Add new features to package:async.

lib/src/stream_completer.dart

+// 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";
+import "delegating_stream_subscription.dart";
+
+/// 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 of this completer.
+  ///
+  /// 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.
+  final Stream<T> stream = new _CompleterStream<T>();

[nweiz, 2015/06/16 01:05:23]

I should have mentioned this in the last pass (sorry!), but: is it worth all the
complexity here to avoid just making this a stream controller and manually
forwarding events to it? That would substantially cut down on the size of the
code here, make it look more like user-authored async code, and avoid
duplicating a lot of stuff that's already in dart:async. I'm sure it wouldn't be
quite as efficient, but I doubt it would be enough to matter.

[Lasse Reichstein Nielsen, 2015/06/18 12:10:12]

Ack. Done.

+
+  /// 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) {

[nweiz, 2015/06/16 01:05:23]

"e, s" -> "_"

[Lasse Reichstein Nielsen, 2015/06/18 12:10:12]

Done.

+                        completer.setSourceStream(streamFuture.asStream());
+                      });
+    return completer.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,
+  /// then nothing further happens. This means that the `sourceStream` may
+  /// never be listened to, even if the [stream] has been listened to.
+  ///
+  /// Otherwise, when the source stream is then set,
+  /// it is immediately listened to.
+  /// If the existing subscription was paused, the source stream subscription
+  /// is paused as many times, and then all events and callbacks are forwarded

[nweiz, 2015/06/16 01:05:23]

It's kind of strange that this forwards the same *number* of pauses to the
source stream. StreamController folds all pauses on its subscription into a
single [onPaused] event, and it feels like that's what should happen here. At
least this shouldn't guarantee the number of pause calls that are passed to the
subscription.

[Lasse Reichstein Nielsen, 2015/06/18 12:10:12]

If you pause three times, you need to resume three times to continue listening.
The controller will only be called once, it doesn't need to know how many times
the subscription is paused, just that it shouldn't push any events on it.
Here the subscription is a proxy for another, later, subscription, so it should
record the actual number of pauses and ensure that the real subscription
requires the same number of resumes.

(Rewritten completely anyway, so no longer an issue).
We have to. But we shouldn't put it in the public documentation - it should just
work behind the scenes.

+  /// between the two subscriptions, so the listener works as if it was
+  /// listening to the source stream subscription directly.
+  ///
+  /// Either [setSourceStream] or [setEmpty] may be called at most once.
+  /// Trying to call either of them again will fail.
+  void setSourceStream(Stream<T> sourceStream) {
+    _CompleterStream completerStream = this.stream;
+    completerStream._linkStream(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() {
+    // TODO(lrn): Optimize this to not actually create the empty stream.
+    _CompleterStream completerStream = this.stream;
+    completerStream._linkStream(new Stream.fromIterable(const []));
+  }
+}
+
+/// Stream that acts as a source stream it is (eventually) linked with.
+///
+/// The linked source stream can be set after a user has started listening on
+/// this stream. No events occur before the source stream is provided.
+///
+/// If a user listens before events are available, the state of the
+/// subscription is maintained, and the subscription is then linked
+/// to the source stream when that becomes available.
+class _CompleterStream<T> extends Stream<T> {
+  // Bit flags used for the value of [_state].
+  /// Flag marking that the source stream has been set.
+  static const int _streamFlag = 1;
+  /// Flag marking that the stream has been listened to.

[nweiz, 2015/06/16 01:05:23]

Nit: separate these with newlines.

[Lasse Reichstein Nielsen, 2015/06/18 12:10:12]

Gone.

+  static const int _listenerFlag = 2;
+
+  // States used as values for _state.
+  /// Initial state with no listener and  no source stream set.
+  static const int _initial = 0;
+  /// State where only the stream has been set.
+  static const int _streamOnly = _streamFlag;
+  /// State where there is only a listener.
+  static const int _listenerOnly = _listenerFlag;
+  /// State where source stream and listener have been linked.
+  static const int _linked = _streamFlag | _listenerFlag;
+
+  /// The current state.
+  ///
+  /// One of [_initial], [_streamOnly], [_listenerOnly],
+  /// or [_linked].
+  int _state = _initial;
+
+  /// Shared variable used to store different values depending on the state.
+  ///
+  /// In the [_streamOnly] state it contains the source stream.
+  /// In the [_listenerOnly] state it contains a delegating subscription
+  /// controller.
+  /// In the [_linked] and [_initial] states it contains `null`.
+  ///
+  /// Do not access this field directly,
+  /// instead use [_stream] or [_controller] to read the value
+  /// appropriate for the current state.
+  var _stateData;
+
+  /// Returns source stream that has been set.
+  ///
+  /// Must only be used when the source stream has been set,
+  /// but the stream has not been listened to yet (state is [_streamOnly]);

[nweiz, 2015/06/16 01:05:23]

";" -> "."

+  Stream get _stream {
+    assert(_state == _streamOnly);
+    return _stateData;
+  }
+
+  /// Returns the mutable subscription controller with the subscription
+  /// on this stream
+  ///
+  /// Must only be used when the stream has been listened to,
+  /// but the source stream has not been set to yet (state is [_listenerOnly]);

[nweiz, 2015/06/16 01:05:23]

";" -> "."

[Lasse Reichstein Nielsen, 2015/06/18 12:10:12]

Acknowledged.

+  MutableDelegatingStreamSubscriptionController get _controller {
+    assert(_state == _listenerOnly);
+    return _stateData;
+  }
+
+  // State transition functions.
+
+  /// Sets the source stream, and enters the [_streamOnly] state.
+  ///
+  /// Must only be called from the initial state, before this stream has
+  /// been listened to.
+  void _setStream(Stream stream) {
+    assert(_state == _initial);
+    _stateData = stream;
+    _state = _streamOnly;
+  }
+
+  /// Sets the listener subscription, and enters the [_listenerOnly] state.
+  ///
+  /// Must only be called from the initial state, before the source stream has
+  /// been set.
+  void _setListened(MutableDelegatingStreamSubscriptionController subscription) {

[nweiz, 2015/06/16 01:05:23]

Long line.

[Lasse Reichstein Nielsen, 2015/06/18 12:10:12]

Class really needs a better name.
I think I'll remove the class again, now that I don't use it.

+    assert(_state == _initial);
+    _stateData = subscription;
+    _state = _listenerOnly;
+  }
+
+  /// Marks the listener and source stream as linked.
+  ///
+  /// Enters the [_isLinked] state.
+  /// This must be called only from either the [_streamOnly]
+  /// or the [_listenerOnly] state after setting up the link.
+  void _setLinked() {
+    assert(_state == _streamOnly || _state == _listenerOnly);
+    _state = _linked;
+    _stateData = null;
+  }
+
+  // end state transition functions.
+
+  /// Called by the controller when the user supplies a stream
+  void _linkStream(Stream stream) {
+    if (_state == _listenerOnly) {
+      var subscription = _controller.sourceSubscription;
+      if (subscription is _CompleterSubscriptionState) {
+        _controller.sourceSubscription = subscription._linkStream(stream);
+      } else {
+        assert(subscription is _CanceledSubscription);
+      }
+      _setLinked();
+    } else if (_state == _initial) {
+      _setStream(stream);
+    } else {
+      throw new StateError("Stream already linked.");
+    }
+  }
+
+  StreamSubscription listen(void onData(T event),
+                            {Function onError,
+                             void onDone(),
+                             bool cancelOnError}) {
+    if (_state == _initial) {
+      if (cancelOnError == null) cancelOnError = false;
+      var controller =
+          new MutableDelegatingStreamSubscriptionController<T>(null);
+      controller.sourceSubscription =

[nweiz, 2015/06/16 01:05:23]

Why are you assigning this rather than passing it in as a parameter?

[Lasse Reichstein Nielsen, 2015/06/18 12:10:12]

Because of the cyclic dependency, the state object refers to the controller so
it can forward delayed calls back through it.

+          new _CompleterSubscriptionState(cancelOnError, controller);
+      var subscription = controller.subscription;
+      subscription.onData(onData);
+      subscription.onError(onError);
+      subscription.onDone(onDone);
+      _setListened(controller);
+      return subscription;
+    }
+    if (_state == _streamOnly) {

[nweiz, 2015/06/16 01:05:23]

Nit: "else if", for compactness and to match other places in the code.

[Lasse Reichstein Nielsen, 2015/06/18 12:10:12]

Then I think I should also put the final throw in an "else" block.
I thought that would make the analyzer complain that my non-void function didn't
end with a return or throw, but apparently that's not a problem.

+      var result = _stream.listen(onData, onError: onError, onDone: onDone,
+                                  cancelOnError: cancelOnError);
+      _setLinked();
+      return result;
+    }
+    throw new StateError("Stream has already been listened to.");
+  }
+}
+
+/// Holds subscription callbacks and state for a [_CompleterSubscription]
+/// until the real subscription is available.
+///
+/// Always used in a [MutableDelegatingStreamSubscriptionController].
+class _CompleterSubscriptionState implements StreamSubscription {
+  /// The mutable subscription wrapper.
+  ///
+  /// Used for handling [pause] with resume futures and [asFuture] which
+  /// both need to call a different function (`resume` and `cancel`
+  /// respectively) at a later point. We let those go through the
+  /// wrapper subscription to ensure that they are forwarded to the
+  /// controller source subscription that is current at that time.
+  final MutableDelegatingStreamSubscriptionController _controller;
+
+  /// Whether the subscription cancels on error.
+  ///
+  /// This is forwarded to the real subscription when that is created.
+  final bool _cancelOnError;
+
+  // Callbacks forwarded to the real subscription when it's created.
+
+  ZoneUnaryCallback _onData;
+  Function _onError;
+  ZoneCallback _onDone;
+
+  /// Future set when cancel is called.
+  /// This both marks the subscription as canceled and allows returning

[nweiz, 2015/06/16 01:05:23]

Nit: newline above this.

[Lasse Reichstein Nielsen, 2015/06/18 12:10:12]

Acknowledged.

+  /// the same future every time the cancel function is called.
+  Future cancelFuture;
+
+  /// Count of active pauses.
+  ///
+  /// When the real subscription is created, it is paused this many times.
+  int pauseCount = 0;
+
+  _CompleterSubscriptionState(this._cancelOnError,
+                              this._controller);
+
+  void onData(void handleData(data)) {
+    _onData = handleData;
+  }
+
+  void onError(Function handleError) {
+    _onError = handleError;
+  }
+
+  void onDone(void handleDone()) {
+    _onDone = handleDone;
+  }
+
+  void pause([Future resumeFuture]) {
+    pauseCount++;
+    if (resumeFuture != null) {
+      // Go through wrapper subscription in case the real subscription
+      // is linked before the future completes.
+      resumeFuture.whenComplete(_controller.subscription.resume);
+    }
+  }
+
+  void resume() {
+    if (pauseCount > 0) pauseCount--;
+  }
+
+  Future cancel() {
+    var cancelFuture = new Future.value();

[nweiz, 2015/06/16 01:05:23]

Why return a future here? cancel() is allowed to return null right?

[Lasse Reichstein Nielsen, 2015/06/18 12:10:12]

Yes, sadly. That's a consequence of backwards compatibility - we added the
future after cancel had already existed for a while with a void return type, so
we had to accept null.

It's annoying to have to check, so I try to not return null, in the naïve hope
that we can eventually get rid of nulls.

Ooooh, and non-null-by-default might actually get us there!

+    // Immediately replace the [_CompleterSubscription._delegate] so
+    // we won't be called again.
+    // This also releases any, now unused, callbacks we are holding on to.
+    _controller.sourceSubscription = new _CanceledSubscription(cancelFuture);
+    return cancelFuture;
+  }
+
+  bool get isPaused {
+    return (cancelFuture != null && pauseCount > 0);
+  }
+
+  Future asFuture([futureValue]) {
+    Completer completer = new Completer();
+    _onDone = () {
+      completer.complete(futureValue);
+    };
+    _onError = (error, StackTrace stackTrace) {
+      // Cancel the wrapper subscription in case the real subscription
+      // is linked before an error triggers this function.
+      _controller.subscription.cancel();
+      completer.completeError(error, stackTrace);
+    };
+    return completer.future;
+  }
+
+  StreamSubscription _linkStream(Stream stream) {
+    if (cancelFuture != null) {
+      return new _CanceledSubscription(cancelFuture);
+    }
+    // If not canceled, create the real subscription and make
+    // sure it has the requested callbacks, cancelOnErrror flag and
+    // number of pauses.
+    var subscription = stream.listen(null, cancelOnError: _cancelOnError);
+    subscription.onData(_onData);
+    subscription.onError(_onError);
+    subscription.onDone(_onDone);
+    while (pauseCount > 0) {
+      subscription.pause();
+      pauseCount--;
+    }
+    return subscription;
+  }
+}
+
+/// A subscription that acts as if it has been canceled.
+///
+/// No events are fired and pausing is ignored.
+/// The [cancel] method always returns the same future.
+class _CanceledSubscription implements StreamSubscription {
+  /// The future returned by [cancel];
+  Future _doneFuture;
+
+  _CanceledSubscription(this._doneFuture);
+
+  void onData(void handleData(data)) {}
+
+  void onError(Function handleError) {}
+
+  void onDone(void handleDone()) {}
+
+  void pause([Future resumeFuture]) {}
+
+  void resume() {}
+
+  Future cancel() => _doneFuture;
+
+  /// Returns future that never completes.
+  ///
+  /// The `asFuture` result is completed by either an error event
+  /// or a done event. A canceled future never produces either.
+  Future asFuture([futureValue]) => new Completer().future;
+
+  bool get isPaused => false;
+}