State transition Guards

The workflow set for a State field defines all the states and transitions that are available for that field. For example, the Commerce Order "Fulfillment" workflow has states: "Draft", "Fulfillment", "Completed", and "Canceled", with transitions: "Place", "Fulfill", and "Cancel". This diagram illustrates the states and transitions of the Order fulfillment workflow:

Order fulfillment workflow

When an order is placed, either at checkout or through the admin UI, its State changes from "Draft" to "Fulfillment". After that, administrative users can manually transition the order's state to "Completed" or "Canceled".

Custom state field config

But what if we wanted to add constraints to prevent administrative users from moving an Order from the Fulfillment state to Completed or Canceled under certain conditions? In the Custom state fields documentation, we created a custom State field for Order items, to track fulfillment on a per-item basis. We might want to prevent users from cancelling an order if any of its order items have already been "filled". And we might want to prevent users from transitioning an order to Completed if any of its items are still "un-filled". The State Machine module's state transition "Guards" provide the ability to incorporate this sort of business logic. State transition Guards limit the transitions available for a State field based on contextual information. Transitions can be restricted based on the current user's permissions, a parent entity field, etc.

Order fulfillment workflow with guards

Using Guards requires writing custom code. Specifically, you will need to write a tagged service that implements Drupal\state_machine\Guard\GuardInterface. This interface defines a single method that returns a boolean TRUE/FALSE value:

public function allowed(WorkflowTransition $transition, WorkflowInterface $workflow, EntityInterface $entity);

If the given transition is allowed for the workflow, return TRUE; otherwise return FALSE.

Here is an example Guard implementation that implements the business logic described above:


namespace Drupal\mymodule\Guard;

use Drupal\Core\Entity\EntityInterface;
use Drupal\state_machine\Guard\GuardInterface;
use Drupal\state_machine\Plugin\Workflow\WorkflowInterface;
use Drupal\state_machine\Plugin\Workflow\WorkflowTransition;

class FulfillmentOrderGuard implements GuardInterface {

  public function allowed(WorkflowTransition $transition, WorkflowInterface $workflow, EntityInterface $entity) {
    if ($transition->getId() == 'cancel') {
      // Do not cancel orders with filled items.
      foreach ($entity->getItems() as $order_item) {
        if ($order_item->get('field_fulfillment_state')->first()->getId() == 'filled') {
          return FALSE;
      return TRUE;
    if ($transition->getId() == 'fulfill') {
      // All items must be filled or backordered. At least one must be filled.
      $has_filled_item = FALSE;
      foreach ($entity->getItems() as $order_item) {
        $inventory_state = $order_item->get('field_fulfillment_state')->first()->getId();
        if ($inventory_state != 'filled' && $inventory_state != 'backordered') {
          return FALSE;
        $has_filled_item = $inventory_state == 'filled' ? TRUE : $has_filled_item;
      return $has_filled_item;     

    // All other transitions.
    return TRUE;


By itself, this FulfillmentOrderGuard class won't do anything. We need to add it to our custom module's services.yml file so that it can be discovered. The group argument allows the guard factory to only instantiate the guards relevant to a specific workflow group. In this case, we want the commerce_order group since our Guard is intended for the Commerce order State field:

    class: Drupal\mymodule\Guard\FulfillmentOrderGuard
      - { name: state_machine.guard, group: commerce_order }

If we wanted to implement a Guard for the custom Order item "Fulfillment state" field we implemented in the Custom state fields documentation, the group value would change to match that custom workflow group:

    class: Drupal\mymodule\Guard\FulfillmentOrderItemGuard
      - { name: state_machine.guard, group: mymodule_order_item }

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