pkgs/stream_transform/lib/src/scan.dart

// Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.
import 'dart:async';

/// Scan is like fold, but instead of producing a single value it yields
/// each intermediate accumulation.
StreamTransformer<S, T> scan<S, T>(
        T initialValue, T combine(T previousValue, S element)) =>
    new StreamTransformer<S, T>((stream, cancelOnError) {
      T accumulated = initialValue;
      return stream
          .map((value) => accumulated = combine(accumulated, value))
          .listen(null, cancelOnError: cancelOnError);
    });