BufferedOutputStream for Dart -

buffering_iosink.dart

// example code, take care
// https://github.com/dart-lang/sdk/issues/32874#issuecomment-1467905810

class BufferingIOSink implements IOSink {
  static const int _defaultBufferSize = 2048;
  final Sink<List<int>> _sink;
  Uint8List _buffer;
  // Current live bytes in buffer.
  int _start = 0;
  int _end = 0;
  Encoding _encoding;
  BufferingIOSink(Sink<List<int>> output, {Encoding? encoding, Uint8List? buffer, int? bufferSize})
     : _sink = output, 
       _encoding = encoding ?? (output is IOSink ? output.encoding : utf8),
       _buffer = buffer ?? Uint8List(bufferSize ?? _defaultBufferSize);

  void _flushBuffer() {
    if (_start >= _end) return;
    _sink.add(Uint8List.sublistView(_buffer, _start, _end));
    _start = _end;
    var capacity = _buffer.length;
    if ((capacity - _end) * 4 < capacity) { // Reallocate if less that 25% free after flush.
      _buffer = Uint8List(capacity); 
      _start = _end = 0;
    }
  }

  Future<void> flush() {
    _flushBuffer();
    var sink = _sink;
    if (sink is IOSink) return sink.flush();
    return Future.value(null);
  }

  void add(List<int> bytes) {
    var totalCapacity = _buffer.length;
    var capacity = totalCapacity - _end;
    var bytesCursor = 0;
    while (bytesCursor + capacity < bytes.length) {
      _buffer.setRange(_end, totalCapacity, bytes, bytesCursor);
      _bytesCursor += capacity;
      _flushBuffer();
      capacity = totalCapacity - _end;
    }
    _buffer.setRange(_end, _end + bytes.length - bytesCursor, bytes, bytesCursor);
  }
  // All the other methods.
    
}