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packages/async/lib/src/stream_completer.dart

Index: packages/async/lib/src/stream_completer.dart
diff --git a/packages/async/lib/src/stream_completer.dart b/packages/async/lib/src/stream_completer.dart
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+// Copyright (c) 2015, 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 async.stream_completer;
+
+import "dart:async";
+
+/// A single-subscription [stream] where the contents are provided later.
+///
+/// It is generally recommended that you never create a `Future<Stream>`
+/// because you can just directly create a stream that doesn't do anything
+/// until it's ready to do so.
+/// This class can be used to create such a stream.
+///
+/// The [stream] is a normal stream that you can listen to immediately,
+/// but until either [setSourceStream] or [setEmpty] is called,
+/// the stream won't produce any events.
+///
+/// The same effect can be achieved by using a [StreamController]
+/// and adding the stream using `addStream` when both
+/// the controller's stream is listened to and the source stream is ready.
+/// This class attempts to shortcut some of the overhead when possible.
+/// For example, if the [stream] is only listened to
+/// after the source stream has been set,
+/// the listen is performed directly on the source stream.
+class StreamCompleter<T> {
+  /// The stream doing the actual work, is returned by [stream].
+  final _CompleterStream _stream = new _CompleterStream<T>();
+
+  /// Convert a `Future<Stream>` to a `Stream`.
+  ///
+  /// This creates a stream using a stream completer,
+  /// and sets the source stream to the result of the future when the
+  /// future completes.
+  ///
+  /// If the future completes with an error, the returned stream will
+  /// instead contain just that error.
+  static Stream fromFuture(Future<Stream> streamFuture) {
+    var completer = new StreamCompleter();
+    streamFuture.then(completer.setSourceStream,
+                      onError: (e, s) {
+                        completer.setSourceStream(streamFuture.asStream());
+                      });
+    return completer.stream;
+  }
+
+  /// The stream of this completer.
+  ///
+  /// This stream is always a single-subscription stream.
+  ///
+  /// When a source stream is provided, its events will be forwarded to
+  /// listeners on this stream.
+  ///
+  /// The stream can be listened either before or after a source stream
+  /// is set.
+  Stream<T> get stream => _stream;
+
+  /// Set a stream as the source of events for the [StreamCompleter]'s
+  /// [stream].
+  ///
+  /// The completer's `stream` will act exactly as [sourceStream].
+  ///
+  /// If the source stream is set before [stream] is listened to,
+  /// the listen call on [stream] is forwarded directly to [sourceStream].
+  ///
+  /// If [stream] is listened to before setting the source stream,
+  /// an intermediate subscription is created. It looks like a completely
+  /// normal subscription, and can be paused or canceled, but it won't
+  /// produce any events until a source stream is provided.
+  ///
+  /// If the `stream` subscription is canceled before a source stream is set,
+  /// the source stream will be listened to and immediately canceled again.
+  ///
+  /// Otherwise, when the source stream is then set,
+  /// it is immediately listened to, and its events are forwarded to the
+  /// existing subscription.
+  ///
+  /// Either [setSourceStream] or [setEmpty] may be called at most once.
+  /// Trying to call either of them again will fail.
+  void setSourceStream(Stream<T> sourceStream) {
+    if (_stream._isSourceStreamSet) {
+      throw new StateError("Source stream already set");
+    }
+    _stream._setSourceStream(sourceStream);
+  }
+
+  /// Equivalent to setting an empty stream using [setSourceStream].
+  ///
+  /// Either [setSourceStream] or [setEmpty] may be called at most once.
+  /// Trying to call either of them again will fail.
+  void setEmpty() {
+    if (_stream._isSourceStreamSet) {
+      throw new StateError("Source stream already set");
+    }
+    _stream._setEmpty();
+  }
+}
+
+/// Stream completed by [StreamCompleter].
+class _CompleterStream<T> extends Stream<T> {
+  /// Controller for an intermediate stream.
+  ///
+  /// Created if the user listens on this stream before the source stream
+  /// is set, or if using [_setEmpty] so there is no source stream.
+  StreamController _controller;
+
+  /// Source stream for the events provided by this stream.
+  ///
+  /// Set when the completer sets the source stream using [_setSourceStream]
+  /// or [_setEmpty].
+  Stream _sourceStream;
+
+  StreamSubscription<T> listen(onData(T data),
+                               {Function onError,
+                                void onDone(),
+                                bool cancelOnError}) {
+    if (_controller == null) {
+      if (_sourceStream != null && !_sourceStream.isBroadcast) {
+        // If the source stream is itself single subscription,
+        // just listen to it directly instead of creating a controller.
+        return _sourceStream.listen(onData, onError: onError, onDone: onDone,
+                                    cancelOnError: cancelOnError);
+      }
+      _createController();
+      if (_sourceStream != null) {
+        _linkStreamToController();
+      }
+    }
+    return _controller.stream.listen(onData, onError: onError, onDone: onDone,
+                                     cancelOnError: cancelOnError);
+  }
+
+  /// Whether a source stream has been set.
+  ///
+  /// Used to throw an error if trying to set a source stream twice.
+  bool get _isSourceStreamSet => _sourceStream != null;
+
+  /// Sets the source stream providing the events for this stream.
+  ///
+  /// If set before the user listens, listen calls will be directed directly
+  /// to the source stream. If the user listenes earlier, and intermediate
+  /// stream is created using a stream controller, and the source stream is
+  /// linked into that stream later.
+  void _setSourceStream(Stream<T> sourceStream) {
+    assert(_sourceStream == null);
+    _sourceStream = sourceStream;
+    if (_controller != null) {
+      // User has already listened, so provide the data through controller.
+      _linkStreamToController();
+    }
+  }
+
+  /// Links source stream to controller when both are available.
+  void _linkStreamToController() {
+    assert(_controller != null);
+    assert(_sourceStream != null);
+    _controller.addStream(_sourceStream, cancelOnError: false)
+               .whenComplete(_controller.close);
+  }
+
+  /// Sets an empty source stream.
+  ///
+  /// Uses [_controller] for the stream, then closes the controller
+  /// immediately.
+  void _setEmpty() {
+    assert(_sourceStream == null);
+    if (_controller == null) {
+      _createController();
+    }
+    _sourceStream = _controller.stream;  // Mark stream as set.
+    _controller.close();
+  }
+
+  // Creates the [_controller].
+  void _createController() {
+    assert(_controller == null);
+    _controller = new StreamController<T>(sync: true);
+  }
+}