apply method

  1. @override
Delta apply(
  1. Delta document,
  2. int index,
  3. int length,
  4. NotusAttribute attribute

Applies heuristic rule to a retain (format) operation on a document and returns resulting Delta.


Delta apply(Delta document, int index, int length, NotusAttribute attribute) {
  if (attribute.key != return null;
  // If user selection is not collapsed we let it fallback to default rule
  // which simply applies the attribute to selected range.
  // This may still not be a bulletproof approach as selection can span
  // multiple lines or be a subset of existing link-formatted text.
  // So certain improvements can be made in the future to account for such
  // edge cases.
  if (length != 0) return null;

  final result = Delta();
  final iter = DeltaIterator(document);
  final before = iter.skip(index);
  final after =;
  var startIndex = index;
  var retain = 0;
  if (before != null && before.hasAttribute(attribute.key)) {
    startIndex -= before.length;
    retain = before.length;
  if (after != null && after.hasAttribute(attribute.key)) {
    retain += after.length;
  // There is no link-styled text around `index` position so it becomes a
  // no-op action.
  if (retain == 0) return null;

  result..retain(startIndex)..retain(retain, attribute.toJson());

  return result;