($state_id)) {
       throw new \InvalidArgumentException("The state '$state_id' already exists in workflow.");
     }
-    if (preg_match('/[^a-z0-9_]+/', $state_id)) {
+    if (preg_match(static::VALID_ID_REGEX, $state_id)) {
       throw new \InvalidArgumentException("The state ID '$state_id' must contain only lowercase letters, numbers, and underscores");
     }
     $this->configuration['states'][$state_id] = [
@@ -172,20 +177,7 @@ public function getStates($state_ids = NULL) {
     }
     /** @var \Drupal\workflows\StateInterface[] $states */
     $states = array_combine($state_ids, array_map([$this, 'getState'], $state_ids));
-    if (count($states) > 1) {
-      // Sort states by weight and then label.
-      $weights = $labels = [];
-      foreach ($states as $id => $state) {
-        $weights[$id] = $state->weight();
-        $labels[$id] = $state->label();
-      }
-      array_multisort(
-        $weights, SORT_NUMERIC, SORT_ASC,
-        $labels, SORT_NATURAL, SORT_ASC
-      );
-      $states = array_replace($weights, $states);
-    }
-    return $states;
+    return static::labelWeightMultisort($states);
   }
 
   /**
@@ -208,7 +200,7 @@ public function getState($state_id) {
    * {@inheritdoc}
    */
   public function setStateLabel($state_id, $label) {
-    if (!isset($this->configuration['states'][$state_id])) {
+    if (!$this->hasState($state_id)) {
       throw new \InvalidArgumentException("The state '$state_id' does not exist in workflow.'");
     }
     $this->configuration['states'][$state_id]['label'] = $label;
@@ -219,7 +211,7 @@ public function setStateLabel($state_id, $label) {
    * {@inheritdoc}
    */
   public function setStateWeight($state_id, $weight) {
-    if (!isset($this->configuration['states'][$state_id])) {
+    if (!$this->hasState($state_id)) {
       throw new \InvalidArgumentException("The state '$state_id' does not exist in workflow.'");
     }
     $this->configuration['states'][$state_id]['weight'] = $weight;
@@ -230,7 +222,7 @@ public function setStateWeight($state_id, $weight) {
    * {@inheritdoc}
    */
   public function deleteState($state_id) {
-    if (!isset($this->configuration['states'][$state_id])) {
+    if (!$this->hasState($state_id)) {
       throw new \InvalidArgumentException("The state '$state_id' does not exist in workflow.'");
     }
     if (count($this->configuration['states']) === 1) {
@@ -258,10 +250,10 @@ public function deleteState($state_id) {
    * {@inheritdoc}
    */
   public function addTransition($transition_id, $label, array $from_state_ids, $to_state_id) {
-    if (isset($this->configuration['transitions'][$transition_id])) {
+    if ($this->hasTransition($transition_id)) {
       throw new \InvalidArgumentException("The transition '$transition_id' already exists in workflow.'");
     }
-    if (preg_match('/[^a-z0-9_]+/', $transition_id)) {
+    if (preg_match(static::VALID_ID_REGEX, $transition_id)) {
       throw new \InvalidArgumentException("The transition ID '$transition_id' must contain only lowercase letters, numbers, and underscores.");
     }
 
@@ -284,6 +276,7 @@ public function addTransition($transition_id, $label, array $from_state_ids, $to
       throw $e;
     }
 
+    ksort($this->configuration['transitions']);
     return $this;
   }
 
@@ -296,10 +289,22 @@ public function getTransitions(array $transition_ids = NULL) {
     }
     /** @var \Drupal\workflows\TransitionInterface[] $transitions */
     $transitions = array_combine($transition_ids, array_map([$this, 'getTransition'], $transition_ids));
-    if (count($transitions) > 1) {
-      // Sort transitions by weights and then labels.
+    return static::labelWeightMultisort($transitions);
+  }
+
+  /**
+   * Sort states or transitions by weight and label.
+   *
+   * @param \Drupal\workflows\StateInterface[]|\Drupal\workflows\TransitionInterface[] $objects
+   *   Objects to multi-sort.
+   *
+   * @return \Drupal\workflows\StateInterface[]|\Drupal\workflows\TransitionInterface[]
+   *   An array of sorted transitions or states.
+   */
+  protected static function labelWeightMultisort($objects) {
+    if (count($objects) > 1) {
       $weights = $labels = [];
-      foreach ($transitions as $id => $transition) {
+      foreach ($objects as $id => $transition) {
         $weights[$id] = $transition->weight();
         $labels[$id] = $transition->label();
       }
@@ -307,16 +312,16 @@ public function getTransitions(array $transition_ids = NULL) {
         $weights, SORT_NUMERIC, SORT_ASC,
         $labels, SORT_NATURAL, SORT_ASC
       );
-      $transitions = array_replace($weights, $transitions);
+      return array_replace($weights, $objects);
     }
-    return $transitions;
+    return $objects;
   }
 
   /**
    * {@inheritdoc}
    */
   public function getTransition($transition_id) {
-    if (!isset($this->configuration['transitions'][$transition_id])) {
+    if (!$this->hasTransition($transition_id)) {
       throw new \InvalidArgumentException("The transition '$transition_id' does not exist in workflow.'");
     }
     $transition = new Transition(
@@ -362,7 +367,7 @@ public function getTransitionFromStateToState($from_state_id, $to_state_id) {
    * {@inheritdoc}
    */
   public function hasTransitionFromStateToState($from_state_id, $to_state_id) {
-    return !empty($this->getTransitionIdFromStateToState($from_state_id, $to_state_id));
+    return $this->getTransitionIdFromStateToState($from_state_id, $to_state_id) !== NULL;
   }
 
   /**
@@ -389,12 +394,10 @@ protected function getTransitionIdFromStateToState($from_state_id, $to_state_id)
    * {@inheritdoc}
    */
   public function setTransitionLabel($transition_id, $label) {
-    if (isset($this->configuration['transitions'][$transition_id])) {
-      $this->configuration['transitions'][$transition_id]['label'] = $label;
-    }
-    else {
+    if (!$this->hasTransition($transition_id)) {
       throw new \InvalidArgumentException("The transition '$transition_id' does not exist in workflow.");
     }
+    $this->configuration['transitions'][$transition_id]['label'] = $label;
     return $this;
   }
 
@@ -402,12 +405,10 @@ public function setTransitionLabel($transition_id, $label) {
    * {@inheritdoc}
    */
   public function setTransitionWeight($transition_id, $weight) {
-    if (isset($this->configuration['transitions'][$transition_id])) {
-      $this->configuration['transitions'][$transition_id]['weight'] = $weight;
-    }
-    else {
+    if (!$this->hasTransition($transition_id)) {
       throw new \InvalidArgumentException("The transition '$transition_id' does not exist in workflow.'");
     }
+    $this->configuration['transitions'][$transition_id]['weight'] = $weight;
     return $this;
   }
 
@@ -415,7 +416,7 @@ public function setTransitionWeight($transition_id, $weight) {
    * {@inheritdoc}
    */
   public function setTransitionFromStates($transition_id, array $from_state_ids) {
-    if (!isset($this->configuration['transitions'][$transition_id])) {
+    if (!$this->hasTransition($transition_id)) {
       throw new \InvalidArgumentException("The transition '$transition_id' does not exist in workflow.");
     }
 
@@ -425,9 +426,9 @@ public function setTransitionFromStates($transition_id, array $from_state_ids) {
         throw new \InvalidArgumentException("The state '$from_state_id' does not exist in workflow.");
       }
       if ($this->hasTransitionFromStateToState($from_state_id, $this->configuration['transitions'][$transition_id]['to'])) {
-        $transition = $this->getTransitionFromStateToState($from_state_id, $this->configuration['transitions'][$transition_id]['to']);
-        if ($transition_id !== $transition->id()) {
-          throw new \InvalidArgumentException("The '{$transition->id()}' transition already allows '$from_state_id' to '{$this->configuration['transitions'][$transition_id]['to']}' transitions in workflow.");
+        $existing_transition_id = $this->getTransitionIdFromStateToState($from_state_id, $this->configuration['transitions'][$transition_id]['to']);
+        if ($transition_id !== $existing_transition_id) {
+          throw new \InvalidArgumentException("The '$existing_transition_id' transition already allows '$from_state_id' to '{$this->configuration['transitions'][$transition_id]['to']}' transitions in workflow.");
         }
       }
     }
@@ -437,7 +438,6 @@ public function setTransitionFromStates($transition_id, array $from_state_ids) {
     $from_state_ids = array_values($from_state_ids);
     sort($from_state_ids);
     $this->configuration['transitions'][$transition_id]['from'] = $from_state_ids;
-    ksort($this->configuration['transitions']);
 
     return $this;
   }
@@ -446,12 +446,10 @@ public function setTransitionFromStates($transition_id, array $from_state_ids) {
    * {@inheritdoc}
    */
   public function deleteTransition($transition_id) {
-    if (isset($this->configuration['transitions'][$transition_id])) {
-      unset($this->configuration['transitions'][$transition_id]);
-    }
-    else {
+    if (!$this->hasTransition($transition_id)) {
       throw new \InvalidArgumentException("The transition '$transition_id' does not exist in workflow.");
     }
+    unset($this->configuration['transitions'][$transition_id]);
     return $this;
   }
 
diff --git a/core/modules/workflows/src/WorkflowTypeInterface.php b/core/modules/workflows/src/WorkflowTypeInterface.php
index 009f41c..d84d1a7 100644
--- a/core/modules/workflows/src/WorkflowTypeInterface.php
+++ b/core/modules/workflows/src/WorkflowTypeInterface.php
@@ -164,6 +164,9 @@ public function onDependencyRemoval(array $dependencies);
    *
    * @return \Drupal\workflows\WorkflowTypeInterface
    *   The workflow type plugin.
+   *
+   * @throws \InvalidArgumentException
+   *   Thrown if a state already exists or state ID is invalid.
    */
   public function addState($state_id, $label);