packages/quiver/lib/src/async/future_stream.dart

// Copyright 2014 Google Inc. All Rights Reserved.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

part of quiver.async;

/// A Stream that will emit the same values as the stream returned by [future]
/// once [future] completes.
///
/// If [future] completes to an error, the return value will emit that error
/// and then close.
///
/// If [broadcast] is true, this will be a broadcast stream. This assumes that
/// the stream returned by [future] will be a broadcast stream as well.
/// [broadcast] defaults to false.
///
/// # Example
///
/// This class is useful when you need to retreive some object via a `Future`,
/// then return a `Stream` from that object:
///
///     var futureOfStream = getResource().then((resource) => resource.stream);
///     return new FutureStream(futureOfStream);
class FutureStream<T> extends Stream<T> {
  static T _identity<T>(T t) => t;

  Future<Stream<T>> _future;
  StreamController<T> _controller;
  StreamSubscription _subscription;

  FutureStream(Future<Stream<T>> future, {bool broadcast: false}) {
    _future = future.then(_identity, onError: (e, stackTrace) {
      // Since [controller] is synchronous, it's likely that emitting an error
      // will cause it to be cancelled before we call close.
      if (_controller != null) {
        _controller.addError(e, stackTrace);
        _controller.close();
      }
      _controller = null;
    });

    if (broadcast == true) {
      _controller = new StreamController.broadcast(
          sync: true, onListen: _onListen, onCancel: _onCancel);
    } else {
      _controller = new StreamController(
          sync: true, onListen: _onListen, onCancel: _onCancel);
    }
  }

  _onListen() {
    _future.then((stream) {
      if (_controller == null) return;
      _subscription = stream.listen(_controller.add,
          onError: _controller.addError, onDone: _controller.close);
    });
  }

  _onCancel() {
    if (_subscription != null) _subscription.cancel();
    _subscription = null;
    _controller = null;
  }

  StreamSubscription<T> listen(void onData(T event),
      {Function onError, void onDone(), bool cancelOnError}) {
    return _controller.stream.listen(onData,
        onError: onError, onDone: onDone, cancelOnError: cancelOnError);
  }

  bool get isBroadcast => _controller.stream.isBroadcast;
}