Add typed wrapper functions to delegate classes.

lib/src/lazy_stream.dart

 // 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`.

[floitsch, 2016/04/12 20:03:08]

Why would it be expensive to create a Stream?
As long as no one is listening there should be no work.

Should we deprecate this class?

[Lasse Reichstein Nielsen, 2016/04/12 20:18:01]

I think an example would be an API that provide the content of an XMLHttpRequest
call. The HTMLHttpRequest body is itself a stream, but there is an overhead (and
async delay) required in order to get access to it. This is an easy way to write
a stream that provides the same events as the request body, without doing the
initialization before you actually need it, and still return a Stream object
synchronously.

It's equivalent to a StreamCompleter with an onListen callback that creates the
stream and completes the completer.

 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 stream = result is Future
-        ? StreamCompleter.fromFuture(result)
-        : result;
+    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);
   }
 }