Automatic silver keep alive (#11067)

* AutomaticKeepAlive

A Widget that listens for notifications from widgets that don't want to die.

* Automatically wrap SliverList and SliverGrid children in AutomaticKeepAlive widgets

* Fixes for review comments

packages/flutter/lib/src/material/ink_well.dart

@@ -238,12 +238,14 @@ class InkResponse extends StatefulWidget {
   }
 }
 
-class _InkResponseState<T extends InkResponse> extends State<T> {
-
+class _InkResponseState<T extends InkResponse> extends State<T> with AutomaticKeepAliveClientMixin {
   Set<InkSplash> _splashes;
   InkSplash _currentSplash;
   InkHighlight _lastHighlight;
 
+  @override
+  bool get wantKeepAlive => _lastHighlight != null || (_splashes != null && _splashes.isNotEmpty);
+
   void updateHighlight(bool value) {
     if (value == (_lastHighlight != null && _lastHighlight.active))
       return;
@@ -260,8 +262,10 @@ class _InkResponseState<T extends InkResponse> extends State<T> {
           onRemoved: () {
             assert(_lastHighlight != null);
             _lastHighlight = null;
+            updateKeepAlive();
           },
         );
+        updateKeepAlive();
       } else {
         _lastHighlight.activate();
       }
@@ -292,12 +296,14 @@ class _InkResponseState<T extends InkResponse> extends State<T> {
           _splashes.remove(splash);
           if (_currentSplash == splash)
             _currentSplash = null;
+          updateKeepAlive();
         } // else we're probably in deactivate()
       }
     );
     _splashes ??= new HashSet<InkSplash>();
     _splashes.add(splash);
     _currentSplash = splash;
+    updateKeepAlive();
     updateHighlight(true);
   }
 
@@ -353,6 +359,7 @@ class _InkResponseState<T extends InkResponse> extends State<T> {
   @override
   Widget build(BuildContext context) {
     assert(widget.debugCheckContext(context));
+    super.build(context); // See AutomaticKeepAliveClientMixin.
     final ThemeData themeData = Theme.of(context);
     _lastHighlight?.color = widget.highlightColor ?? themeData.highlightColor;
     _currentSplash?.color = widget.splashColor ?? themeData.splashColor;

packages/flutter/lib/src/material/input_decorator.dart

@@ -12,6 +12,11 @@ import 'theme.dart';
 const Duration _kTransitionDuration = const Duration(milliseconds: 200);
 const Curve _kTransitionCurve = Curves.fastOutSlowIn;
 
+// See the InputDecorator.build method, where this is used.
+class _InputDecoratorChildGlobalKey extends GlobalObjectKey {
+  const _InputDecoratorChildGlobalKey(BuildContext value) : super(value);
+}
+
 /// Text and styles used to label an input field.
 ///
 /// See also:
@@ -482,7 +487,19 @@ class InputDecorator extends StatelessWidget {
       );
     }
 
-    Widget inputChild;
+    Widget inputChild = new KeyedSubtree(
+      // It's important that we maintain the state of our child subtree, as it
+      // may be stateful (e.g. containing text selections). Since our build
+      // function risks changing the depth of the tree, we preserve the subtree
+      // using global keys.
+      // GlobalObjectKey(context) will always be the same whenever we are built.
+      // Additionally, we use a subclass of GlobalObjectKey to avoid clashes
+      // with anyone else using our BuildContext as their global object key
+      // value.
+      key: new _InputDecoratorChildGlobalKey(context),
+      child: child,
+    );
+
     if (!hasInlineLabel && (!isEmpty || hintText == null) &&
         (decoration?.prefixText != null || decoration?.suffixText != null)) {
       final List<Widget> rowContents = <Widget>[];
@@ -492,7 +509,7 @@ class InputDecorator extends StatelessWidget {
             style: decoration.prefixStyle ?? hintStyle)
         );
       }
-      rowContents.add(new Expanded(child: child));
+      rowContents.add(new Expanded(child: inputChild));
       if (decoration.suffixText != null) {
         rowContents.add(
             new Text(decoration.suffixText,
@@ -500,8 +517,6 @@ class InputDecorator extends StatelessWidget {
         );
       }
       inputChild = new Row(children: rowContents);
-    } else {
-      inputChild = child;
     }
 
     if (isCollapsed) {

packages/flutter/lib/src/material/material.dart

@@ -294,24 +294,26 @@ class _RenderInkFeatures extends RenderProxyBox implements MaterialInkController
   @override
   Color color;
 
-  final List<InkFeature> _inkFeatures = <InkFeature>[];
+  List<InkFeature> _inkFeatures;
 
   @override
   void addInkFeature(InkFeature feature) {
     assert(!feature._debugDisposed);
     assert(feature._controller == this);
+    _inkFeatures ??= <InkFeature>[];
     assert(!_inkFeatures.contains(feature));
     _inkFeatures.add(feature);
     markNeedsPaint();
   }
 
   void _removeFeature(InkFeature feature) {
+    assert(_inkFeatures != null);
     _inkFeatures.remove(feature);
     markNeedsPaint();
   }
 
   void _didChangeLayout() {
-    if (_inkFeatures.isNotEmpty)
+    if (_inkFeatures != null && _inkFeatures.isNotEmpty)
       markNeedsPaint();
   }
 
@@ -320,7 +322,7 @@ class _RenderInkFeatures extends RenderProxyBox implements MaterialInkController
 
   @override
   void paint(PaintingContext context, Offset offset) {
-    if (_inkFeatures.isNotEmpty) {
+    if (_inkFeatures != null && _inkFeatures.isNotEmpty) {
       final Canvas canvas = context.canvas;
       canvas.save();
       canvas.translate(offset.dx, offset.dy);
@@ -334,7 +336,12 @@ class _RenderInkFeatures extends RenderProxyBox implements MaterialInkController
 }
 
 class _InkFeatures extends SingleChildRenderObjectWidget {
-  const _InkFeatures({ Key key, this.color, Widget child, @required this.vsync }) : super(key: key, child: child);
+  const _InkFeatures({
+    Key key,
+    this.color,
+    @required this.vsync,
+    Widget child,
+  }) : super(key: key, child: child);
 
   // This widget must be owned by a MaterialState, which must be provided as the vsync.
   // This relationship must be 1:1 and cannot change for the lifetime of the MaterialState.
@@ -347,7 +354,7 @@ class _InkFeatures extends SingleChildRenderObjectWidget {
   _RenderInkFeatures createRenderObject(BuildContext context) {
     return new _RenderInkFeatures(
       color: color,
-      vsync: vsync
+      vsync: vsync,
     );
   }
 
@@ -368,7 +375,7 @@ abstract class InkFeature {
   InkFeature({
     @required MaterialInkController controller,
     @required this.referenceBox,
-    this.onRemoved
+    this.onRemoved,
   }) : assert(controller != null),
        assert(referenceBox != null),
        _controller = controller;

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

@@ -4,6 +4,7 @@
 
 import 'package:flutter/foundation.dart';
 
+import 'automatic_keep_alive.dart';
 import 'basic.dart';
 import 'framework.dart';
 import 'gesture_detector.dart';
@@ -164,7 +165,7 @@ class _DismissibleClipper extends CustomClipper<Rect> {
   }
 }
 
-class _DismissibleState extends State<Dismissible> with TickerProviderStateMixin {
+class _DismissibleState extends State<Dismissible> with TickerProviderStateMixin, AutomaticKeepAliveClientMixin {
   @override
   void initState() {
     super.initState();
@@ -183,6 +184,9 @@ class _DismissibleState extends State<Dismissible> with TickerProviderStateMixin
   bool _dragUnderway = false;
   Size _sizePriorToCollapse;
 
+  @override
+  bool get wantKeepAlive => _moveController?.isAnimating == true || _resizeController?.isAnimating == true;
+
   @override
   void dispose() {
     _moveController.dispose();
@@ -323,6 +327,7 @@ class _DismissibleState extends State<Dismissible> with TickerProviderStateMixin
   void _handleDismissStatusChanged(AnimationStatus status) {
     if (status == AnimationStatus.completed && !_dragUnderway)
       _startResizeAnimation();
+    updateKeepAlive();
   }
 
   void _startResizeAnimation() {
@@ -335,7 +340,8 @@ class _DismissibleState extends State<Dismissible> with TickerProviderStateMixin
         widget.onDismissed(_dismissDirection);
     } else {
       _resizeController = new AnimationController(duration: widget.resizeDuration, vsync: this)
-        ..addListener(_handleResizeProgressChanged);
+        ..addListener(_handleResizeProgressChanged)
+        ..addStatusListener((AnimationStatus status) => updateKeepAlive());
       _resizeController.forward();
       setState(() {
         _sizePriorToCollapse = context.size;
@@ -362,6 +368,7 @@ class _DismissibleState extends State<Dismissible> with TickerProviderStateMixin
 
   @override
   Widget build(BuildContext context) {
+    super.build(context); // See AutomaticKeepAliveClientMixin.
     Widget background = widget.background;
     if (widget.secondaryBackground != null) {
       final DismissDirection direction = _dismissDirection;

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

@@ -8,6 +8,7 @@ import 'package:flutter/foundation.dart';
 import 'package:flutter/rendering.dart';
 import 'package:flutter/services.dart';
 
+import 'automatic_keep_alive.dart';
 import 'basic.dart';
 import 'focus_manager.dart';
 import 'focus_scope.dart';
@@ -252,7 +253,7 @@ class EditableText extends StatefulWidget {
     description.add('focusNode: $focusNode');
     if (obscureText != false)
       description.add('obscureText: $obscureText');
-    description.add('$style');
+    description.add('${style.toString().split("\n").join(", ")}');
     if (textAlign != null)
       description.add('$textAlign');
     if (textScaleFactor != null)
@@ -267,7 +268,7 @@ class EditableText extends StatefulWidget {
 }
 
 /// State for a [EditableText].
-class EditableTextState extends State<EditableText> implements TextInputClient {
+class EditableTextState extends State<EditableText> with AutomaticKeepAliveClientMixin implements TextInputClient {
   Timer _cursorTimer;
   final ValueNotifier<bool> _showCursor = new ValueNotifier<bool>(false);
 
@@ -278,6 +279,9 @@ class EditableTextState extends State<EditableText> implements TextInputClient {
   final LayerLink _layerLink = new LayerLink();
   bool _didAutoFocus = false;
 
+  @override
+  bool get wantKeepAlive => widget.focusNode.hasFocus;
+
   // State lifecycle:
 
   @override
@@ -308,6 +312,7 @@ class EditableTextState extends State<EditableText> implements TextInputClient {
     if (widget.focusNode != oldWidget.focusNode) {
       oldWidget.focusNode.removeListener(_handleFocusChanged);
       widget.focusNode.addListener(_handleFocusChanged);
+      updateKeepAlive();
     }
   }
 
@@ -549,11 +554,13 @@ class EditableTextState extends State<EditableText> implements TextInputClient {
       // Clear the selection and composition state if this widget lost focus.
       _value = new TextEditingValue(text: _value.text);
     }
+    updateKeepAlive();
   }
 
   @override
   Widget build(BuildContext context) {
     FocusScope.of(context).reparentIfNeeded(widget.focusNode);
+    super.build(context); // See AutomaticKeepAliveClientMixin.
     return new Scrollable(
       axisDirection: _isMultiline ? AxisDirection.down : AxisDirection.right,
       controller: _scrollController,

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

@@ -343,6 +343,22 @@ class LabeledGlobalKey<T extends State<StatefulWidget>> extends GlobalKey<T> {
 /// Used to tie the identity of a widget to the identity of an object used to
 /// generate that widget.
 ///
+/// If the object is not private, then it is possible that collisions will occur
+/// where independent widgets will reuse the same object as their
+/// [GlobalObjectKey] value in a different part of the tree, leading to a global
+/// key conflict. To avoid this problem, create a private [GlobalObjectKey]
+/// subclass, as in:
+///
+/// ```dart
+/// class _MyKey extends GlobalObjectKey {
+///   const _MyKey(Object value) : super(value);
+/// }
+/// ```
+///
+/// Since the [runtimeType] of the key is part of its identity, this will
+/// prevent clashes with other [GlobalObjectKey]s even if they have the same
+/// value.
+///
 /// Any [GlobalObjectKey] created for the same value will match.
 @optionalTypeArgs
 class GlobalObjectKey<T extends State<StatefulWidget>> extends GlobalKey<T> {
@@ -1418,6 +1434,19 @@ abstract class ParentDataWidget<T extends RenderObjectWidget> extends ProxyWidge
   /// parent, as appropriate.
   @protected
   void applyParentData(RenderObject renderObject);
+
+  /// Whether the [ParentDataElement.applyWidgetOutOfTurn] method is allowed
+  /// with this widget.
+  ///
+  /// This should only return true if this widget represents a [ParentData]
+  /// configuration that will have no impact on the layout or paint phase.
+  ///
+  /// See also:
+  ///
+  ///  * [ParentDataElement.applyWidgetOutOfTurn], which verifies this in debug
+  ///    mode.
+  @protected
+  bool debugCanApplyOutOfTurn() => false;
 }
 
 /// Base class for widgets that efficiently propagate information down the tree.
@@ -2470,6 +2499,11 @@ abstract class Element implements BuildContext {
   ///
   /// There is no guaranteed order in which the children will be visited, though
   /// it should be consistent over time.
+  ///
+  /// Calling this during build is dangerous: the child list might still be
+  /// being updated at that point, so the children might not be constructed yet,
+  /// or might be old children that are going to be replaced. This method should
+  /// only be called if it is provable that the children are available.
   void visitChildren(ElementVisitor visitor) { }
 
   /// Calls the argument for each child that is relevant for semantics. By
@@ -2477,11 +2511,20 @@ abstract class Element implements BuildContext {
   /// to hide their children.
   void visitChildrenForSemantics(ElementVisitor visitor) => visitChildren(visitor);
 
-  /// Wrapper around visitChildren for BuildContext.
+  /// Wrapper around [visitChildren] for [BuildContext].
   @override
   void visitChildElements(ElementVisitor visitor) {
-    // don't allow visitChildElements() during build, since children aren't necessarily built yet
-    assert(owner == null || !owner._debugStateLocked);
+    assert(() {
+      if (owner == null || !owner._debugStateLocked)
+        return true;
+      throw new FlutterError(
+        'visitChildElements() called during build.\n'
+        'The BuildContext.visitChildElements() method can\'t be called during '
+        'build because the child list is still being updated at that point, '
+        'so the children might not be constructed yet, or might be old children '
+        'that are going to be replaced.'
+      );
+    });
     visitChildren(visitor);
   }
 
@@ -3685,18 +3728,60 @@ class ParentDataElement<T extends RenderObjectWidget> extends ProxyElement {
     super.mount(parent, slot);
   }
 
-  void _notifyChildren(Element child) {
-    if (child is RenderObjectElement) {
-      child._updateParentData(widget);
-    } else {
-      assert(child is! ParentDataElement<RenderObjectWidget>);
-      child.visitChildren(_notifyChildren);
+  void _applyParentData(ParentDataWidget<T> widget) {
+    void applyParentDataToChild(Element child) {
+      if (child is RenderObjectElement) {
+        child._updateParentData(widget);
+      } else {
+        assert(child is! ParentDataElement<RenderObjectWidget>);
+        child.visitChildren(applyParentDataToChild);
+      }
     }
+    visitChildren(applyParentDataToChild);
+  }
+
+  /// Calls [ParentDataWidget.applyParentData] on the given widget, passing it
+  /// the [RenderObject] whose parent data this element is ultimately
+  /// responsible for.
+  ///
+  /// This allows a render object's [RenderObject.parentData] to be modified
+  /// without triggering a build. This is generally ill-advised, but makes sense
+  /// in situations such as the following:
+  ///
+  ///  * Build and layout are currently under way, but the [ParentData] in question
+  ///    does not affect layout, and the value to be applied could not be
+  ///    determined before build and layout (e.g. it depends on the layout of a
+  ///    descendant).
+  ///
+  ///  * Paint is currently under way, but the [ParentData] in question does not
+  ///    affect layour or paint, and the value to be applied could not be
+  ///    determined before paint (e.g. it depends on the compositing phase).
+  ///
+  /// In either case, the next build is expected to cause this element to be
+  /// configured with the given new widget (or a widget with equivalent data).
+  ///
+  /// Only [ParentDataWidget]s that return true for
+  /// [ParentDataWidget.debugCanApplyOutOfTurn] can be applied this way.
+  ///
+  /// The new widget must have the same child as the current widget.
+  ///
+  /// An example of when this is used is the [AutomaticKeepAlive] widget. If it
+  /// receives a notification during the build of one of its descendants saying
+  /// that its child must be kept alive, it will apply a [KeepAlive] widget out
+  /// of turn. This is safe, because by definition the child is already alive,
+  /// and therefore this will not change the behavior of the parent this frame.
+  /// It is more efficient than requesting an additional frame just for the
+  /// purpose of updating the [KeepAlive] widget.
+  void applyWidgetOutOfTurn(ParentDataWidget<T> newWidget) {
+    assert(newWidget != null);
+    assert(newWidget.debugCanApplyOutOfTurn());
+    assert(newWidget.child == widget.child);
+    _applyParentData(newWidget);
   }
 
   @override
   void notifyClients(ParentDataWidget<T> oldWidget) {
-    visitChildren(_notifyChildren);
+    _applyParentData(widget);
   }
 }
 

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

@@ -26,6 +26,8 @@ typedef bool NotificationListenerCallback<T extends Notification>(T notification
 /// widgets with the appropriate type parameters that are ancestors of the given
 /// [BuildContext].
 abstract class Notification {
+  const Notification();
+
   /// Applied to each ancestor of the [dispatch] target.
   ///
   /// The [Notification] class implementation of this method dispatches the
@@ -89,7 +91,7 @@ class NotificationListener<T extends Notification> extends StatelessWidget {
   const NotificationListener({
     Key key,
     @required this.child,
-    this.onNotification
+    this.onNotification,
   }) : super(key: key);
 
   /// The widget below this widget in the tree.

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

@@ -515,8 +515,10 @@ class ListView extends BoxScrollView {
   /// It is usually more efficient to create children on demand using [new
   /// ListView.builder].
   ///
-  /// The `addRepaintBoundaries` argument corresponds to the
-  /// [SliverChildListDelegate.addRepaintBoundaries] property and must not be
+  /// The `addAutomaticKeepAlives` argument corresponds to the
+  /// [SliverChildListDelegate.addAutomaticKeepAlives] property. The
+  /// `addRepaintBoundaries` argument corresponds to the
+  /// [SliverChildListDelegate.addRepaintBoundaries] property. Both must not be
   /// null.
   ListView({
     Key key,
@@ -528,10 +530,12 @@ class ListView extends BoxScrollView {
     bool shrinkWrap: false,
     EdgeInsets padding,
     this.itemExtent,
+    bool addAutomaticKeepAlives: true,
     bool addRepaintBoundaries: true,
     List<Widget> children: const <Widget>[],
   }) : childrenDelegate = new SliverChildListDelegate(
          children,
+         addAutomaticKeepAlives: addAutomaticKeepAlives,
          addRepaintBoundaries: addRepaintBoundaries,
        ), super(
     key: key,
@@ -563,9 +567,11 @@ class ListView extends BoxScrollView {
   /// Even more efficient, however, is to create the instances on demand using
   /// this constructor's `itemBuilder` callback.
   ///
-  /// The `addRepaintBoundaries` argument corresponds to the
-  /// [SliverChildBuilderDelegate.addRepaintBoundaries] property and must not be
-  /// null.
+  /// The `addAutomaticKeepAlives` argument corresponds to the
+  /// [SliverChildBuilderDelegate.addAutomaticKeepAlives] property. The
+  /// `addRepaintBoundaries` argument corresponds to the
+  /// [SliverChildBuilderDelegate.addRepaintBoundaries] property. Both must not
+  /// be null.
   ListView.builder({
     Key key,
     Axis scrollDirection: Axis.vertical,
@@ -578,10 +584,12 @@ class ListView extends BoxScrollView {
     this.itemExtent,
     @required IndexedWidgetBuilder itemBuilder,
     int itemCount,
+    bool addAutomaticKeepAlives: true,
     bool addRepaintBoundaries: true,
   }) : childrenDelegate = new SliverChildBuilderDelegate(
          itemBuilder,
          childCount: itemCount,
+         addAutomaticKeepAlives: addAutomaticKeepAlives,
          addRepaintBoundaries: addRepaintBoundaries,
        ), super(
     key: key,
@@ -783,8 +791,10 @@ class GridView extends BoxScrollView {
   ///
   /// The [gridDelegate] argument must not be null.
   ///
-  /// The `addRepaintBoundaries` argument corresponds to the
-  /// [SliverChildListDelegate.addRepaintBoundaries] property and must not be
+  /// The `addAutomaticKeepAlives` argument corresponds to the
+  /// [SliverChildListDelegate.addAutomaticKeepAlives] property. The
+  /// `addRepaintBoundaries` argument corresponds to the
+  /// [SliverChildListDelegate.addRepaintBoundaries] property. Both must not be
   /// null.
   GridView({
     Key key,
@@ -796,11 +806,13 @@ class GridView extends BoxScrollView {
     bool shrinkWrap: false,
     EdgeInsets padding,
     @required this.gridDelegate,
+    bool addAutomaticKeepAlives: true,
     bool addRepaintBoundaries: true,
     List<Widget> children: const <Widget>[],
   }) : assert(gridDelegate != null),
        childrenDelegate = new SliverChildListDelegate(
          children,
+         addAutomaticKeepAlives: addAutomaticKeepAlives,
          addRepaintBoundaries: addRepaintBoundaries,
        ),
        super(
@@ -828,9 +840,11 @@ class GridView extends BoxScrollView {
   ///
   /// The [gridDelegate] argument must not be null.
   ///
-  /// The `addRepaintBoundaries` argument corresponds to the
-  /// [SliverChildBuilderDelegate.addRepaintBoundaries] property and must not be
-  /// null.
+  /// The `addAutomaticKeepAlives` argument corresponds to the
+  /// [SliverChildBuilderDelegate.addAutomaticKeepAlives] property. The
+  /// `addRepaintBoundaries` argument corresponds to the
+  /// [SliverChildBuilderDelegate.addRepaintBoundaries] property. Both must not
+  /// be null.
   GridView.builder({
     Key key,
     Axis scrollDirection: Axis.vertical,
@@ -843,11 +857,13 @@ class GridView extends BoxScrollView {
     @required this.gridDelegate,
     @required IndexedWidgetBuilder itemBuilder,
     int itemCount,
+    bool addAutomaticKeepAlives: true,
     bool addRepaintBoundaries: true,
   }) : assert(gridDelegate != null),
        childrenDelegate = new SliverChildBuilderDelegate(
          itemBuilder,
          childCount: itemCount,
+         addAutomaticKeepAlives: addAutomaticKeepAlives,
          addRepaintBoundaries: addRepaintBoundaries,
        ),
        super(
@@ -897,8 +913,10 @@ class GridView extends BoxScrollView {
   ///
   /// Uses a [SliverGridDelegateWithFixedCrossAxisCount] as the [gridDelegate].
   ///
-  /// The `addRepaintBoundaries` argument corresponds to the
-  /// [SliverChildListDelegate.addRepaintBoundaries] property and must not be
+  /// The `addAutomaticKeepAlives` argument corresponds to the
+  /// [SliverChildListDelegate.addAutomaticKeepAlives] property. The
+  /// `addRepaintBoundaries` argument corresponds to the
+  /// [SliverChildListDelegate.addRepaintBoundaries] property. Both must not be
   /// null.
   ///
   /// See also:
@@ -917,6 +935,7 @@ class GridView extends BoxScrollView {
     double mainAxisSpacing: 0.0,
     double crossAxisSpacing: 0.0,
     double childAspectRatio: 1.0,
+    bool addAutomaticKeepAlives: true,
     bool addRepaintBoundaries: true,
     List<Widget> children: const <Widget>[],
   }) : gridDelegate = new SliverGridDelegateWithFixedCrossAxisCount(
@@ -927,6 +946,7 @@ class GridView extends BoxScrollView {
        ),
        childrenDelegate = new SliverChildListDelegate(
          children,
+         addAutomaticKeepAlives: addAutomaticKeepAlives,
          addRepaintBoundaries: addRepaintBoundaries,
        ), super(
     key: key,
@@ -944,8 +964,10 @@ class GridView extends BoxScrollView {
   ///
   /// Uses a [SliverGridDelegateWithMaxCrossAxisExtent] as the [gridDelegate].
   ///
-  /// The `addRepaintBoundaries` argument corresponds to the
-  /// [SliverChildListDelegate.addRepaintBoundaries] property and must not be
+  /// The `addAutomaticKeepAlives` argument corresponds to the
+  /// [SliverChildListDelegate.addAutomaticKeepAlives] property. The
+  /// `addRepaintBoundaries` argument corresponds to the
+  /// [SliverChildListDelegate.addRepaintBoundaries] property. Both must not be
   /// null.
   ///
   /// See also:
@@ -964,6 +986,7 @@ class GridView extends BoxScrollView {
     double mainAxisSpacing: 0.0,
     double crossAxisSpacing: 0.0,
     double childAspectRatio: 1.0,
+    bool addAutomaticKeepAlives: true,
     bool addRepaintBoundaries: true,
     List<Widget> children: const <Widget>[],
   }) : gridDelegate = new SliverGridDelegateWithMaxCrossAxisExtent(
@@ -974,6 +997,7 @@ class GridView extends BoxScrollView {
        ),
        childrenDelegate = new SliverChildListDelegate(
          children,
+         addAutomaticKeepAlives: addAutomaticKeepAlives,
          addRepaintBoundaries: addRepaintBoundaries,
        ), super(
     key: key,

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

@@ -7,6 +7,7 @@ import 'dart:collection' show SplayTreeMap, HashMap;
 import 'package:flutter/foundation.dart';
 import 'package:flutter/rendering.dart';
 
+import 'automatic_keep_alive.dart';
 import 'basic.dart';
 import 'framework.dart';
 
@@ -43,7 +44,7 @@ abstract class SliverChildDelegate {
   /// exists.
   ///
   /// Subclasses typically override this function and wrap their children in
-  /// [RepaintBoundary] widgets.
+  /// [AutomaticKeepAlive] and [RepaintBoundary] widgets.
   Widget build(BuildContext context, int index);
 
   /// Returns an estimate of the number of children this delegate will build.
@@ -119,7 +120,9 @@ abstract class SliverChildDelegate {
 /// not even have to be built until they are displayed.
 ///
 /// The widgets returned from the builder callback are automatically wrapped in
-/// [RepaintBoundary] widgets if [addRepaintBoundaries] is true (the default).
+/// [AutomaticKeepAlive] widgets if [addAutomaticKeepAlives] is true (the
+/// default) and in [RepaintBoundary] widgets if [addRepaintBoundaries] is true
+/// (also the default).
 ///
 /// See also:
 ///
@@ -129,12 +132,15 @@ class SliverChildBuilderDelegate extends SliverChildDelegate {
   /// Creates a delegate that supplies children for slivers using the given
   /// builder callback.
   ///
-  /// The [builder] and [addRepaintBoundaries] arguments must not be null.
+  /// The [builder], [addAutomaticKeepAlives], and [addRepaintBoundaries]
+  /// arguments must not be null.
   const SliverChildBuilderDelegate(
     this.builder, {
     this.childCount,
+    this.addAutomaticKeepAlives: true,
     this.addRepaintBoundaries: true,
   }) : assert(builder != null),
+       assert(addAutomaticKeepAlives != null),
        assert(addRepaintBoundaries != null);
 
   /// Called to build children for the sliver.
@@ -155,6 +161,20 @@ class SliverChildBuilderDelegate extends SliverChildDelegate {
   /// [builder] returns null.
   final int childCount;
 
+  /// Whether to wrap each child in an [AutomaticKeepAlive].
+  ///
+  /// Typically, children in lazy list are wrapped in [AutomaticKeepAlive]
+  /// widgets so that children can use [KeepAliveNotification]s to preserve
+  /// their state when they would otherwise be garbage collected off-screen.
+  ///
+  /// This feature (and [addRepaintBoundaries]) must be disabled if the children
+  /// are going to manually maintain their [KeepAlive] state. It may also be
+  /// more efficient to disable this feature if it is known ahead of time that
+  /// none of the children will ever try to keep themselves alive.
+  ///
+  /// Defaults to true.
+  final bool addAutomaticKeepAlives;
+
   /// Whether to wrap each child in a [RepaintBoundary].
   ///
   /// Typically, children in a scrolling container are wrapped in repaint
@@ -171,10 +191,14 @@ class SliverChildBuilderDelegate extends SliverChildDelegate {
     assert(builder != null);
     if (index < 0 || (childCount != null && index >= childCount))
       return null;
-    final Widget child = builder(context, index);
+    Widget child = builder(context, index);
     if (child == null)
       return null;
-    return addRepaintBoundaries ? new RepaintBoundary.wrap(child, index) : child;
+    if (addRepaintBoundaries)
+      child = new RepaintBoundary.wrap(child, index);
+    if (addAutomaticKeepAlives)
+      child = new AutomaticKeepAlive(child: child);
+    return child;
   }
 
   @override
@@ -205,7 +229,9 @@ class SliverChildBuilderDelegate extends SliverChildDelegate {
 /// conditions.
 ///
 /// The widgets in the given [children] list are automatically wrapped in
-/// [RepaintBoundary] widgets if [addRepaintBoundaries] is true (the default).
+/// [AutomaticKeepAlive] widgets if [addAutomaticKeepAlives] is true (the
+/// default) and in [RepaintBoundary] widgets if [addRepaintBoundaries] is true
+/// (also the default).
 ///
 /// See also:
 ///
@@ -215,13 +241,30 @@ class SliverChildListDelegate extends SliverChildDelegate {
   /// Creates a delegate that supplies children for slivers using the given
   /// list.
   ///
-  /// The [children] and [addRepaintBoundaries] arguments must not be null.
+  /// The [children], [addAutomaticKeepAlives], and [addRepaintBoundaries]
+  /// arguments must not be null.
   const SliverChildListDelegate(
     this.children, {
+    this.addAutomaticKeepAlives: true,
     this.addRepaintBoundaries: true,
   }) : assert(children != null),
+       assert(addAutomaticKeepAlives != null),
        assert(addRepaintBoundaries != null);
 
+  /// Whether to wrap each child in an [AutomaticKeepAlive].
+  ///
+  /// Typically, children in lazy list are wrapped in [AutomaticKeepAlive]
+  /// widgets so that children can use [KeepAliveNotification]s to preserve
+  /// their state when they would otherwise be garbage collected off-screen.
+  ///
+  /// This feature (and [addRepaintBoundaries]) must be disabled if the children
+  /// are going to manually maintain their [KeepAlive] state. It may also be
+  /// more efficient to disable this feature if it is known ahead of time that
+  /// none of the children will ever try to keep themselves alive.
+  ///
+  /// Defaults to true.
+  final bool addAutomaticKeepAlives;
+
   /// Whether to wrap each child in a [RepaintBoundary].
   ///
   /// Typically, children in a scrolling container are wrapped in repaint
@@ -241,9 +284,13 @@ class SliverChildListDelegate extends SliverChildDelegate {
     assert(children != null);
     if (index < 0 || index >= children.length)
       return null;
-    final Widget child = children[index];
+    Widget child = children[index];
     assert(child != null);
-    return addRepaintBoundaries ? new RepaintBoundary.wrap(child, index) : child;
+    if (addRepaintBoundaries)
+      child = new RepaintBoundary.wrap(child, index);
+    if (addAutomaticKeepAlives)
+      child = new AutomaticKeepAlive(child: child);
+    return child;
   }
 
   @override
@@ -875,11 +922,18 @@ class KeepAlive extends ParentDataWidget<SliverMultiBoxAdaptorWidget> {
     if (parentData.keepAlive != keepAlive) {
       parentData.keepAlive = keepAlive;
       final AbstractNode targetParent = renderObject.parent;
-      if (targetParent is RenderObject)
-        targetParent.markNeedsLayout();
+      if (targetParent is RenderObject && !keepAlive)
+        targetParent.markNeedsLayout(); // No need to redo layout if it became true.
     }
   }
 
+  // We only return true if [keepAlive] is true, because turning _off_ keep
+  // alive requires a layout to do the garbage collection (but turning it on
+  // requires nothing, since by definition the widget is already alive and won't
+  // go away _unless_ we do a layout).
+  @override
+  bool debugCanApplyOutOfTurn() => keepAlive;
+
   @override
   void debugFillDescription(List<String> description) {
     super.debugFillDescription(description);

packages/flutter/lib/widgets.dart

@@ -19,6 +19,7 @@ export 'src/widgets/animated_list.dart';
 export 'src/widgets/animated_size.dart';
 export 'src/widgets/app.dart';
 export 'src/widgets/async.dart';
+export 'src/widgets/automatic_keep_alive.dart';
 export 'src/widgets/banner.dart';
 export 'src/widgets/basic.dart';
 export 'src/widgets/binding.dart';

packages/flutter/test/material/ink_well_test.dart

@@ -3,8 +3,10 @@
 // found in the LICENSE file.
 
 import 'package:flutter/material.dart';
+import 'package:flutter/rendering.dart';
 import 'package:flutter_test/flutter_test.dart';
 
+import '../rendering/mock_canvas.dart';
 import 'feedback_tester.dart';
 
 void main() {
@@ -116,4 +118,37 @@ void main() {
       expect(feedback.hapticCount, 0);
     });
   });
+
+  testWidgets('splashing survives scrolling when keep-alive is enabled', (WidgetTester tester) async {
+    Future<Null> runTest(bool keepAlive) async {
+      await tester.pumpWidget(new Material(
+        child: new CompositedTransformFollower( // forces a layer, which makes the paints easier to separate out
+          link: new LayerLink(),
+          child: new ListView(
+            addAutomaticKeepAlives: keepAlive,
+            children: <Widget>[
+              new Container(height: 500.0, child: new InkWell(onTap: () { }, child: const Placeholder())),
+              new Container(height: 500.0),
+              new Container(height: 500.0),
+            ],
+          ),
+        ),
+      ));
+      expect(tester.renderObject<RenderProxyBox>(find.byType(PhysicalModel)).child, paintsNothing);
+      await tester.tap(find.byType(InkWell));
+      await tester.pump();
+      await tester.pump(const Duration(milliseconds: 10));
+      expect(tester.renderObject<RenderProxyBox>(find.byType(PhysicalModel)).child, paints..circle());
+      await tester.drag(find.byType(ListView), const Offset(0.0, -1000.0));
+      await tester.pump(const Duration(milliseconds: 10));
+      await tester.drag(find.byType(ListView), const Offset(0.0, 1000.0));
+      await tester.pump(const Duration(milliseconds: 10));
+      expect(
+        tester.renderObject<RenderProxyBox>(find.byType(PhysicalModel)).child,
+        keepAlive ? (paints..circle()) : paintsNothing,
+      );
+    }
+    await runTest(true);
+    await runTest(false);
+  });
 }

packages/flutter/test/material/text_field_focus_test.dart

@@ -132,4 +132,78 @@ void main() {
     expect(tester.testTextInput.isVisible, isFalse);
   });
 
+  testWidgets('Focus triggers keep-alive', (WidgetTester tester) async {
+    final FocusNode focusNode = new FocusNode();
+
+    await tester.pumpWidget(
+      new MaterialApp(
+        home: new Material(
+          child: new ListView(
+            children: <Widget>[
+              new TextField(
+                focusNode: focusNode,
+              ),
+              new Container(
+                height: 1000.0,
+              ),
+            ],
+          ),
+        ),
+      ),
+    );
+
+    expect(find.byType(TextField), findsOneWidget);
+    expect(tester.testTextInput.isVisible, isFalse);
+
+    FocusScope.of(tester.element(find.byType(TextField))).requestFocus(focusNode);
+    await tester.pump();
+    expect(find.byType(TextField), findsOneWidget);
+    expect(tester.testTextInput.isVisible, isTrue);
+
+    await tester.drag(find.byType(ListView), const Offset(0.0, -1000.0));
+    await tester.pump();
+    expect(find.byType(TextField), findsOneWidget);
+    expect(tester.testTextInput.isVisible, isTrue);
+
+    focusNode.unfocus();
+    await tester.pump();
+
+    expect(find.byType(TextField), findsNothing);
+    expect(tester.testTextInput.isVisible, isFalse);
+  });
+
+  testWidgets('Focus keep-alive works with GlobalKey reparenting', (WidgetTester tester) async {
+    final FocusNode focusNode = new FocusNode();
+
+    Widget makeTest(String prefix) {
+      return new MaterialApp(
+        home: new Material(
+          child: new ListView(
+            children: <Widget>[
+              new TextField(
+                focusNode: focusNode,
+                decoration: new InputDecoration(
+                  prefixText: prefix,
+                ),
+              ),
+              new Container(
+                height: 1000.0,
+              ),
+            ],
+          ),
+        ),
+      );
+    }
+
+    await tester.pumpWidget(makeTest(null));
+    FocusScope.of(tester.element(find.byType(TextField))).requestFocus(focusNode);
+    await tester.pump();
+    expect(find.byType(TextField), findsOneWidget);
+    await tester.drag(find.byType(ListView), const Offset(0.0, -1000.0));
+    await tester.pump();
+    expect(find.byType(TextField), findsOneWidget);
+    await tester.pumpWidget(makeTest('test'));
+    await tester.pump(); // in case the AutomaticKeepAlive widget thinks it needs a cleanup frame
+    expect(find.byType(TextField), findsOneWidget);
+  });
 }

packages/flutter/test/rendering/mock_canvas.dart

@@ -36,8 +36,13 @@ import 'recording_canvas.dart';
 /// order).
 ///
 /// See [PaintPattern] for a discussion of the semantics of paint patterns.
+///
+/// To match something which paints nothing, see [paintsNothing].
 PaintPattern get paints => new _TestRecordingCanvasPatternMatcher();
 
+/// Matches objects or functions that paint an empty display list.
+Matcher get paintsNothing => new _TestRecordingCanvasPaintsNothingMatcher();
+
 /// Signature for [PaintPattern.something] predicate argument.
 ///
 /// Used by the [paints] matcher.
@@ -243,7 +248,73 @@ abstract class PaintPattern {
   void something(PaintPatternPredicate predicate);
 }
 
-class _TestRecordingCanvasPatternMatcher extends Matcher implements PaintPattern {
+abstract class _TestRecordingCanvasMatcher extends Matcher {
+  @override
+  bool matches(Object object, Map<dynamic, dynamic> matchState) {
+    final TestRecordingCanvas canvas = new TestRecordingCanvas();
+    final TestRecordingPaintingContext context = new TestRecordingPaintingContext(canvas);
+    if (object is _ContextPainterFunction) {
+      final _ContextPainterFunction function = object;
+      function(context, Offset.zero);
+    } else if (object is _CanvasPainterFunction) {
+      final _CanvasPainterFunction function = object;
+      function(canvas);
+    } else {
+      if (object is Finder) {
+        TestAsyncUtils.guardSync();
+        final Finder finder = object;
+        object = finder.evaluate().single.renderObject;
+      }
+      if (object is RenderObject) {
+        final RenderObject renderObject = object;
+        renderObject.paint(context, Offset.zero);
+      } else {
+        matchState[this] = 'was not one of the supported objects for the "paints" matcher.';
+        return false;
+      }
+    }
+    final StringBuffer description = new StringBuffer();
+    final bool result = _evaluatePredicates(canvas.invocations, description);
+    if (!result) {
+      const String indent = '\n            '; // the length of '   Which: ' in spaces, plus two more
+      if (canvas.invocations.isNotEmpty)
+        description.write(' The complete display list was:');
+        for (Invocation call in canvas.invocations)
+          description.write('$indent${_describeInvocation(call)}');
+    }
+    matchState[this] = description.toString();
+    return result;
+  }
+
+  bool _evaluatePredicates(Iterable<Invocation> calls, StringBuffer description);
+
+  @override
+  Description describeMismatch(
+    dynamic item,
+    Description description,
+    Map<dynamic, dynamic> matchState,
+    bool verbose,
+  ) {
+    return description.add(matchState[this]);
+  }
+}
+
+class _TestRecordingCanvasPaintsNothingMatcher extends _TestRecordingCanvasMatcher {
+  @override
+  Description describe(Description description) {
+    return description.add('An object or closure that paints nothing.');
+  }
+
+  @override
+  bool _evaluatePredicates(Iterable<Invocation> calls, StringBuffer description) {
+    if (calls.isEmpty)
+      return true;
+    description.write('painted the following.');
+    return false;
+  }
+}
+
+class _TestRecordingCanvasPatternMatcher extends _TestRecordingCanvasMatcher implements PaintPattern {
   final List<_PaintPredicate> _predicates = <_PaintPredicate>[];
 
   @override
@@ -326,43 +397,6 @@ class _TestRecordingCanvasPatternMatcher extends Matcher implements PaintPattern
     _predicates.add(new _SomethingPaintPredicate(predicate));
   }
 
-  @override
-  bool matches(Object object, Map<dynamic, dynamic> matchState) {
-    final TestRecordingCanvas canvas = new TestRecordingCanvas();
-    final TestRecordingPaintingContext context = new TestRecordingPaintingContext(canvas);
-    if (object is _ContextPainterFunction) {
-      final _ContextPainterFunction function = object;
-      function(context, Offset.zero);
-    } else if (object is _CanvasPainterFunction) {
-      final _CanvasPainterFunction function = object;
-      function(canvas);
-    } else {
-      if (object is Finder) {
-        TestAsyncUtils.guardSync();
-        final Finder finder = object;
-        object = finder.evaluate().single.renderObject;
-      }
-      if (object is RenderObject) {
-        final RenderObject renderObject = object;
-        renderObject.paint(context, Offset.zero);
-      } else {
-        matchState[this] = 'was not one of the supported objects for the "paints" matcher.';
-        return false;
-      }
-    }
-    final StringBuffer description = new StringBuffer();
-    final bool result = _evaluatePredicates(canvas.invocations, description);
-    if (!result) {
-      const String indent = '\n            '; // the length of '   Which: ' in spaces, plus two more
-      if (canvas.invocations.isNotEmpty)
-        description.write(' The complete display list was:');
-        for (Invocation call in canvas.invocations)
-          description.write('$indent${_describeInvocation(call)}');
-    }
-    matchState[this] = description.toString();
-    return result;
-  }
-
   @override
   Description describe(Description description) {
     if (_predicates.isEmpty)
@@ -375,18 +409,11 @@ class _TestRecordingCanvasPatternMatcher extends Matcher implements PaintPattern
   }
 
   @override
-  Description describeMismatch(
-    dynamic item,
-    Description description,
-    Map<dynamic, dynamic> matchState,
-    bool verbose,
-  ) {
-    return description.add(matchState[this]);
-  }
-
   bool _evaluatePredicates(Iterable<Invocation> calls, StringBuffer description) {
-    // If we ever want to have a matcher for painting nothing, create a separate
-    // paintsNothing matcher.
+    if (calls.isEmpty) {
+      description.write('painted nothing.');
+      return false;
+    }
     if (_predicates.isEmpty) {
       description.write(
         'painted something, but you must now add a pattern to the paints matcher '
@@ -394,10 +421,6 @@ class _TestRecordingCanvasPatternMatcher extends Matcher implements PaintPattern
       );
       return false;
     }
-    if (calls.isEmpty) {
-      description.write('painted nothing.');
-      return false;
-    }
     final Iterator<_PaintPredicate> predicate = _predicates.iterator;
     final Iterator<Invocation> call = calls.iterator..moveNext();
     try {

packages/flutter/test/widgets/keep_alive_test.dart

@@ -8,8 +8,7 @@ import 'package:flutter_test/flutter_test.dart';
 import 'package:flutter/widgets.dart';
 
 class Leaf extends StatefulWidget {
-  Leaf({ Key key, this.index, this.child }) : super(key: key);
-  final int index;
+  Leaf({ Key key, this.child }) : super(key: key);
   final Widget child;
   @override
   _LeafState createState() => new _LeafState();
@@ -36,7 +35,6 @@ List<Widget> generateList(Widget child) {
     100,
     (int index) => new Leaf(
       key: new GlobalObjectKey<_LeafState>(index),
-      index: index,
       child: child,
     ),
     growable: false,
@@ -46,6 +44,7 @@ List<Widget> generateList(Widget child) {
 void main() {
   testWidgets('KeepAlive with ListView with itemExtent', (WidgetTester tester) async {
     await tester.pumpWidget(new ListView(
+      addAutomaticKeepAlives: false,
       addRepaintBoundaries: false,
       itemExtent: 12.3, // about 50 widgets visible
       children: generateList(const Placeholder()),
@@ -85,6 +84,7 @@ void main() {
 
   testWidgets('KeepAlive with ListView without itemExtent', (WidgetTester tester) async {
     await tester.pumpWidget(new ListView(
+      addAutomaticKeepAlives: false,
       addRepaintBoundaries: false,
       children: generateList(new Container(height: 12.3, child: const Placeholder())), // about 50 widgets visible
     ));
@@ -123,6 +123,7 @@ void main() {
 
   testWidgets('KeepAlive with GridView', (WidgetTester tester) async {
     await tester.pumpWidget(new GridView.count(
+      addAutomaticKeepAlives: false,
       addRepaintBoundaries: false,
       crossAxisCount: 2,
       childAspectRatio: 400.0 / 24.6, // about 50 widgets visible
@@ -163,6 +164,7 @@ void main() {
 
   testWidgets('KeepAlive render tree description', (WidgetTester tester) async {
     await tester.pumpWidget(new ListView(
+      addAutomaticKeepAlives: false,
       addRepaintBoundaries: false,
       itemExtent: 400.0, // 2 visible children
       children: generateList(const Placeholder()),

packages/flutter/test/widgets/list_view_viewporting_test.dart

@@ -260,6 +260,7 @@ void main() {
 
   testWidgets('ListView underflow extents', (WidgetTester tester) async {
     await tester.pumpWidget(new ListView(
+      addAutomaticKeepAlives: false,
       children: <Widget>[
         new Container(height: 100.0),
         new Container(height: 100.0),

packages/flutter/test/widgets/sliver_fill_viewport_test.dart

@@ -15,7 +15,7 @@ void main() {
       new CustomScrollView(
         slivers: <Widget>[
           new SliverFillViewport(
-            delegate: new SliverChildListDelegate(children),
+            delegate: new SliverChildListDelegate(children, addAutomaticKeepAlives: false),
           ),
         ],
       ),