Solution to assignment https://workflowy.com/s/render-tree/rUSXOW3O9NKYBv6Z

main.dart

import 'package:flutter/material.dart';

/// Globals
Key selectedNode = UniqueKey();
Set<Key> selectedSubnodes = Set();
List<Node> nodes = [
  Node('A', [
    Node('A1'),
    Node('A2', [
      Node('A21'),
      Node('A22'),
      Node('A23', [Node('A231')]),
      Node('A24'),
    ]),
  ]),
  Node('B'),
  Node('C', [
    Node('C1'),
    Node('C2'),
    Node('C3'),
  ]),
];

/// Model
class Node {
  final Key key = UniqueKey();
  String title = '';
  List<Node> nodes;

  Node(this.title, [_nodes]) : this.nodes = _nodes == null ? [] : _nodes;
}

/// App
void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'TreeRendererWidget'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key? key, required this.title}) : super(key: key);
  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
        appBar: AppBar(
          title: Text(widget.title),
        ),
        body: TreeRendererWidget(nodes: nodes));
  }
}

class TreeRendererWidget extends StatefulWidget {
  final List<Node> nodes;
  final int depth;

  TreeRendererWidget({Key? key, required this.nodes, this.depth = 0}) : super(key: key);

  @override
  _TreeRendererWidgetState createState() => _TreeRendererWidgetState();
}

class _TreeRendererWidgetState extends State<TreeRendererWidget> {
  @override
  Widget build(BuildContext context) {
    var ancestralState = context.findRootAncestorStateOfType<_MyHomePageState>();
    return Column(
      crossAxisAlignment: CrossAxisAlignment.start,
      children: widget.nodes.map((Node n) {
        return Column(
          crossAxisAlignment: CrossAxisAlignment.start,
          children: [
            GestureDetector(
              onTap: () {
                ancestralState?.setState(() {
                  if (selectedNode == n.key) {
                    selectedNode = UniqueKey();
                    selectedSubnodes.clear();
                  } else {
                    selectedNode = n.key;
                    selectedSubnodes.clear();
                    _addSubnode(n);
                  }
                });
              },
              child: Container(
                color: selectedNode == n.key
                    ? Colors.red
                    : selectedSubnodes.contains(n.key)
                        ? Colors.orange
                        : Colors.white,
                child: Text(
                  ' ' * widget.depth * 5 + '• ' + n.title,
                  style: TextStyle(fontSize: 30),
                ),
              ),
            ),
            TreeRendererWidget(nodes: n.nodes, depth: widget.depth + 1)
          ],
        );
      }).toList(),
    );
  }

  void _addSubnode(Node n) {
    n.nodes.forEach((nn) {
      selectedSubnodes.add(nn.key);
      _addSubnode(nn);
    });
  }
}