blob: 147fa8bd5f2984b66335bc4a78f8b9b8e2648a3b [file] [log] [blame]
// 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 "stream_completer.dart";
import "delegate/stream.dart";
/// A [Stream] wrapper that forwards to another [Stream] that's initialized
/// lazily.
/// This class allows a concrete `Stream` to be created only once it has a
/// listener. It's useful to wrapping APIs that do expensive computation to
/// produce a `Stream`.
class LazyStream<T> extends Stream<T> {
/// The callback that's called to create the inner stream.
ZoneCallback _callback;
/// Creates a single-subscription `Stream` that calls [callback] when it gets
/// a listener and forwards to the returned stream.
/// The [callback] may return a `Stream` or a `Future<Stream>`.
LazyStream(callback()) : _callback = callback {
// Explicitly check for null because we null out [_callback] internally.
if (_callback == null) throw new ArgumentError.notNull('callback');
StreamSubscription<T> listen(void onData(T event),
{Function onError,
void onDone(),
bool cancelOnError}) {
if (_callback == null) {
throw new StateError("Stream has already been listened to.");
// Null out the callback before we invoke it to ensure that even while
// running it, this can't be called twice.
var callback = _callback;
_callback = null;
var result = callback();
Stream<T> stream;
if (result is Future) {
stream = StreamCompleter.fromFuture(result.then((stream) {
return DelegatingStream.typed/*<T>*/(stream as Stream);
} else {
stream = DelegatingStream.typed/*<T>*/(result as Stream);
return stream.listen(onData,
onError: onError, onDone: onDone, cancelOnError: cancelOnError);