readAt method

@override Future<int> readAt(int offset, Uint8List dst, { int start = 0, int? end, })

Reads into dst[start..end) at offset. Returns bytes read (0 at EOF).

Implementation

@override
Future<int> readAt(
  int offset,
  Uint8List dst, {
  int start = 0,
  int? end,
}) async {
  final e = end ?? dst.length;
  RangeError.checkValidRange(start, e, dst.length);
  if (offset < 0) throw RangeError.value(offset, 'offset');
  if (e == start) return 0;
  _checkOpen();
  return _serialize(() async {
    if (offset >= _length) return 0;
    final want = math.min(e - start, _length - offset);
    if (want <= 0) return 0;
    final first = offset ~/ chunkSize;
    final last = (offset + want - 1) ~/ chunkSize;
    await _loadChunks([
      for (var i = first; i <= last; i++) i,
    ], readOnly: true);
    var done = 0;
    while (done < want) {
      final absolute = offset + done;
      final index = absolute ~/ chunkSize;
      final inChunk = absolute % chunkSize;
      final take = math.min(want - done, chunkSize - inChunk);
      final cached = _cache[index];
      if (cached == null) {
        dst.fillRange(start + done, start + done + take, 0);
      } else {
        dst.setRange(start + done, start + done + take, cached, inChunk);
      }
      done += take;
    }
    return done;
  });
}