feat: node traverser

frontend/app_flowy/packages/flowy_editor/lib/document/node_traverser.dart

@@ -0,0 +1,42 @@
+import 'package:flowy_editor/document/node.dart';
+
+import './state_tree.dart';
+import './node.dart';
+
+/// [NodeTraverser] is used to traverse the nodes in visual order.
+class NodeTraverser {
+  final StateTree stateTree;
+  Node? currentNode;
+
+  NodeTraverser(this.stateTree, Node beginNode) : currentNode = beginNode;
+
+  Node? next() {
+    final node = currentNode;
+    if (node == null) {
+      return null;
+    }
+
+    if (node.children.isNotEmpty) {
+      currentNode = _findLeadingChild(node);
+    } else if (node.next != null) {
+      currentNode = node.next!;
+    } else {
+      final parent = node.parent!;
+      final nextOfParent = parent.next;
+      if (nextOfParent == null) {
+        currentNode = null;
+      } else {
+        currentNode = _findLeadingChild(node);
+      }
+    }
+
+    return node;
+  }
+
+  Node _findLeadingChild(Node node) {
+    while (node.children.isNotEmpty) {
+      node = node.children.first;
+    }
+    return node;
+  }
+}