addAdditionalPropertiesCallback property Null safety

Callback to add additional experimental serialization properties.

This callback can be used to customize the serialization of DiagnosticsNode objects for experimental features in widget inspector clients such as Dart DevTools. For example, Dart DevTools can evaluate the following expression to register a VM Service API with a custom serialization to experiment with visualizing layouts.

The following code samples demonstrates adding the RenderObject associated with an Element to the serialized data for all elements in the tree:

Map<String, Object> getDetailsSubtreeWithRenderObject(
  String id,
  String groupName,
  int subtreeDepth,
) {
  return _nodeToJson(
    root,
    InspectorSerializationDelegate(
      groupName: groupName,
      summaryTree: false,
      subtreeDepth: subtreeDepth,
      includeProperties: true,
      service: this,
      addAdditionalPropertiesCallback: (DiagnosticsNode node, _SerializationDelegate delegate) {
        final Map<String, Object> additionalJson = <String, Object>{};
        final Object value = node.value;
        if (value is Element) {
          final renderObject = value.renderObject;
          additionalJson['renderObject'] = renderObject?.toDiagnosticsNode()?.toJsonMap(
            delegate.copyWith(
              subtreeDepth: 0,
              includeProperties: true,
            ),
          );
        }
        return additionalJson;
      },
    ),
 );
}