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.
///
/// If [combine] returns a Future it will not be called again for subsequent
/// events from the source until it completes, therefor the combine callback is
/// always called for elements in order, and the result stream always maintains
/// the same order as the original.
StreamTransformer<S, T> scan<S, T>(
        T initialValue, FutureOr<T> combine(T previousValue, S element)) =>
    StreamTransformer.fromBind((source) {
      var accumulated = initialValue;
      return source.asyncMap((value) {
        var result = combine(accumulated, value);
        if (result is Future<T>) {
          return result.then((r) => accumulated = r);
        } else {
          return accumulated = result as T;
        }
      });
    });