Add an AssetStream class to Barback.

pkg/barback/lib/src/stream_replayer.dart

Index: pkg/barback/lib/src/stream_replayer.dart
diff --git a/pkg/barback/lib/src/stream_replayer.dart b/pkg/barback/lib/src/stream_replayer.dart
new file mode 100644
index 0000000000000000000000000000000000000000..177e13d609f6cc9bb0fafa00ec85ae8d4a8007b6
--- /dev/null
+++ b/pkg/barback/lib/src/stream_replayer.dart
@@ -0,0 +1,67 @@
+// Copyright (c) 2013, 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.
+
+library barback.stream_replayer;
+
+import 'dart:async';
+import 'dart:collection';
+
+import 'utils.dart';
+
+/// Records the values and errors that are sent through a stream and allows them
+/// to be replayed arbitrarily many times.
+class StreamReplayer<T> {
+  /// The wrapped stream.
+  final Stream<T> _stream;
+
+  /// Whether or not [_stream] has been closed.
+  bool _isClosed = false;
+
+  /// The buffer of events or errors that have already been emitted by
+  /// [_stream].
+  ///
+  /// Each element is a [Either] that's either a value or an error sent through
+  /// the stream.
+  final _buffer = new Queue<Either<T, dynamic>>();
+
+  /// The controllers that are listening for future events from [_stream].
+  final _controllers = new Set<StreamController<T>>();
+
+  StreamReplayer(this._stream) {
+    _stream.listen((data) {
+      _buffer.add(new Either<T, dynamic>.withFirst(data));
+      for (var controller in _controllers) {
+        controller.add(data);
+      }
+    }, onError: (error) {
+      _buffer.add(new Either<T, dynamic>.withSecond(error));
+      for (var controller in _controllers) {
+        controller.addError(error);
+      }
+    }, onDone: () {
+      _isClosed = true;
+      for (var controller in _controllers) {
+        controller.close();
+      }
+      _controllers.clear();
+    });
+  }
+
+  /// Returns a stream that replays the values and errors of the input stream.
+  ///
+  /// This stream is a buffered stream regardless of whether the input stream
+  /// was broadcast or buffered.
+  Stream<T> getReplay() {
+    var controller = new StreamController<T>();
+    for (var eventOrError in _buffer) {
+      eventOrError.match(controller.add, controller.addError);

[Bob Nystrom, 2013/08/27 17:59:57]

Oh, that cleaned up nicely.

+    }
+    if (_isClosed) {
+      controller.close();
+    } else {
+      _controllers.add(controller);
+    }
+    return controller.stream;
+  }
+}