blob: e6779ee5949e82fc7dadf2f8d47650a8b3c7cc3d [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 'utils.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.
FutureOrCallback<Stream<T>>? _callback;
/// Creates a single-subscription `Stream` that calls [callback] when it gets
/// a listener and forwards to the returned stream.
LazyStream(FutureOr<Stream<T>> Function() callback) : _callback = callback {
// Explicitly check for null because we null out [_callback] internally.
if (_callback == null) throw ArgumentError.notNull('callback');
StreamSubscription<T> listen(void Function(T)? onData,
{Function? onError, void Function()? onDone, bool? cancelOnError}) {
var callback = _callback;
if (callback == null) {
throw 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.
_callback = null;
var result = callback();
Stream<T> stream;
if (result is Future<Stream<T>>) {
stream = StreamCompleter.fromFuture(result);
} else {
stream = result;
return stream.listen(onData,
onError: onError, onDone: onDone, cancelOnError: cancelOnError);