Idea: Provide initial data to the StreamBuilder (#13820)

packages/flutter/lib/src/widgets/async.dart

@@ -267,13 +267,13 @@ typedef Widget AsyncWidgetBuilder<T>(BuildContext context, AsyncSnapshot<T> snap
 /// a [Stream].
 ///
 /// Widget rebuilding is scheduled by each interaction, using [State.setState],
-/// but is otherwise decoupled from the timing of the stream. The [build] method
+/// but is otherwise decoupled from the timing of the stream. The [builder]
 /// is called at the discretion of the Flutter pipeline, and will thus receive a
 /// timing-dependent sub-sequence of the snapshots that represent the
 /// interaction with the stream.
 ///
 /// As an example, when interacting with a stream producing the integers
-/// 0 through 9, the [build] method may be called with any ordered sub-sequence
+/// 0 through 9, the [builder] may be called with any ordered sub-sequence
 /// of the following snapshots that includes the last one (the one with
 /// ConnectionState.done):
 ///
@@ -284,8 +284,9 @@ typedef Widget AsyncWidgetBuilder<T>(BuildContext context, AsyncSnapshot<T> snap
 /// * `new AsyncSnapshot<int>.withData(ConnectionState.active, 9)`
 /// * `new AsyncSnapshot<int>.withData(ConnectionState.done, 9)`
 ///
-/// The actual sequence of invocations of [build] depends on the relative timing
-/// of events produced by the stream and the build rate of the Flutter pipeline.
+/// The actual sequence of invocations of the [builder] depends on the relative
+/// timing of events produced by the stream and the build rate of the Flutter
+/// pipeline.
 ///
 /// Changing the [StreamBuilder] configuration to another stream during event
 /// generation introduces snapshot pairs of the form
@@ -303,6 +304,11 @@ typedef Widget AsyncWidgetBuilder<T>(BuildContext context, AsyncSnapshot<T> snap
 /// The data and error fields of snapshots produced are only changed when the
 /// state is `ConnectionState.active`.
 ///
+/// The initial snapshot data can be controlled by specifying [initialData].
+/// You would use this facility to ensure that if the [builder] is invoked
+/// before the first event arrives on the stream, the snapshot carries data of
+/// your choice rather than the default null value.
+///
 /// See also:
 ///
 /// * [StreamBuilderBase], which supports widget building based on a computation
@@ -333,19 +339,25 @@ typedef Widget AsyncWidgetBuilder<T>(BuildContext context, AsyncSnapshot<T> snap
 class StreamBuilder<T> extends StreamBuilderBase<T, AsyncSnapshot<T>> {
   /// Creates a new [StreamBuilder] that builds itself based on the latest
   /// snapshot of interaction with the specified [stream] and whose build
-  /// strategy is given by [builder].
+  /// strategy is given by [builder]. The [initialData] is used to create the
+  /// initial snapshot. It is null by default.
   const StreamBuilder({
     Key key,
+    this.initialData,
     Stream<T> stream,
     @required this.builder
-  }) : assert(builder != null),
-       super(key: key, stream: stream);
+  })
+      : assert(builder != null),
+        super(key: key, stream: stream);
 
   /// The build strategy currently used by this builder. Cannot be null.
   final AsyncWidgetBuilder<T> builder;
 
+  /// The data that will be used to create the initial snapshot. Null by default.
+  final T initialData;
+
   @override
-  AsyncSnapshot<T> initial() => new AsyncSnapshot<T>.nothing(); // ignore: prefer_const_constructors
+  AsyncSnapshot<T> initial() => new AsyncSnapshot<T>.withData(ConnectionState.none, initialData);
 
   @override
   AsyncSnapshot<T> afterConnected(AsyncSnapshot<T> current) => current.inState(ConnectionState.waiting);
@@ -391,6 +403,11 @@ class StreamBuilder<T> extends StreamBuilderBase<T, AsyncSnapshot<T>> {
 /// * `new AsyncSnapshot<String>.withData(ConnectionState.waiting, null)`
 /// * `new AsyncSnapshot<String>.withError(ConnectionState.done, 'some error')`
 ///
+/// The initial snapshot data can be controlled by specifying [initialData]. You
+/// would use this facility to ensure that if the [builder] is invoked before
+/// the future completes, the snapshot carries data of your choice rather than
+/// the default null value.
+///
 /// The data and error fields of the snapshot change only as the connection
 /// state field transitions from `waiting` to `done`, and they will be retained
 /// when changing the [FutureBuilder] configuration to another future. If the
@@ -437,6 +454,7 @@ class FutureBuilder<T> extends StatefulWidget {
   const FutureBuilder({
     Key key,
     this.future,
+    this.initialData,
     @required this.builder
   }) : assert(builder != null),
        super(key: key);
@@ -448,6 +466,9 @@ class FutureBuilder<T> extends StatefulWidget {
   /// The build strategy currently used by this builder. Cannot be null.
   final AsyncWidgetBuilder<T> builder;
 
+  /// The data that will be used to create the initial snapshot. Null by default.
+  final T initialData;
+
   @override
   State<FutureBuilder<T>> createState() => new _FutureBuilderState<T>();
 }
@@ -458,11 +479,12 @@ class _FutureBuilderState<T> extends State<FutureBuilder<T>> {
   /// calling setState from stale callbacks, e.g. after disposal of this state,
   /// or after widget reconfiguration to a new Future.
   Object _activeCallbackIdentity;
-  AsyncSnapshot<T> _snapshot = new AsyncSnapshot<T>.nothing(); // ignore: prefer_const_constructors
+  AsyncSnapshot<T> _snapshot;
 
   @override
   void initState() {
     super.initState();
+    _snapshot = new AsyncSnapshot<T>.withData(ConnectionState.none, widget.initialData);
     _subscribe();
   }
 

packages/flutter/test/widgets/async_test.dart

@@ -118,6 +118,34 @@ void main() {
       await eventFiring(tester);
       expect(find.text('AsyncSnapshot(ConnectionState.done, null, bad)'), findsOneWidget);
     });
+    testWidgets('runs the builder using given initial data', (WidgetTester tester) async {
+      final GlobalKey key = new GlobalKey();
+      await tester.pumpWidget(new FutureBuilder<String>(
+        key: key,
+        future: null,
+        builder: snapshotText,
+        initialData: 'I',
+      ));
+      expect(find.text('AsyncSnapshot(ConnectionState.none, I, null)'), findsOneWidget);
+    });
+    testWidgets('ignores initialData when reconfiguring', (WidgetTester tester) async {
+      final GlobalKey key = new GlobalKey();
+      await tester.pumpWidget(new FutureBuilder<String>(
+        key: key,
+        future: null,
+        builder: snapshotText,
+        initialData: 'I',
+      ));
+      expect(find.text('AsyncSnapshot(ConnectionState.none, I, null)'), findsOneWidget);
+      final Completer<String> completer = new Completer<String>();
+      await tester.pumpWidget(new FutureBuilder<String>(
+        key: key,
+        future: completer.future,
+        builder: snapshotText,
+        initialData: 'Ignored',
+      ));
+      expect(find.text('AsyncSnapshot(ConnectionState.waiting, I, null)'), findsOneWidget);
+    });
   });
   group('StreamBuilder', () {
     testWidgets('gracefully handles transition from null stream', (WidgetTester tester) async {
@@ -180,6 +208,33 @@ void main() {
       await eventFiring(tester);
       expect(find.text('AsyncSnapshot(ConnectionState.done, 4, null)'), findsOneWidget);
     });
+    testWidgets('runs the builder using given initial data', (WidgetTester tester) async {
+      final StreamController<String> controller = new StreamController<String>();
+      await tester.pumpWidget(new StreamBuilder<String>(
+        stream: controller.stream,
+        builder: snapshotText,
+        initialData: 'I',
+      ));
+      expect(find.text('AsyncSnapshot(ConnectionState.waiting, I, null)'), findsOneWidget);
+    });
+    testWidgets('ignores initialData when reconfiguring', (WidgetTester tester) async {
+      final GlobalKey key = new GlobalKey();
+      await tester.pumpWidget(new StreamBuilder<String>(
+        key: key,
+        stream: null,
+        builder: snapshotText,
+        initialData: 'I',
+      ));
+      expect(find.text('AsyncSnapshot(ConnectionState.none, I, null)'), findsOneWidget);
+      final StreamController<String> controller = new StreamController<String>();
+      await tester.pumpWidget(new StreamBuilder<String>(
+        key: key,
+        stream: controller.stream,
+        builder: snapshotText,
+        initialData: 'Ignored',
+      ));
+      expect(find.text('AsyncSnapshot(ConnectionState.waiting, I, null)'), findsOneWidget);
+    });
   });
   group('FutureBuilder and StreamBuilder behave identically on Stream from Future', () {
     testWidgets('when completing with data', (WidgetTester tester) async {
@@ -211,6 +266,24 @@ void main() {
       ]));
       expect(find.text('AsyncSnapshot(ConnectionState.none, null, null)'), findsNWidgets(2));
     });
+    testWidgets('when initialData is used with null Future and Stream', (WidgetTester tester) async {
+      await tester.pumpWidget(new Column(children: <Widget>[
+        new FutureBuilder<String>(future: null, builder: snapshotText, initialData: 'I'),
+        new StreamBuilder<String>(stream: null, builder: snapshotText, initialData: 'I'),
+      ]));
+      expect(find.text('AsyncSnapshot(ConnectionState.none, I, null)'), findsNWidgets(2));
+    });
+    testWidgets('when using initialData and completing with data', (WidgetTester tester) async {
+      final Completer<String> completer = new Completer<String>();
+      await tester.pumpWidget(new Column(children: <Widget>[
+        new FutureBuilder<String>(future: completer.future, builder: snapshotText, initialData: 'I'),
+        new StreamBuilder<String>(stream: completer.future.asStream(), builder: snapshotText, initialData: 'I'),
+      ]));
+      expect(find.text('AsyncSnapshot(ConnectionState.waiting, I, null)'), findsNWidgets(2));
+      completer.complete('hello');
+      await eventFiring(tester);
+      expect(find.text('AsyncSnapshot(ConnectionState.done, hello, null)'), findsNWidgets(2));
+    });
   });
   group('StreamBuilderBase', () {
     testWidgets('gracefully handles transition from null stream', (WidgetTester tester) async {