applyRule method

@override Delta? applyRule(Delta document, int index, { int? len, Object? data, Attribute? attribute, })

Applies heuristic rule to an operation on a document and returns resulting Delta.

Implementation

@override
Delta? applyRule(Delta document, int index, {int? len, Object? data, Attribute? attribute}) {
  if (data is! String || data != '\n') {
    return null;
  }

  final itr = DeltaIterator(document);
  final prev = itr.skip(index), cur = itr.next();
  final blockStyle = Style.fromJson(cur.attributes).getBlockExceptHeader();
  // We are not in a block, ignore.
  if (cur.isPlain || blockStyle == null) {
    return null;
  }
  // We are not on an empty line, ignore.
  if (!_isEmptyLine(prev, cur)) {
    return null;
  }

  // We are on an empty line. Now we need to determine if we are on the
  // last line of a block.
  // First check if `cur` length is greater than 1, this would indicate
  // that it contains multiple newline characters which share the same style.
  // This would mean we are not on the last line yet.
  // `cur.value as String` is safe since we already called isEmptyLine and
  // know it contains a newline
  if ((cur.value as String).length > 1) {
    // We are not on the last line of this block, ignore.
    return null;
  }

  // Keep looking for the next newline character to see if it shares the same
  // block style as `cur`.
  final nextNewLine = _getNextNewLine(itr);
  if (nextNewLine.operation != null &&
      nextNewLine.operation!.attributes != null &&
      Style.fromJson(nextNewLine.operation!.attributes).getBlockExceptHeader() == blockStyle) {
    // We are not at the end of this block, ignore.
    return null;
  }

  // Here we now know that the line after `cur` is not in the same block
  // therefore we can exit this block.
  final attributes = cur.attributes ?? <String, dynamic>{};
  final k = attributes.keys.firstWhere(Attribute.blockKeysExceptHeader.contains);
  attributes[k] = null;
  // retain(1) should be '\n', set it with no attribute
  return Delta()
    ..retain(index + (len ?? 0))
    ..retain(1, attributes);
}