Remove ModalPosition (#3734)

Clients of getPosition should just use a one-child custom layout delegate
instead.

Fixes #2484

examples/material_gallery/lib/demo/grid_list_demo.dart

@@ -221,9 +221,8 @@ class GridListDemoState extends State<GridListDemo> {
     ];
 
     final EdgeInsets padding = MediaQuery.of(context).padding;
-    final ModalPosition position = new ModalPosition(
+    final RelativeRect position = new RelativeRect.fromLTRB(
-      right: padding.right + 16.0,
+      0.0, padding.top + 16.0, padding.right + 16.0, 0.0
-      top: padding.top + 16.0
     );
 
     showMenu(context: context, position: position, items: items).then((GridDemoTileStyle value) {

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

@@ -119,12 +119,12 @@ class _DropDownMenu<T> extends StatusTransitionWidget {
     );
 
     final Tween<double> menuTop = new Tween<double>(
-      begin: route.rect.top,
+      begin: route.buttonRect.top,
-      end: route.rect.top - route.selectedIndex * route.rect.height - _kMenuVerticalPadding.top
+      end: route.buttonRect.top - route.selectedIndex * route.buttonRect.height - _kMenuVerticalPadding.top
     );
     final Tween<double> menuBottom = new Tween<double>(
-      begin: route.rect.bottom,
+      begin: route.buttonRect.bottom,
-      end: menuTop.end + route.items.length * route.rect.height + _kMenuVerticalPadding.vertical
+      end: menuTop.end + route.items.length * route.buttonRect.height + _kMenuVerticalPadding.vertical
     );
 
     Widget child = new Material(
@@ -156,6 +156,37 @@ class _DropDownMenu<T> extends StatusTransitionWidget {
   }
 }
 
+class _DropDownMenuRouteLayout extends SingleChildLayoutDelegate {
+  _DropDownMenuRouteLayout(this.buttonRect, this.selectedIndex);
+
+  final Rect buttonRect;
+  final int selectedIndex;
+
+  @override
+  BoxConstraints getConstraintsForChild(BoxConstraints constraints) {
+    return new BoxConstraints(
+      minWidth: buttonRect.width,
+      maxWidth: buttonRect.width,
+      minHeight: 0.0,
+      maxHeight: constraints.maxHeight
+    );
+  }
+
+  @override
+  Offset getPositionForChild(Size size, Size childSize) {
+    return new Offset(
+      buttonRect.left,
+      buttonRect.top - selectedIndex * buttonRect.height - _kMenuVerticalPadding.top
+    );
+  }
+
+  @override
+  bool shouldRelayout(_DropDownMenuRouteLayout oldDelegate) {
+    return oldDelegate.buttonRect != buttonRect
+        || oldDelegate.selectedIndex != selectedIndex;
+  }
+}
+
 // We box the return value so that the return value can be null. Otherwise,
 // canceling the route (which returns null) would get confused with actually
 // returning a real null value.
@@ -180,14 +211,14 @@ class _DropDownRoute<T> extends PopupRoute<_DropDownRouteResult<T>> {
   _DropDownRoute({
     Completer<_DropDownRouteResult<T>> completer,
     this.items,
+    this.buttonRect,
     this.selectedIndex,
-    this.rect,
     this.elevation: 8
   }) : super(completer: completer);
 
   final List<DropDownMenuItem<T>> items;
+  final Rect buttonRect;
   final int selectedIndex;
-  final Rect rect;
   final int elevation;
 
   @override
@@ -199,22 +230,12 @@ class _DropDownRoute<T> extends PopupRoute<_DropDownRouteResult<T>> {
   @override
   Color get barrierColor => null;
 
-  @override
-  ModalPosition getPosition(BuildContext context) {
-    RenderBox overlayBox = Overlay.of(context).context.findRenderObject();
-    assert(overlayBox != null); // can't be null; routes get inserted by Navigator which has its own Overlay
-    Size overlaySize = overlayBox.size;
-    RelativeRect menuRect = new RelativeRect.fromSize(rect, overlaySize);
-    return new ModalPosition(
-      top: menuRect.top - selectedIndex * rect.height - _kMenuVerticalPadding.top,
-      left: menuRect.left,
-      right: menuRect.right
-    );
-  }
-
   @override
   Widget buildPage(BuildContext context, Animation<double> animation, Animation<double> forwardAnimation) {
-    return new _DropDownMenu<T>(route: this);
+    return new CustomSingleChildLayout(
+      delegate: new _DropDownMenuRouteLayout(buttonRect, selectedIndex),
+      child: new _DropDownMenu<T>(route: this)
+    );
   }
 }
 
@@ -359,13 +380,13 @@ class _DropDownButtonState<T> extends State<DropDownButton<T>> {
 
   void _handleTap() {
     final RenderBox renderBox = indexedStackKey.currentContext.findRenderObject();
-    final Rect rect = renderBox.localToGlobal(Point.origin) & renderBox.size;
+    final Rect buttonRect = renderBox.localToGlobal(Point.origin) & renderBox.size;
     final Completer<_DropDownRouteResult<T>> completer = new Completer<_DropDownRouteResult<T>>();
     Navigator.push(context, new _DropDownRoute<T>(
       completer: completer,
       items: config.items,
+      buttonRect: _kMenuHorizontalPadding.inflateRect(buttonRect),
       selectedIndex: _selectedIndex,
-      rect: _kMenuHorizontalPadding.inflateRect(rect),
       elevation: config.elevation
     ));
     completer.future.then((_DropDownRouteResult<T> newValue) {

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

@@ -328,17 +328,12 @@ class _PopupMenu<T> extends StatelessWidget {
 class _PopupMenuRouteLayout extends SingleChildLayoutDelegate {
   _PopupMenuRouteLayout(this.position, this.selectedItemOffset);
 
-  final ModalPosition position;
+  final RelativeRect position;
   final double selectedItemOffset;
 
   @override
   BoxConstraints getConstraintsForChild(BoxConstraints constraints) {
-    return new BoxConstraints(
+    return constraints.loosen();
-      minWidth: 0.0,
-      maxWidth: constraints.maxWidth,
-      minHeight: 0.0,
-      maxHeight: constraints.maxHeight
-    );
   }
 
   // Put the child wherever position specifies, so long as it will fit within the
@@ -380,14 +375,11 @@ class _PopupMenuRoute<T> extends PopupRoute<T> {
     this.elevation
   }) : super(completer: completer);
 
-  final ModalPosition position;
+  final RelativeRect position;
   final List<PopupMenuEntry<T>> items;
   final dynamic initialValue;
   final int elevation;
 
-  @override
-  ModalPosition getPosition(BuildContext context) => null;
-
   @override
   Animation<double> createAnimation() {
     return new CurvedAnimation(
@@ -416,13 +408,9 @@ class _PopupMenuRoute<T> extends PopupRoute<T> {
         selectedItemOffset += items[i].height;
       }
     }
-    final Size screenSize = MediaQuery.of(context).size;
+    return new CustomSingleChildLayout(
-    return new ConstrainedBox(
+      delegate: new _PopupMenuRouteLayout(position, selectedItemOffset),
-      constraints: new BoxConstraints(maxWidth: screenSize.width, maxHeight: screenSize.height),
+      child: new _PopupMenu<T>(route: this)
-      child: new CustomSingleChildLayout(
-        delegate: new _PopupMenuRouteLayout(position, selectedItemOffset),
-        child: new _PopupMenu<T>(route: this)
-      )
     );
   }
 }
@@ -434,7 +422,7 @@ class _PopupMenuRoute<T> extends PopupRoute<T> {
 /// implies the menu's origin.
 Future<dynamic/*=T*/> showMenu/*<T>*/({
   BuildContext context,
-  ModalPosition position,
+  RelativeRect position,
   List<PopupMenuEntry<dynamic/*=T*/>> items,
   dynamic/*=T*/ initialValue,
   int elevation: 8
@@ -518,9 +506,9 @@ class _PopupMenuButtonState<T> extends State<PopupMenuButton<T>> {
       elevation: config.elevation,
       items: config.itemBuilder(context),
       initialValue: config.initialValue,
-      position: new ModalPosition(
+      position: new RelativeRect.fromLTRB(
-        left: topLeft.x,
+        topLeft.x, topLeft.y + (config.initialValue != null ? renderBox.size.height / 2.0 : 0.0),
-        top: topLeft.y + (config.initialValue != null ? renderBox.size.height / 2.0 : 0.0)
+        0.0, 0.0
       )
     )
     .then((T value) {