Add PageStorage support to Navigator2

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

@@ -8,6 +8,7 @@ import 'basic.dart';
 import 'framework.dart';
 import 'navigator2.dart';
 import 'overlay.dart';
+import 'page_storage.dart';
 import 'transitions.dart';
 
 // TODO(abarth): Should we add a type for the result?
@@ -81,8 +82,9 @@ class _PageState extends State<_Page> {
   Widget build(BuildContext context) {
     if (config.route._offstage) {
       return new OffStage(
-        child: new KeyedSubtree(
+        child: new PageStorage(
           key: _subtreeKey,
+          bucket: config.route._storageBucket,
           child: _invokeBuilder()
         )
       );
@@ -93,8 +95,9 @@ class _PageState extends State<_Page> {
       child: new FadeTransition(
         performance: config.route.performance,
         opacity: _opacity,
-        child: new KeyedSubtree(
+        child: new PageStorage(
           key: _subtreeKey,
+          bucket: config.route._storageBucket,
           child: _invokeBuilder()
         )
       )
@@ -132,6 +135,8 @@ class PageRoute extends TransitionRoute {
   Duration get transitionDuration => const Duration(milliseconds: 150);
   List<Widget> createWidgets() => [ new _Page(key: pageKey, route: this) ];
 
+  final PageStorageBucket _storageBucket = new PageStorageBucket();
+
   bool get offstage => _offstage;
   bool _offstage = false;
   void set offstage (bool value) {

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

@@ -0,0 +1,102 @@
+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'framework.dart';
+
+class _StorageEntryIdentifier {
+  Type clientType;
+  List<Key> keys;
+  void addKey(Key key) {
+    assert(key != null);
+    assert(key is! GlobalKey);
+    keys ??= <Key>[];
+    keys.add(key);
+  }
+  GlobalKey scopeKey;
+  bool operator ==(dynamic other) {
+    if (other is! _StorageEntryIdentifier)
+      return false;
+    final _StorageEntryIdentifier typedOther = other;
+    if (clientType != typedOther.clientType ||
+        scopeKey != typedOther.scopeKey ||
+        keys?.length != typedOther.keys?.length)
+      return false;
+    if (keys != null) {
+      for (int index = 0; index < keys.length; index += 1) {
+        if (keys[index] != typedOther.keys[index])
+          return false;
+      }
+    }
+    return true;
+  }
+  int get hashCode {
+    int value = 373;
+    value = 37 * value + clientType.hashCode;
+    value = 37 * value + scopeKey.hashCode;
+    if (keys != null) {
+      for (Key key in keys)
+        value = 37 * value + key.hashCode;
+    }
+    return value;
+  }
+}
+
+class PageStorageBucket {
+  _StorageEntryIdentifier _computeStorageIdentifier(BuildContext context) {
+    _StorageEntryIdentifier result = new _StorageEntryIdentifier();
+    result.clientType = context.widget.runtimeType;
+    Key lastKey = context.widget.key;
+    if (lastKey is! GlobalKey) {
+      context.visitAncestorElements((Element element) {
+        if (element.widget.key is GlobalKey) {
+          lastKey = element.widget.key;
+          return false;
+        } else if (element.widget.key != null) {
+          result.addKey(element.widget.key);
+        }
+        return true;
+      });
+      return result;
+    }
+    assert(lastKey is GlobalKey);
+    result.scopeKey = lastKey;
+    return result;
+  }
+
+  Map<_StorageEntryIdentifier, dynamic> _storage;
+  void writeState(BuildContext context, dynamic data) {
+    _storage ??= <_StorageEntryIdentifier, dynamic>{};
+    _storage[_computeStorageIdentifier(context)] = data;
+  }
+  dynamic readState(BuildContext context) {
+    return _storage != null ? _storage[_computeStorageIdentifier(context)] : null;
+  }
+}
+
+class PageStorage extends StatelessComponent {
+  PageStorage({
+    Key key,
+    this.child,
+    this.bucket
+  }) : super(key: key);
+
+  final Widget child;
+  final PageStorageBucket bucket;
+
+  /// Might return null if there is no PageStorage in this context.
+  static PageStorageBucket of(BuildContext context) {
+    PageStorageBucket result;
+    context.visitAncestorElements((Element element) {
+      Widget widget = element.widget;
+      if (widget is PageStorage) {
+        result = widget.bucket;
+        return false;
+      }
+      return true;
+    });
+    return result;
+  }
+
+  Widget build(BuildContext context) => child;
+}