range_span.dart

abstract class Annotation {
  ui.TextStyle? get style;
  void build(...);
}

class Range {
  // ...
}

@immutable
class RangeSpan extends InlineSpan implements HitTestTarget, MouseTrackerAnnotation {
  const TextSpan({
    this.text,
    this.annotations,
  });

  final String? text;

  final List<(Range, Annotation)> annotations;

  @override
  void handleEvent(PointerEvent event, HitTestEntry entry) {
    ...for each annotation that overlaps the destination:
       annotation.handleEvent(...)
  }

  @override
  void build(
    ui.ParagraphBuilder builder, {
    TextScaler textScaler = TextScaler.noScaling,
    List<PlaceholderDimensions>? dimensions,
  }) {
    split the text into which subranges are affected by different annotations
    for each subrange:
       for each annotation with a style:
          builder.pushStyle(annotation.style)
    for each subrange:
       for each annotation that wants to build something (e.g. insert a WidgetSpan):
          annotation.build(...)
    builder.addText(text of subrange);
    for each subrange:
       for each annotation with a style:
          builder.pop
  }

  @override
  bool visitChildren(InlineSpanVisitor visitor) {
    walk each annotation that wants to build something (e.g. insert a WidgetSpan)
  }

  ...
}