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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.
import 'dart:async';
import 'future_group.dart';
import 'result/result.dart';
/// A class that splits a single source stream into an arbitrary number of
/// (single-subscription) streams (called "branch") that emit the same events.
/// Each branch will emit all the same values and errors as the source stream,
/// regardless of which values have been emitted on other branches. This means
/// that the splitter stores every event that has been emitted so far, which may
/// consume a lot of memory. The user can call [close] to indicate that no more
/// branches will be created, and this memory will be released.
/// The source stream is only listened to once a branch is created *and listened
/// to*. It's paused when all branches are paused *or when all branches are
/// canceled*, and resumed once there's at least one branch that's listening and
/// unpaused. It's not canceled unless no branches are listening and [close] has
/// been called.
class StreamSplitter<T> {
/// The wrapped stream.
final Stream<T> _stream;
/// The subscription to [_stream].
/// This will be `null` until a branch has a listener.
StreamSubscription<T> _subscription;
/// The buffer of events or errors that have already been emitted by
/// [_stream].
final _buffer = <Result<T>>[];
/// The controllers for branches that are listening for future events from
/// [_stream].
/// Once a branch is canceled, it's removed from this list. When [_stream] is
/// done, all branches are removed.
final _controllers = <StreamController<T>>{};
/// A group of futures returned by [close].
/// This is used to ensure that [close] doesn't complete until all
/// [StreamController.close] and [StreamSubscription.cancel] calls complete.
final _closeGroup = FutureGroup();
/// Whether [_stream] is done emitting events.
var _isDone = false;
/// Whether [close] has been called.
var _isClosed = false;
/// Splits [stream] into [count] identical streams.
/// [count] defaults to 2. This is the same as creating [count] branches and
/// then closing the [StreamSplitter].
static List<Stream<T>> splitFrom<T>(Stream<T> stream, [int count]) {
count ??= 2;
var splitter = StreamSplitter<T>(stream);
var streams = List<Stream<T>>.generate(count, (_) => splitter.split());
return streams;
/// Returns a single-subscription stream that's a copy of the input stream.
/// This will throw a [StateError] if [close] has been called.
Stream<T> split() {
if (_isClosed) {
throw StateError("Can't call split() on a closed StreamSplitter.");
var controller = StreamController<T>(
onListen: _onListen, onPause: _onPause, onResume: _onResume);
controller.onCancel = () => _onCancel(controller);
for (var result in _buffer) {
if (_isDone) {
} else {
/// Indicates that no more branches will be requested via [split].
/// This clears the internal buffer of events. If there are no branches or all
/// branches have been canceled, this cancels the subscription to the input
/// stream.
/// Returns a [Future] that completes once all events have been processed by
/// all branches and (if applicable) the subscription to the input stream has
/// been canceled.
Future close() {
if (_isClosed) return _closeGroup.future;
_isClosed = true;
if (_controllers.isEmpty) _cancelSubscription();
return _closeGroup.future;
/// Cancel [_subscription] and close [_closeGroup].
/// This should be called after all the branches' subscriptions have been
/// canceled and the splitter has been closed. In that case, we won't use the
/// events from [_subscription] any more, since there's nothing to pipe them
/// to and no more branches will be created. If [_subscription] is done,
/// canceling it will be a no-op.
/// This may also be called before any branches have been created, in which
/// case [_subscription] will be `null`.
void _cancelSubscription() {
Future future;
if (_subscription != null) future = _subscription.cancel();
if (future != null) _closeGroup.add(future);
// StreamController events
/// Subscribe to [_stream] if we haven't yet done so, and resume the
/// subscription if we have.
void _onListen() {
if (_isDone) return;
if (_subscription != null) {
// Resume the subscription in case it was paused, either because all the
// controllers were paused or because the last one was canceled. If it
// wasn't paused, this will be a no-op.
} else {
_subscription =
_stream.listen(_onData, onError: _onError, onDone: _onDone);
/// Pauses [_subscription] if every controller is paused.
void _onPause() {
if (!_controllers.every((controller) => controller.isPaused)) return;
/// Resumes [_subscription].
/// If [_subscription] wasn't paused, this is a no-op.
void _onResume() {
/// Removes [controller] from [_controllers] and cancels or pauses
/// [_subscription] as appropriate.
/// Since the controller emitting a done event will cause it to register as
/// canceled, this is the only way that a controller is ever removed from
/// [_controllers].
void _onCancel(StreamController controller) {
if (_controllers.isNotEmpty) return;
if (_isClosed) {
} else {
// Stream events
/// Buffers [data] and passes it to [_controllers].
void _onData(T data) {
if (!_isClosed) _buffer.add(Result.value(data));
for (var controller in _controllers) {
/// Buffers [error] and passes it to [_controllers].
void _onError(Object error, StackTrace stackTrace) {
if (!_isClosed) _buffer.add(Result.error(error, stackTrace));
for (var controller in _controllers) {
controller.addError(error, stackTrace);
/// Marks [_controllers] as done.
void _onDone() {
_isDone = true;
for (var controller in _controllers) {