Type mismatch on generic Nodes

[Gustav Gussi]

You use generic Node<T> but you never address this T type in your code. That is why sometimes TypeError is occurred.

For example I have a function:

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void _onNodeSelected(Node<Zone> node) {
 ...
}

When this code is executing

final node = controller.getNode('key');
_onNodeSelected(node); // Crash is here because Node<Zone> is expected but Node<dynamic> is provided.

Please respect the type T everywhere where Node type is used, i.e. Node<T> instead of Node.

P.S.: Also you created `nodeBuilder` (thanks for that!) but you use

Function (BuildContext context, Node node)

and it is better to use:

Widget Function (BuildContext context, Node node)

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[Kevin Armstrong]

Hi Gustav, The T type is addressed in the place where it is defined: on the Node class. In order to use Node<T> throughout the code, I would need to have and define T throughout the code. That isn’t feasible.

If you define your type Node<Zone>, not just Node, when you create the node, getNode will return Node<Zone> and not Node<dynamic>.

[Kevin Armstrong]

Thanks for the suggestion on using Widget Function(BuildContext context, Node node)!

[Gustav Gussi]

If you define your type Node<Zone>, not just Node, when you create the node, getNode will return Node<Zone> and not Node<dynamic>.

Unfortunately it is not true. And also `copyWith` returns Node<dynamic> instead of Node<Zone> but I clonned Node<Zone>.

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[Kevin Armstrong]

Gustav, can you provide me with a code sample where this is not true for you. I am willing to make the changes.

[Gustav Gussi]

Kevin, unfortunately I cannot copy my code as I work via VPN (w/o Internet) so I can just provide some lines from my real code:

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// Somewhere in StreamBuilder's builder:

final zones = snapshot.data.items; // zones is a List<Zone>
final treeNodes = _buildTree(zones); // treeNodes is a List<Node<Zone>>
treeController = TreeViewController(
  children: treeNodes, // Here children already a List<Node<dynamic>>
  selectedKey: _selectedKey,
);

return TreeView(
  controller: treeControoller,
  ...
  onNodeTap: _nodeSelectHandler,
);

// Node selection handler
Future<void> _nodeSelectHandler(String key) async {
  final node = treeController.getNode(key); // already here `node` is Node<dynamic>
  final updatedZone = await zoneBloc.getEntity((node.data as Zone).id);
  widget.onNodeChanged?.call(node.cast(updateZone)); // Here I am forced to call a `cast` method. Otherwise I have a TypeError because the `onNodeChanged` is ValueChanged<Node<Zone>>
}

//
extension NodeExtension<T> on Node<T> {
  Node<T> cast([T data]) => Node<T>(
    key: key,
    label: label,
    ...
    data: data ?? this.data,
  );
}

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[吕彦]

Please respect the type T everywhere where Node type is used, i.e. Node<T> instead of Node.

I also agree with that

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[Jennie Browne]

Also having this issue when using copyWith it returns Node<dynamic> rather than Node<T>. Please add all generic types as in the example…

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Please change this

Node<T> copyWith({
  String? key,
  String? label,
  List<Node<T>>? children,
  bool? expanded,
  bool? parent,
  IconData? icon,
  T? data,
}) =>
    Node<T>(
      key: key ?? this.key,
      label: label ?? this.label,
      icon: icon ?? this.icon,
      expanded: expanded ?? this.expanded,
      parent: parent ?? this.parent,
      children: children ?? this.children,
      data: data ?? this.data,
    );

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[Kevin Armstrong]

Fixes #32: Updated Node copyWith method to properly use Generic for children nodes Fixes #44: Removed hardcoded hoverColor when creating the TreeView widget

→ <<cset 104d77b8c2d3>>

[Kevin Armstrong]

Thanks for this update Jennie. I am updating the code now.

[Jennie Browne]

Thanks so much Kevin. While you are in the TreeViewController class…there were quite a few other references that return Node rather than Node<T> if you can fix those too?

Thanks again!!

[Kevin Armstrong]

I moved too fast Jennie. I already published those updates.

I think I mentioned this previously but unless I’m missing something, there is no way for the TreeViewController to know the generic T so I can’t add it throughout without providing a means for you to pass that generic at method invocation. e.g.

var node = controller.getNode<double>(key);

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[Jennie Browne]

Hi Kevin,

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You can pass T through on the method. If you are creating a Node without a type (such as copyWith()) inside TreeViewController it defeats the purpose of a strongly typed Node.

For example in copyWith you could change the method signature to →

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Node copyWith<T>({
})

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And then you call it with:

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copyWith<String>();

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If you are calling it within TreeViewController you need to make sure all method sigs pass through the type T. For example for updateNode it would look like this:

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List<Node<T>> updateNode<T>(String key, Node<T> newNode, {Node<T>? parent}) {
  List<Node<T>> _children = parent == null ? this.children : parent.children;
  return _children.map((Node<T> child) {
    if (child.key == key) {
      return newNode;
    } else {
      if (child.isParent) {
        return child.copyWith<T>(
          children: updateNode<T>(
            key,
            newNode,
            parent: child,
          ),
        );
      }
      return child;
    }
  }).toList();

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The root issue I am trying to address it when we lose the typed Node during a copyWith (basically where you created a Node() without a type.)

Thanks Kevin!

[Kevin Armstrong]

Thanks Jennie, that certainly clears up what Gustav was trying to convey. And it’s something I didn’t think about when I added the data property. I will work on getting that updated ASAP.

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[Jennie Browne]

Thanks Kevin!!