Add a platform field to Theme (#5024)

We'll use this field to adapt material widgets to iOS.

packages/flutter/lib/foundation.dart

@@ -14,5 +14,6 @@ export 'src/foundation/basic_types.dart';
 export 'src/foundation/binding.dart';
 export 'src/foundation/change_notifier.dart';
 export 'src/foundation/licenses.dart';
+export 'src/foundation/platform.dart';
 export 'src/foundation/print.dart';
 export 'src/foundation/synchronous_future.dart';

packages/flutter/lib/src/foundation/platform.dart

@@ -0,0 +1,12 @@
+// Copyright 2016 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.
+
+/// The platform that user interaction should adapt to target.
+enum TargetPlatform {
+  /// Android: <https://www.android.com/>
+  android,
+
+  /// iOS: <http://www.apple.com/ios/>
+  iOS,
+}

packages/flutter/lib/src/material/app.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:io' show Platform;
-
 import 'package:flutter/rendering.dart';
 import 'package:flutter/widgets.dart';
 
@@ -25,6 +23,15 @@ const TextStyle _errorTextStyle = const TextStyle(
   decorationStyle: TextDecorationStyle.double
 );
 
+/// The visual and interaction design for overscroll.
+enum OverscrollStyle {
+  /// Overscrolls are clamped and indicated with a glow.
+  glow,
+
+  /// Overscrolls are not clamped and indicated with elastic physics.
+  bounce
+}
+
 /// An application that uses material design.
 ///
 /// A convenience widget that wraps a number of widgets that are commonly
@@ -53,6 +60,7 @@ class MaterialApp extends StatefulWidget {
     this.theme,
     this.home,
     this.routes: const <String, WidgetBuilder>{},
+    this.overscrollStyle,
     this.onGenerateRoute,
     this.onLocaleChanged,
     this.debugShowMaterialGrid: false,
@@ -104,6 +112,11 @@ class MaterialApp extends StatefulWidget {
   /// build the page instead.
   final Map<String, WidgetBuilder> routes;
 
+  /// The visual and interaction design for overscroll.
+  ///
+  /// Defaults to being adapted to the current [TargetPlatform].
+  final OverscrollStyle overscrollStyle;
+
   /// The route generator callback used when the app is navigated to a
   /// named route.
   final RouteFactory onGenerateRoute;
@@ -149,7 +162,8 @@ class _IndicatorScrollConfigurationDelegate extends ScrollConfigurationDelegate
   @override
   Widget wrapScrollWidget(Widget scrollWidget) => new OverscrollIndicator(child: scrollWidget);
 }
-final ScrollConfigurationDelegate _indicatorScroll = new _IndicatorScrollConfigurationDelegate();
+
+final ScrollConfigurationDelegate _glowScroll = new _IndicatorScrollConfigurationDelegate();
 final ScrollConfigurationDelegate _bounceScroll = new ScrollConfigurationDelegate();
 
 class _MaterialAppState extends State<MaterialApp> {
@@ -180,6 +194,24 @@ class _MaterialAppState extends State<MaterialApp> {
     return null;
   }
 
+  ScrollConfigurationDelegate _getScrollDelegate(TargetPlatform platform) {
+    if (config.overscrollStyle != null) {
+      switch (config.overscrollStyle) {
+        case OverscrollStyle.glow:
+          return _glowScroll;
+        case OverscrollStyle.bounce:
+          return _bounceScroll;
+      }
+    }
+    switch (platform) {
+      case TargetPlatform.android:
+        return _glowScroll;
+      case TargetPlatform.iOS:
+        return _bounceScroll;
+    }
+    return null;
+  }
+
   @override
   Widget build(BuildContext context) {
     ThemeData theme = config.theme ?? new ThemeData.fallback();
@@ -213,7 +245,7 @@ class _MaterialAppState extends State<MaterialApp> {
     });
 
     return new ScrollConfiguration(
-      delegate: (Platform.isIOS || Platform.isMacOS) ? _bounceScroll : _indicatorScroll,
+      delegate: _getScrollDelegate(theme.platform),
       child: result
     );
   }

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

@@ -2,8 +2,11 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'dart:io' show Platform;
 import 'dart:ui' show Color, hashValues;
 
+import 'package:flutter/foundation.dart';
+
 import 'colors.dart';
 import 'icon_theme_data.dart';
 import 'typography.dart';
@@ -90,7 +93,8 @@ class ThemeData {
     TextTheme textTheme,
     TextTheme primaryTextTheme,
     IconThemeData iconTheme,
-    IconThemeData primaryIconTheme
+    IconThemeData primaryIconTheme,
+    TargetPlatform platform
   }) {
     brightness ??= Brightness.light;
     final bool isDark = brightness == Brightness.dark;
@@ -121,6 +125,7 @@ class ThemeData {
     primaryTextTheme ??= primaryIsDark ? Typography.white : Typography.black;
     iconTheme ??= isDark ? const IconThemeData(color: Colors.white) : const IconThemeData(color: Colors.black);
     primaryIconTheme ??= primaryIsDark ? const IconThemeData(color: Colors.white) : const IconThemeData(color: Colors.black);
+    platform ??= (Platform.isIOS || Platform.isMacOS) ? TargetPlatform.iOS : TargetPlatform.android;
     return new ThemeData.raw(
       brightness: brightness,
       primaryColor: primaryColor,
@@ -146,7 +151,8 @@ class ThemeData {
       textTheme: textTheme,
       primaryTextTheme: primaryTextTheme,
       iconTheme: iconTheme,
-      primaryIconTheme: primaryIconTheme
+      primaryIconTheme: primaryIconTheme,
+      platform: platform
     );
   }
 
@@ -181,7 +187,8 @@ class ThemeData {
     this.textTheme,
     this.primaryTextTheme,
     this.iconTheme,
-    this.primaryIconTheme
+    this.primaryIconTheme,
+    this.platform
   }) {
     assert(brightness != null);
     assert(primaryColor != null);
@@ -208,6 +215,7 @@ class ThemeData {
     assert(primaryTextTheme != null);
     assert(iconTheme != null);
     assert(primaryIconTheme != null);
+    assert(platform != null);
   }
 
   /// A default light blue theme.
@@ -320,6 +328,11 @@ class ThemeData {
   /// An icon theme that contrasts with the primary color.
   final IconThemeData primaryIconTheme;
 
+  /// The platform the material widgets should adapt to target.
+  ///
+  /// Defaults to the current platform.
+  final TargetPlatform platform;
+
   /// Linearly interpolate between two themes.
   static ThemeData lerp(ThemeData begin, ThemeData end, double t) {
     return new ThemeData.raw(
@@ -347,7 +360,8 @@ class ThemeData {
       textTheme: TextTheme.lerp(begin.textTheme, end.textTheme, t),
       primaryTextTheme: TextTheme.lerp(begin.primaryTextTheme, end.primaryTextTheme, t),
       iconTheme: IconThemeData.lerp(begin.iconTheme, end.iconTheme, t),
-      primaryIconTheme: IconThemeData.lerp(begin.primaryIconTheme, end.primaryIconTheme, t)
+      primaryIconTheme: IconThemeData.lerp(begin.primaryIconTheme, end.primaryIconTheme, t),
+      platform: t < 0.5 ? begin.platform : end.platform
     );
   }