apply method

@override Delta? apply(Delta document, int index, int length)

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

Implementation

@override
Delta? apply(Delta document, int index, int length) {
  final iter = DeltaIterator(document);

  // First, check if line-break deleted after an embed.
  var op = iter.skip(index);
  var indexDelta = 0;
  var lengthDelta = 0;
  var remaining = length;
  var foundEmbed = false;
  var hasLineBreakBefore = false;
  if (op != null && op.data.endsWith(kZeroWidthSpace)) {
    foundEmbed = true;
    var candidate = iter.next(1);
    remaining--;
    if (candidate?.data == '\n') {
      indexDelta += 1;
      lengthDelta -= 1;

      /// Check if it's an empty line
      candidate = iter.next(1);
      remaining--;
      if (candidate!.data == '\n') {
        // Allow deleting empty line after an embed.
        lengthDelta += 1;
      }
    }
  } else {
    // If op is `null` it's a beginning of the doc, e.g. implicit line break.
    hasLineBreakBefore = op == null || op.data.endsWith('\n');
  }

  // Second, check if line-break deleted before an embed.
  op = iter.skip(remaining);
  if (op != null && op.data.endsWith('\n')) {
    final candidate = iter.next(1);
    // If there is a line-break before deleted range we allow the operation
    // since it results in a correctly formatted line with single embed in it.
    if (candidate?.data == kZeroWidthSpace && !hasLineBreakBefore) {
      foundEmbed = true;
      lengthDelta -= 1;
    }
  }

  if (foundEmbed) {
    return Delta()
      ..retain(index + indexDelta)
      ..delete(length + lengthDelta);
  }

  return null; // fallback
}