DataModel.update does not parse adjacency-list contents for non-root paths

[ymmt2005]

Describe the bug

DataModel.update only calls _parseDataModelContents when the path is root (/ or empty). For non-root paths (e.g. /todos), the raw contents List is passed directly to _updateValue, which stores it as-is. This causes a TypeError when a widget subscribes to that path expecting Map<String, Object?> — because the stored value is still the raw List<dynamic> (the A2UI adjacency-list format).

The error manifests as:

type 'List<dynamic>' is not a subtype of type 'Map<String, Object?>?' in type cast

thrown from DataModel.getValue (data_model.dart:190) when ComponentChildrenBuilder subscribes to a template's dataBinding path.

To Reproduce

1. Send a dataModelUpdate A2UI message with a non-root path (e.g. "/todos") and contents in the standard adjacency-list format:

{
  "dataModelUpdate": {
    "surfaceId": "todo-main",
    "path": "/todos",
    "contents": [
      {"key": "0", "valueMap": [
        {"key": "id", "valueString": "abc"},
        {"key": "title", "valueString": "Buy groceries"}
      ]},
      {"key": "1", "valueMap": [
        {"key": "id", "valueString": "def"},
        {"key": "title", "valueString": "Finish report"}
      ]}
    ]
  }
}

2. Have a List component with a template bound to /todos:

{
  "List": {
    "children": {
      "template": {
        "dataBinding": "/todos",
        "componentId": "item-template"
      }
    }
  }
}

3. The app crashes with type 'List<dynamic>' is not a subtype of type 'Map<String, Object?>?' in type cast.

Expected behavior

DataModel.update should parse the adjacency-list contents into a Map<String, Object?> for all paths, not just the root. The relevant code in data_model.dart:

void update(DataPath? absolutePath, Object? contents) {
  if (absolutePath == null || absolutePath.segments.isEmpty) {
    if (contents is List) {
      _data = _parseDataModelContents(contents); // ← only called for root
    }
    // ...
    return;
  }
  _updateValue(_data, absolutePath.segments, contents); // ← raw List stored as-is
}

For non-root paths, contents should also be parsed through _parseDataModelContents before being stored, e.g.:

void update(DataPath? absolutePath, Object? contents) {
  Object? parsedContents = contents;
  if (contents is List) {
    parsedContents = _parseDataModelContents(contents);
  }
  if (absolutePath == null || absolutePath.segments.isEmpty) {
    if (parsedContents is Map) {
      _data = Map<String, Object?>.from(parsedContents);
    }
    // ...
    return;
  }
  _updateValue(_data, absolutePath.segments, parsedContents);
}

Additional context

• Package version: genui 0.7.0
• The A2UI v0.8 specification defines dataModelUpdate.contents as an adjacency-list array regardless of whether path is specified.
• ComponentChildrenBuilder (widget_helpers.dart:103) subscribes with subscribe<Map<String, Object?>>, which calls getValue<Map<String, Object?>> → _getValue(...) as Map<String, Object?>?, triggering the cast error.
• Workaround: intercept DataModelUpdate messages at the ContentGenerator stream level and pre-parse contents from the adjacency-list to a plain Map before the SDK processes them.

[andrewkolos]

Thanks for the report. At first I was surprised this wasn't encountered earlier, but I suppose it makes sense given that to encounter this,

1. the agent decided to use the template feature over explicitList,
2. the path being bound to was not the root, and
3. the agent sent a dataModelUpdate to that non-root path.

Condition 1 might have been rare as LLMs might "prefer" to write out a static explicitList of items rather than use template (i.e. lists are massively more common in training data than the abstract concept of a template+data+a binding path). Condition 2 made this impossible to repro in simpler apps; and--even in complex UIs--I have a hunch that LLMs are biased to emit the entire state (i.e. update root) even when unnecessary because the idea of partial state updates is less common in their training data.

Hopefully we'll see less of this type of problem when we migrate to the A2UI v0.9 spec, which uses everyday-looking JSON object for bound values rather than a bespoke data-mapping schema encoded in adjacency lists.