Line data Source code
1 : // Copyright (c) 2015, the Dart project authors. Please see the AUTHORS file
2 : // for details. All rights reserved. Use of this source code is governed by a
3 : // BSD-style license that can be found in the LICENSE file.
4 :
5 : import "dart:async";
6 :
7 : import "delegate/stream.dart";
8 : import "stream_completer.dart";
9 : import "utils.dart";
10 :
11 : /// A [Stream] wrapper that forwards to another [Stream] that's initialized
12 : /// lazily.
13 : ///
14 : /// This class allows a concrete `Stream` to be created only once it has a
15 : /// listener. It's useful to wrapping APIs that do expensive computation to
16 : /// produce a `Stream`.
17 : class LazyStream<T> extends Stream<T> {
18 : /// The callback that's called to create the inner stream.
19 : FutureOrCallback<Stream<T>> _callback;
20 :
21 : /// Creates a single-subscription `Stream` that calls [callback] when it gets
22 : /// a listener and forwards to the returned stream.
23 0 : LazyStream(FutureOr<Stream<T>> callback()) : _callback = callback {
24 : // Explicitly check for null because we null out [_callback] internally.
25 0 : if (_callback == null) throw new ArgumentError.notNull('callback');
26 : }
27 :
28 : StreamSubscription<T> listen(void onData(T event),
29 : {Function onError, void onDone(), bool cancelOnError}) {
30 0 : if (_callback == null) {
31 0 : throw new StateError("Stream has already been listened to.");
32 : }
33 :
34 : // Null out the callback before we invoke it to ensure that even while
35 : // running it, this can't be called twice.
36 0 : var callback = _callback;
37 0 : _callback = null;
38 0 : var result = callback();
39 :
40 : Stream<T> stream;
41 0 : if (result is Future<Stream<T>>) {
42 0 : stream = StreamCompleter.fromFuture(result.then((stream) {
43 0 : return DelegatingStream.typed<T>(stream);
44 : }));
45 : } else {
46 0 : stream = DelegatingStream.typed<T>(result as Stream);
47 : }
48 :
49 0 : return stream.listen(onData,
50 : onError: onError, onDone: onDone, cancelOnError: cancelOnError);
51 : }
52 : }
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