Automated g4 rollback of changelist 173332168.

*** Reason for rollback ***

Broke Google internal application.

*** Original change description ***

Add some convenient(?) Observable-related utils.

- ObservableReference.listen(Stream), which acts like StreamController.addStream
- ObservableView.fromStream, which stashes the most recently emitted value
- ObservableView.map, which lazily transforms an ObservableView using a function

***

PiperOrigin-RevId: 173339037

Author: Googler

lib/model/observable/observable.dart

@@ -164,69 +164,20 @@ class ChangeNotificationProvider<T> implements ChangeAware<T>, Disposable {
 
 /// A read-only 'view' of something. Allows getting the current value and
 /// listening for values and changes.
-abstract class ObservableView<T> extends ChangeAware<T> implements Disposable {
+abstract class ObservableView<T> extends ChangeAware<T> {
   T get value;
-
-  /// Returns a new [ObservableView] which is created by lazily calling [mapper]
-  /// on this view's [value], [stream], and [changes] properties.
-  ///
-  /// Disposing of the mapped view doesn't affect this view in any way.
-  ObservableView<M> map<M>(M Function(T) mapper);
-
-  /// An [ObservableView] of the most-recently-published value on the given
-  /// [stream].
-  factory ObservableView.fromStream(Stream<T> stream, {T initialValue}) =>
-      new ObservableReference(initialValue)..listen(stream);
-}
-
-/// Implements methods of [ObservableView] in terms of the basic [value] and
-/// [stream] properties.
-abstract class ObservableViewMixin<T> implements ObservableView<T> {
-  @override
-  Stream<Change<T>> get changes {
-    var last = value;
-    // Want to do this using stream.map so that the `changes` stream has the
-    // same broadcastness/syncness as the `stream` stream.
-    return stream.map((v) {
-      var change = new Change(last, v);
-      last = v;
-      return change;
-    });
-  }
-
-  @override
-  ObservableView<M> map<M>(M Function(T) mapper) =>
-      new _MappedView<T, M>(this, mapper);
-}
-
-/// An [ObservableView] that just points at an existing [ObservableView] and
-/// passes it through a mapping function.
-class _MappedView<I, O> extends ObservableViewMixin<O> {
-  final ObservableView<I> _delegate;
-  final O Function(I) _mapper;
-  _MappedView(this._delegate, this._mapper);
-
-  @override
-  O get value => _mapper(_delegate.value);
-
-  @override
-  Stream<O> get stream => _delegate.stream.map(_mapper);
-
-  @override
-  void dispose() {}
 }
 
 /// A mutable object holder that allows listening on a stream of changes.
 ///
 /// Changes to the value using `value=` are added to a broadcast stream. If the
 /// new value is equivalent to the current value, nothing's added to the stream.
 class ObservableReference<T> extends ChangeNotificationProvider<T>
-    with ObservableViewMixin<T> {
+    implements ObservableView<T>, ObserveAware<T>, Disposable {
   static bool _defaultEq(a, b) => a == b;
 
-  final EqualsFn _equalsFn;
-  StreamSubscription _listenSub;
   T _value;
+  EqualsFn _equalsFn;
 
   /// Creates a listenable value holder, starting with the given value.
   /// Optionally takes custom equality function.
@@ -249,23 +200,9 @@ class ObservableReference<T> extends ChangeNotificationProvider<T>
     notifyChange(previous, value);
   }
 
-  /// Listens to the given stream and sets the reference's value to whatever is
-  /// emitted on the stream.
-  ///
-  /// Returns a [Future] that completes when the stream is done, analogous to
-  /// `StreamController.addStream`.
-  Future listen(Stream<T> stream) {
-    _listenSub?.cancel();
-    _listenSub = stream.listen((v) {
-      value = v;
-    });
-    return _listenSub.asFuture();
-  }
-
   @override
   void dispose() {
     super.dispose();
-    _listenSub?.cancel();
     _value = null;
   }
 }