Remove the "most valuable keys" Hero feature (#5500)

examples/flutter_gallery/lib/demo/grid_list_demo.dart

@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-import 'dart:collection';
-
 import 'package:flutter/material.dart';
 import 'package:flutter/widgets.dart';
 
@@ -21,12 +19,11 @@ class Photo {
   final String caption;
 
   bool isFavorite;
+  String get tag => assetName; // Assuming that all asset names are unique.
 
   bool get isValid => assetName != null && title != null && caption != null && isFavorite != null;
 }
 
-const String photoHeroTag = 'Photo';
-
 typedef void BannerTapCallback(Photo photo);
 
 class GridDemoPhotoItem extends StatelessWidget {
@@ -46,21 +43,14 @@ class GridDemoPhotoItem extends StatelessWidget {
   final BannerTapCallback onBannerTap; // User taps on the photo's header or footer.
 
   void showPhoto(BuildContext context) {
-    Key photoKey = new Key(photo.assetName);
-    Set<Key> mostValuableKeys = new HashSet<Key>();
-    mostValuableKeys.add(photoKey);
-
     Navigator.push(context, new MaterialPageRoute<Null>(
-      settings: new RouteSettings(
-        mostValuableKeys: mostValuableKeys
-      ),
       builder: (BuildContext context) {
         return new Scaffold(
           appBar: new AppBar(
             title: new Text(photo.title)
           ),
           body: new Hero(
-            tag: photoHeroTag,
+            tag: photo.tag,
             child: new Image.asset(photo.assetName, fit: ImageFit.cover)
           )
         );
@@ -74,7 +64,7 @@ class GridDemoPhotoItem extends StatelessWidget {
       onTap: () { showPhoto(context); },
       child: new Hero(
         key: new Key(photo.assetName),
-        tag: photoHeroTag,
+        tag: photo.tag,
         child: new Image.asset(photo.assetName, fit: ImageFit.cover)
       )
     );

examples/flutter_gallery/lib/demo/shrine/shrine_home.dart

@@ -292,7 +292,7 @@ class ShrineHome extends StatefulWidget {
 }
 
 class _ShrineHomeState extends State<ShrineHome> {
-  static final GlobalKey<ScaffoldState> scaffoldKey = new GlobalKey<ScaffoldState>(debugLabel: 'Order page');
+  static final GlobalKey<ScaffoldState> scaffoldKey = new GlobalKey<ScaffoldState>(debugLabel: 'Shrine Home');
   static final GlobalKey<ScrollableState> scrollableKey = new GlobalKey<ScrollableState>();
   static final GridDelegate gridDelegate = new ShrineGridDelegate();
 

examples/flutter_gallery/lib/demo/shrine/shrine_order.dart

@@ -136,7 +136,7 @@ class OrderPage extends StatefulWidget {
 /// arranged in two columns. Enables the user to specify a quantity and add an
 /// order to the shopping cart.
 class _OrderPageState extends State<OrderPage> {
-  static final GlobalKey<ScaffoldState> scaffoldKey = new GlobalKey<ScaffoldState>(debugLabel: 'Order page');
+  static final GlobalKey<ScaffoldState> scaffoldKey = new GlobalKey<ScaffoldState>(debugLabel: 'Shrine Order');
   static final GlobalKey<ScrollableState> scrollableKey = new GlobalKey<ScrollableState>();
 
   Order get currentOrder => ShrineOrderRoute.of(context).order;

examples/stocks/lib/stock_home.dart

@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-import 'dart:collection';
-
 import 'package:flutter/material.dart';
 import 'package:flutter/rendering.dart' show debugDumpRenderTree, debugDumpLayerTree, debugDumpSemanticsTree;
 import 'package:flutter/scheduler.dart' show timeDilation;
@@ -245,7 +243,7 @@ class StockHomeState extends State<StockHome> {
     return stocks.where((Stock stock) => stock.symbol.contains(regexp));
   }
 
-  void _buyStock(Stock stock, Key arrowKey) {
+  void _buyStock(Stock stock) {
     setState(() {
       stock.percentChange = 100.0 * (1.0 / stock.lastSale);
       stock.lastSale += 1.0;
@@ -255,7 +253,7 @@ class StockHomeState extends State<StockHome> {
       action: new SnackBarAction(
         label: "BUY MORE",
         onPressed: () {
-          _buyStock(stock, arrowKey);
+          _buyStock(stock);
         }
       )
     ));
@@ -263,15 +261,12 @@ class StockHomeState extends State<StockHome> {
 
   Widget _buildStockList(BuildContext context, Iterable<Stock> stocks, StockHomeTab tab) {
     return new StockList(
-      keySalt: tab,
       stocks: stocks.toList(),
       onAction: _buyStock,
-      onOpen: (Stock stock, Key arrowKey) {
-        Set<Key> mostValuableKeys = new HashSet<Key>();
-        mostValuableKeys.add(arrowKey);
-        Navigator.pushNamed(context, '/stock/${stock.symbol}', mostValuableKeys: mostValuableKeys);
+      onOpen: (Stock stock) {
+        Navigator.pushNamed(context, '/stock/${stock.symbol}');
       },
-      onShow: (Stock stock, Key arrowKey) {
+      onShow: (Stock stock) {
         _scaffoldKey.currentState.showBottomSheet((BuildContext context) => new StockSymbolBottomSheet(stock: stock));
       }
     );

examples/stocks/lib/stock_list.dart

@@ -8,9 +8,8 @@ import 'stock_data.dart';
 import 'stock_row.dart';
 
 class StockList extends StatelessWidget {
-  StockList({ Key key, this.keySalt, this.stocks, this.onOpen, this.onShow, this.onAction }) : super(key: key);
+  StockList({ Key key, this.stocks, this.onOpen, this.onShow, this.onAction }) : super(key: key);
 
-  final Object keySalt;
   final List<Stock> stocks;
   final StockRowActionCallback onOpen;
   final StockRowActionCallback onShow;
@@ -23,7 +22,6 @@ class StockList extends StatelessWidget {
       itemExtent: StockRow.kHeight,
       children: stocks.map((Stock stock) {
         return new StockRow(
-          keySalt: keySalt,
           stock: stock,
           onPressed: onOpen,
           onDoubleTap: onShow,

examples/stocks/lib/stock_row.dart

@@ -7,58 +7,25 @@ import 'package:flutter/material.dart';
 import 'stock_data.dart';
 import 'stock_arrow.dart';
 
-enum StockRowPartKind { arrow }
-
-class StockRowPartKey extends LocalKey {
-  const StockRowPartKey(this.keySalt, this.stock, this.part);
-
-  final Object keySalt;
-  final Stock stock;
-  final StockRowPartKind part;
-
-  @override
-  bool operator ==(dynamic other) {
-    if (identical(this, other))
-      return true;
-    if (other.runtimeType != runtimeType)
-      return false;
-    final StockRowPartKey typedOther = other;
-    return keySalt == typedOther.keySalt
-        && stock == typedOther.stock
-        && part == typedOther.part;
-  }
-
-  @override
-  int get hashCode => hashValues(keySalt, stock, part);
-
-  @override
-  String toString() => '[$runtimeType ${keySalt.toString().split(".")[1]}:${stock.symbol}:${part.toString().split(".")[1]}]';
-}
-
-typedef void StockRowActionCallback(Stock stock, Key arrowKey);
+typedef void StockRowActionCallback(Stock stock);
 
 class StockRow extends StatelessWidget {
   StockRow({
     Stock stock,
-    Object keySalt,
     this.onPressed,
     this.onDoubleTap,
     this.onLongPressed
-  }) : this.stock = stock,
-       _arrowKey = new StockRowPartKey(keySalt, stock, StockRowPartKind.arrow),
-       super(key: new ObjectKey(stock));
+  }) : this.stock = stock, super(key: new ObjectKey(stock));
 
   final Stock stock;
   final StockRowActionCallback onPressed;
   final StockRowActionCallback onDoubleTap;
   final StockRowActionCallback onLongPressed;
 
-  final Key _arrowKey;
-
   static const double kHeight = 79.0;
 
   GestureTapCallback _getHandler(StockRowActionCallback callback) {
-    return callback == null ? null : () => callback(stock, _arrowKey);
+    return callback == null ? null : () => callback(stock);
   }
 
   @override
@@ -83,8 +50,7 @@ class StockRow extends StatelessWidget {
             new Container(
               margin: const EdgeInsets.only(right: 5.0),
               child: new Hero(
-                tag: StockRowPartKind.arrow,
-                key: _arrowKey,
+                tag: stock,
                 child: new StockArrow(percentChange: stock.percentChange)
               )
             ),

examples/stocks/lib/stock_symbol_viewer.dart

@@ -6,12 +6,12 @@ import 'package:flutter/material.dart';
 
 import 'stock_data.dart';
 import 'stock_arrow.dart';
-import 'stock_row.dart';
 
-class StockSymbolView extends StatelessWidget {
-  StockSymbolView({ this.stock });
+class _StockSymbolView extends StatelessWidget {
+  _StockSymbolView({ this.stock, this.arrow });
 
   final Stock stock;
+  final Widget arrow;
 
   @override
   Widget build(BuildContext context) {
@@ -31,12 +31,7 @@ class StockSymbolView extends StatelessWidget {
                 '${stock.symbol}',
                 style: Theme.of(context).textTheme.display2
               ),
-              new Hero(
-                key: new ObjectKey(stock),
-                tag: StockRowPartKind.arrow,
-                turns: 2,
-                child: new StockArrow(percentChange: stock.percentChange)
-              ),
+              arrow,
             ],
             mainAxisAlignment: MainAxisAlignment.spaceBetween
           ),
@@ -82,7 +77,16 @@ class StockSymbolPage extends StatelessWidget {
         children: <Widget>[
           new Container(
             margin: new EdgeInsets.all(20.0),
-            child: new Card(child: new StockSymbolView(stock: stock))
+            child: new Card(
+              child: new _StockSymbolView(
+                stock: stock,
+                arrow: new Hero(
+                  tag: stock,
+                  turns: 2,
+                  child: new StockArrow(percentChange: stock.percentChange)
+                )
+              )
+            )
           )
         ]
       )
@@ -102,7 +106,10 @@ class StockSymbolBottomSheet extends StatelessWidget {
       decoration: new BoxDecoration(
         border: new Border(top: new BorderSide(color: Colors.black26))
       ),
-      child: new StockSymbolView(stock: stock)
+      child: new _StockSymbolView(
+        stock: stock,
+        arrow: new StockArrow(percentChange: stock.percentChange)
+      )
    );
   }
 }

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

@@ -20,14 +20,9 @@ import 'transitions.dart';
 // album's details view. In this context, a screen is a navigator ModalRoute.
 
 // To get this effect, all you have to do is wrap each hero on each route with a
-// Hero widget, and give each hero a tag. Tag must either be unique within the
-// current route's widget subtree, or all the Heroes with that tag on a
-// particular route must have a key. When the app transitions from one route to
-// another, each tag present is animated. When there's exactly one hero with
-// that tag, that hero will be animated for that tag. When there are multiple
-// heroes in a route with the same tag, then whichever hero has a key that
-// matches one of the keys in the "most important key" list given to the
-// navigator when the route was pushed will be animated. If a hero is only
+// Hero widget, and give each hero a tag. The tag must either be unique within the
+// current route's widget subtree. When the app transitions from one route to
+// another, each hero is animated to its new location. If a hero is only
 // present on one of the routes and not the other, then it will be made to
 // appear or disappear as needed.
 
@@ -96,57 +91,35 @@ class Hero extends StatefulWidget {
   final int turns;
 
   /// If true, the hero will always animate, even if it has no matching hero to
-  /// animate to or from. (This only applies if the hero is relevant; if there
-  /// are multiple heroes with the same tag, only the one whose key matches the
-  /// "most valuable keys" will be used.)
+  /// animate to or from.
   final bool alwaysAnimate;
 
-  static Map<Object, HeroHandle> of(BuildContext context, Set<Key> mostValuableKeys) {
-    mostValuableKeys ??= new HashSet<Key>();
-    assert(!mostValuableKeys.contains(null));
-    // first we collect ALL the heroes, sorted by their tags
-    Map<Object, Map<Key, HeroState>> heroes = <Object, Map<Key, HeroState>>{};
+  /// Return a hero tag to HeroState map of all of the heroes within the given subtree.
+  static Map<Object, HeroHandle> of(BuildContext context) {
+    final Map<Object, HeroHandle> result = <Object, HeroHandle>{};
     void visitor(Element element) {
       if (element.widget is Hero) {
         StatefulElement hero = element;
         Hero heroWidget = element.widget;
         Object tag = heroWidget.tag;
         assert(tag != null);
-        Key key = heroWidget.key;
-        final Map<Key, HeroState> tagHeroes = heroes.putIfAbsent(tag, () => <Key, HeroState>{});
         assert(() {
-          if (tagHeroes.containsKey(key)) {
+          if (result.containsKey(tag)) {
             new FlutterError(
-              'There are multiple heroes that share the same key within the same subtree.\n'
+              'There are multiple heroes that share the same tag within a subtree.\n'
               'Within each subtree for which heroes are to be animated (typically a PageRoute subtree), '
-              'either each Hero must have a unique tag, or, all the heroes with a particular tag must '
-              'have different keys.\n'
-              'In this case, the tag "$tag" had multiple heroes with the key "$key".'
+              'each Hero must have a unique non-null tag.\n'
+              'In this case, multiple heroes had the tag "$tag".'
             );
           }
           return true;
         });
-        tagHeroes[key] = hero.state;
+        HeroState heroState = hero.state;
+        result[tag] = heroState;
       }
       element.visitChildren(visitor);
     }
     context.visitChildElements(visitor);
-    // next, for each tag, we're going to decide on the one hero we care about for that tag
-    Map<Object, HeroHandle> result = <Object, HeroHandle>{};
-    for (Object tag in heroes.keys) {
-      assert(tag != null);
-      if (heroes[tag].length == 1) {
-        result[tag] = heroes[tag].values.first;
-      } else {
-        assert(heroes[tag].length > 1);
-        assert(!heroes[tag].containsKey(null));
-        assert(heroes[tag].keys.where((Key key) => mostValuableKeys.contains(key)).length <= 1);
-        Key mostValuableKey = mostValuableKeys.firstWhere((Key key) => heroes[tag].containsKey(key), orElse: () => null);
-        if (mostValuableKey != null)
-          result[tag] = heroes[tag][mostValuableKey];
-      }
-    }
-    assert(!result.containsKey(null));
     return result;
   }
 
@@ -532,27 +505,15 @@ class HeroController extends NavigatorObserver {
     return entry;
   }
 
-  Set<Key> _getMostValuableKeys() {
-    assert(_from != null);
-    assert(_to != null);
-    Set<Key> result = new HashSet<Key>();
-    if (_from.settings.mostValuableKeys != null)
-      result.addAll(_from.settings.mostValuableKeys);
-    if (_to.settings.mostValuableKeys != null)
-      result.addAll(_to.settings.mostValuableKeys);
-    return result;
-  }
-
   void _updateQuest(Duration timeStamp) {
     if (navigator == null) {
       // The navigator was removed before this end-of-frame callback was called.
       return;
     }
-    Set<Key> mostValuableKeys = _getMostValuableKeys();
     Map<Object, HeroHandle> heroesFrom = _party.isEmpty ?
-        Hero.of(_from.subtreeContext, mostValuableKeys) : _party.getHeroesToAnimate();
+        Hero.of(_from.subtreeContext) : _party.getHeroesToAnimate();
 
-    Map<Object, HeroHandle> heroesTo = Hero.of(_to.subtreeContext, mostValuableKeys);
+    Map<Object, HeroHandle> heroesTo = Hero.of(_to.subtreeContext);
     _to.offstage = false;
 
     Animation<double> animation = _animation;

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

@@ -111,7 +111,6 @@ class RouteSettings {
   /// Creates data used to construct routes.
   const RouteSettings({
     this.name,
-    this.mostValuableKeys,
     this.isInitialRoute: false
   });
 
@@ -120,29 +119,13 @@ class RouteSettings {
   /// If null, the route is anonymous.
   final String name;
 
-  /// The set of keys that are most relevant for constructoring [Hero]
-  /// transitions. For example, if the current route contains a list of music
-  /// albums and the user triggered this navigation by tapping one of the
-  /// albums, the most valuable album cover is the one associated with the album
-  /// the user tapped and is the one that should heroically transition when
-  /// opening the details page for that album.
-  final Set<Key> mostValuableKeys;
-
   /// Whether this route is the very first route being pushed onto this [Navigator].
   ///
   /// The initial route typically skips any entrance transition to speed startup.
   final bool isInitialRoute;
 
   @override
-  String toString() {
-    String result = '"$name"';
-    if (mostValuableKeys != null && mostValuableKeys.isNotEmpty) {
-      result += '; keys:';
-      for (Key key in mostValuableKeys)
-        result += ' $key';
-    }
-    return result;
-  }
+  String toString() => '"$name"';
 }
 
 /// Creates a route for the given route settings.
@@ -211,10 +194,9 @@ class Navigator extends StatefulWidget {
   /// Push a named route onto the navigator that most tightly encloses the given context.
   ///
   /// The route name will be passed to that navigator's [onGenerateRoute]
-  /// callback. The returned route will be pushed into the navigator. The set of
-  /// most valuable keys will be used to construct an appropriate [Hero] transition.
-  static void pushNamed(BuildContext context, String routeName, { Set<Key> mostValuableKeys }) {
-    Navigator.of(context).pushNamed(routeName, mostValuableKeys: mostValuableKeys);
+  /// callback. The returned route will be pushed into the navigator.
+  static void pushNamed(BuildContext context, String routeName) {
+    Navigator.of(context).pushNamed(routeName);
   }
 
   /// Push a route onto the navigator that most tightly encloses the given context.
@@ -265,10 +247,10 @@ class Navigator extends StatefulWidget {
 
   /// Executes a simple transaction that both pops the current route off and
   /// pushes a named route into the navigator that most tightly encloses the given context.
-  static void popAndPushNamed(BuildContext context, String routeName, { Set<Key> mostValuableKeys }) {
+  static void popAndPushNamed(BuildContext context, String routeName) {
     Navigator.of(context)
       ..pop()
-      ..pushNamed(routeName, mostValuableKeys: mostValuableKeys);
+      ..pushNamed(routeName);
   }
 
   static NavigatorState of(BuildContext context) {
@@ -340,13 +322,10 @@ class NavigatorState extends State<Navigator> {
 
   bool _debugLocked = false; // used to prevent re-entrant calls to push, pop, and friends
 
-  void pushNamed(String name, { Set<Key> mostValuableKeys }) {
+  void pushNamed(String name) {
     assert(!_debugLocked);
     assert(name != null);
-    RouteSettings settings = new RouteSettings(
-      name: name,
-      mostValuableKeys: mostValuableKeys
-    );
+    RouteSettings settings = new RouteSettings(name: name);
     Route<dynamic> route = config.onGenerateRoute(settings);
     if (route == null) {
       assert(config.onUnknownRoute != null);