Add triggers under root rule for workflow validation

[MAILPOET-4629]

mailpoet/lib/Automation/Engine/Validation/WorkflowRules/TriggersUnderRootRule.php

@@ -0,0 +1,40 @@
+<?php declare(strict_types = 1);
+
+namespace MailPoet\Automation\Engine\Validation\WorkflowRules;
+
+use MailPoet\Automation\Engine\Data\Step;
+use MailPoet\Automation\Engine\Data\Workflow;
+use MailPoet\Automation\Engine\Exceptions;
+use MailPoet\Automation\Engine\Validation\WorkflowGraph\WorkflowNode;
+use MailPoet\Automation\Engine\Validation\WorkflowGraph\WorkflowNodeVisitor;
+
+class TriggersUnderRootRule implements WorkflowNodeVisitor {
+  /** @var array<string, Step> $triggersMap */
+  private $triggersMap = [];
+
+  public function initialize(Workflow $workflow): void {
+    $this->triggersMap = [];
+    foreach ($workflow->getSteps() as $step) {
+      if ($step->getType() === 'trigger') {
+        $this->triggersMap[$step->getId()] = $step;
+      }
+    }
+  }
+
+  public function visitNode(Workflow $workflow, WorkflowNode $node): void {
+    $step = $node->getStep();
+    if ($step->getType() === Step::TYPE_ROOT) {
+      return;
+    }
+
+    foreach ($step->getNextSteps() as $nextStep) {
+      $nextStepId = $nextStep->getId();
+      if (isset($this->triggersMap[$nextStepId])) {
+        throw Exceptions::workflowStructureNotValid(__('Trigger must be a direct descendant of workflow root', 'mailpoet'));
+      }
+    }
+  }
+
+  public function complete(Workflow $workflow): void {
+  }
+}