Add new method NodeInterface::level() and its tests.

spec/drupol/phptree/Node/NodeSpec.php

@@ -308,6 +308,38 @@ public function it_can_use_withChildren(): void
             ->shouldYield(new ArrayIterator([]));
     }
 
+    public function it_can_get_the_tree_levels(): void
+    {
+        $nodeLevel3_1 = new Node();
+        $nodeLevel3_2 = new Node();
+        $nodeLevel2 = new Node();
+        $nodeLevel1 = new Node();
+
+        $nodeLevel2->add($nodeLevel3_1, $nodeLevel3_2);
+        $nodeLevel1->add($nodeLevel2);
+        $this->add($nodeLevel1);
+
+        $this
+            ->level(0)
+            ->shouldYield([$this]);
+
+        $this
+            ->level(1)
+            ->shouldYield([$nodeLevel1]);
+
+        $this
+            ->level(2)
+            ->shouldYield([$nodeLevel2]);
+
+        $this
+            ->level(3)
+            ->shouldYield([$nodeLevel3_1, $nodeLevel3_2]);
+
+        $this
+            ->level(4)
+            ->shouldYield([]);
+    }
+
     public function it_has_a_degree(): void
     {
         $this

src/Node/Node.php

@@ -230,6 +230,19 @@ public function isRoot(): bool
         return null === $this->storage()->getParent();
     }
 
+    /**
+     * {@inheritdoc}
+     */
+    public function level(int $level): Traversable
+    {
+        /** @var \drupol\phptree\Node\NodeInterface $node */
+        foreach ($this->all() as $node) {
+            if ($node->depth() === $level) {
+                yield $node;
+            }
+        }
+    }
+
     /**
      * {@inheritdoc}
      */

src/Node/NodeInterface.php

@@ -135,6 +135,13 @@ public function isLeaf(): bool;
      */
     public function isRoot(): bool;
 
+    /**
+     * @param int $level
+     *
+     * @return Traversable<\drupol\phptree\Node\NodeInterface>
+     */
+    public function level(int $level): Traversable;
+
     /**
      * Remove children.
      *