Material gallery crashes when you press the drop-down button

Now use use the route's getPosition function to position the drop-down menu.
Also, fix a number of other related bugs that blocked the dropdown button from
working correctly. The dropdown menu still has the following issues:

1) In the exit animation, the background of the menu disappears too quickly
   because of incorrect paint bounds computations in the layer tree.
2) The drop down menu isn't positioned correctly after the device rotates.
   We'll need to address this issue in a separate patch.

Fixes #630

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

@@ -13,6 +13,7 @@ import 'icon.dart';
 import 'ink_well.dart';
 import 'shadows.dart';
 import 'theme.dart';
+import 'material.dart';
 
 const Duration _kDropDownMenuDuration = const Duration(milliseconds: 300);
 const double _kMenuItemHeight = 48.0;
@@ -100,32 +101,22 @@ class _DropDownMenu<T> extends StatusTransitionComponent {
 
     final AnimatedValue<double> menuOpacity = new AnimatedValue<double>(0.0,
       end: 1.0,
-      curve: new Interval(0.0, 0.25),
-      reverseCurve: new Interval(0.75, 1.0)
+      curve: const Interval(0.0, 0.25),
+      reverseCurve: const Interval(0.75, 1.0)
     );
 
     final AnimatedValue<double> menuTop = new AnimatedValue<double>(route.rect.top,
       end: route.rect.top - route.selectedIndex * route.rect.height,
-      curve: new Interval(0.25, 0.5),
+      curve: const Interval(0.25, 0.5),
       reverseCurve: const Interval(0.0, 0.001)
     );
     final AnimatedValue<double> menuBottom = new AnimatedValue<double>(route.rect.bottom,
       end: menuTop.end + route.items.length * route.rect.height,
-      curve: new Interval(0.25, 0.5),
+      curve: const Interval(0.25, 0.5),
       reverseCurve: const Interval(0.0, 0.001)
     );
 
-    final RenderBox renderBox = route.navigator.context.findRenderObject();
-    final Size navigatorSize = renderBox.size;
-    final RelativeRect menuRect = new RelativeRect.fromSize(route.rect, navigatorSize);
-
-    return new Positioned(
-      top: menuRect.top - (route.selectedIndex * route.rect.height),
-      right: menuRect.right,
-      left: menuRect.left,
-      child: new Focus(
-        key: new GlobalObjectKey(route),
-        child: new FadeTransition(
+    return new FadeTransition(
       performance: route.performance,
       opacity: menuOpacity,
       child: new BuilderTransition(
@@ -140,12 +131,13 @@ class _DropDownMenu<T> extends StatusTransitionComponent {
               menuBottom: menuBottom.value,
               renderBox: context.findRenderObject()
             ),
+            child: new Material(
+              type: MaterialType.transparency,
               child: new Block(children)
+            )
           );
         }
       )
-        )
-      )
     );
   }
 }
@@ -168,6 +160,17 @@ class _DropDownRoute<T> extends PopupRoute<T> {
   bool get barrierDismissable => true;
   Color get barrierColor => null;
 
+  ModalPosition getPosition(BuildContext context) {
+    RenderBox overlayBox = Overlay.of(context).context.findRenderObject();
+    Size overlaySize = overlayBox.size;
+    RelativeRect menuRect = new RelativeRect.fromSize(rect, overlaySize);
+    return new ModalPosition(
+      top: menuRect.top - selectedIndex * rect.height,
+      left: menuRect.left,
+      right: menuRect.right
+    );
+  }
+
   Widget buildPage(BuildContext context, PerformanceView performance, PerformanceView forwardPerformance) {
     return new _DropDownMenu(route: this);
   }
@@ -198,7 +201,7 @@ class DropDownMenuItem<T> extends StatelessComponent {
   }
 }
 
-class DropDownButton<T> extends StatelessComponent {
+class DropDownButton<T> extends StatefulComponent {
   DropDownButton({
     Key key,
     this.items,
@@ -212,40 +215,62 @@ class DropDownButton<T> extends StatelessComponent {
   final ValueChanged<T> onChanged;
   final int elevation;
 
-  void _showDropDown(BuildContext context, int selectedIndex, GlobalKey indexedStackKey) {
+  _DropDownButtonState<T> createState() => new _DropDownButtonState<T>();
+}
+
+class _DropDownButtonState<T> extends State<DropDownButton<T>> {
+  final GlobalKey indexedStackKey = new GlobalKey(debugLabel: 'DropDownButton.IndexedStack');
+
+  void initState() {
+    super.initState();
+    _updateSelectedIndex();
+  }
+
+  void didUpdateConfig(DropDownButton<T> oldConfig) {
+    if (config.items[_selectedIndex].value != config.value)
+      _updateSelectedIndex();
+  }
+
+  int _selectedIndex;
+
+  void _updateSelectedIndex() {
+    for (int itemIndex = 0; itemIndex < config.items.length; itemIndex++) {
+      if (config.items[itemIndex].value == config.value) {
+        _selectedIndex = itemIndex;
+        return;
+      }
+    }
+  }
+
+  void _handleTap() {
     final RenderBox renderBox = indexedStackKey.currentContext.findRenderObject();
     final Rect rect = renderBox.localToGlobal(Point.origin) & renderBox.size;
     final Completer completer = new Completer<T>();
     Navigator.push(context, new _DropDownRoute<T>(
       completer: completer,
-      items: items,
-      selectedIndex: selectedIndex,
+      items: config.items,
+      selectedIndex: _selectedIndex,
       rect: _kMenuHorizontalPadding.inflateRect(rect),
-      elevation: elevation
+      elevation: config.elevation
     ));
     completer.future.then((T newValue) {
-      if (onChanged != null)
-        onChanged(newValue);
+      if (!mounted)
+        return;
+      if (config.onChanged != null)
+        config.onChanged(newValue);
     });
   }
 
   Widget build(BuildContext context) {
-    GlobalKey indexedStackKey = new GlobalKey(debugLabel: 'DropDownButton.IndexedStack');
-    int selectedIndex = 0;
-    for (int itemIndex = 0; itemIndex < items.length; itemIndex++) {
-      if (items[itemIndex].value == value) {
-        selectedIndex = itemIndex;
-        break;
-      }
-    }
     return new GestureDetector(
+      onTap: _handleTap,
       child: new Container(
         decoration: new BoxDecoration(border: _kDropDownUnderline),
         child: new Row(<Widget>[
           new IndexedStack(
-            items,
+            config.items,
             key: indexedStackKey,
-            index: selectedIndex,
+            index: _selectedIndex,
             alignment: const FractionalOffset(0.5, 0.0)
           ),
           new Container(
@@ -255,10 +280,7 @@ class DropDownButton<T> extends StatelessComponent {
         ],
           justifyContent: FlexJustifyContent.collapse
         )
-      ),
-      onTap: () {
-        _showDropDown(context, selectedIndex, indexedStackKey);
-      }
+      )
     );
   }
 }

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

@@ -25,7 +25,10 @@ enum MaterialType {
   circle,
 
   /// Rounded edges, no color by default (used for MaterialButton buttons).
-  button
+  button,
+
+  /// A transparent piece of material that draws ink splashes and highlights.
+  transparency
 }
 
 const Map<MaterialType, double> kMaterialEdges = const <MaterialType, double>{
@@ -33,6 +36,7 @@ const Map<MaterialType, double> kMaterialEdges = const <MaterialType, double>{
   MaterialType.card: 2.0,
   MaterialType.circle: null,
   MaterialType.button: 2.0,
+  MaterialType.transparency: null,
 };
 
 abstract class InkSplash {
@@ -141,6 +145,7 @@ class _MaterialState extends State<Material> {
         child: contents
       );
     }
+    if (config.type != MaterialType.transparency) {
       contents = new AnimatedContainer(
         curve: Curves.ease,
         duration: kThemeChangeDuration,
@@ -152,6 +157,7 @@ class _MaterialState extends State<Material> {
         ),
         child: contents
       );
+    }
     return contents;
   }
 }

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

@@ -123,6 +123,10 @@ class _PopupMenuRoute<T> extends PopupRoute<T> {
   final List<PopupMenuItem<T>> items;
   final int elevation;
 
+  ModalPosition getPosition(BuildContext context) {
+    return position;
+  }
+
   PerformanceView createPerformance() {
     return new CurvedPerformance(
       super.createPerformance(),

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

@@ -349,7 +349,7 @@ class _ModalScopeState extends State<_ModalScope> {
       );
     }
     contents = new RepaintBoundary(child: contents);
-    ModalPosition position = config.route.position;
+    ModalPosition position = config.route.getPosition(context);
     if (position == null)
       return contents;
     return new Positioned(
@@ -388,7 +388,7 @@ abstract class ModalRoute<T> extends TransitionRoute<T> with LocalHistoryRoute<T
 
   // The API for subclasses to override - used by _ModalScope
 
-  ModalPosition get position => null;
+  ModalPosition getPosition(BuildContext context) => null;
   Widget buildPage(BuildContext context, PerformanceView performance, PerformanceView forwardPerformance);
   Widget buildTransitions(BuildContext context, PerformanceView performance, PerformanceView forwardPerformance, Widget child) {
     return child;