// 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 'package:flutter/rendering.dart';
import 'package:flutter/widgets.dart';

import 'arc.dart';
import 'colors.dart';
import 'floating_action_button.dart';
import 'icons.dart';
import 'material_localizations.dart';
import 'page.dart';
import 'theme.dart';

/// [MaterialApp] uses this [TextStyle] as its [DefaultTextStyle] to encourage
/// developers to be intentional about their [DefaultTextStyle].
///
/// In Material Design, most [Text] widgets are contained in [Material] widgets,
/// which sets a specific [DefaultTextStyle]. If you're seeing text that uses
/// this text style, consider putting your text in a [Material] widget (or
/// another widget that sets a [DefaultTextStyle]).
const TextStyle _errorTextStyle = const TextStyle(
  color: const Color(0xD0FF0000),
  fontFamily: 'monospace',
  fontSize: 48.0,
  fontWeight: FontWeight.w900,
  decoration: TextDecoration.underline,
  decorationColor: const Color(0xFFFFFF00),
  decorationStyle: TextDecorationStyle.double,
  debugLabel: 'fallback style; consider putting your text in a Material',
);

/// An application that uses material design.
///
/// A convenience widget that wraps a number of widgets that are commonly
/// required for material design applications. It builds upon a [WidgetsApp] by
/// adding material-design specific functionality, such as [AnimatedTheme] and
/// [GridPaper].
///
/// The [MaterialApp] configures the top-level [Navigator] to search for routes
/// in the following order:
///
///  1. For the `/` route, the [home] property, if non-null, is used.
///
///  2. Otherwise, the [routes] table is used, if it has an entry for the route.
///
///  3. Otherwise, [onGenerateRoute] is called, if provided. It should return a
///     non-null value for any _valid_ route not handled by [home] and [routes].
///
///  4. Finally if all else fails [onUnknownRoute] is called.
///
/// If a [Navigator] is created, at least one of these options must handle the
/// `/` route, since it is used when an invalid [initialRoute] is specified on
/// startup (e.g. by another application launching this one with an intent on
/// Android; see [Window.defaultRouteName]).
///
/// This widget also configures the observer of the top-level [Navigator] (if
/// any) to perform [Hero] animations.
///
/// If [home], [routes], [onGenerateRoute], and [onUnknownRoute] are all null,
/// and [builder] is not null, then no [Navigator] is created.
///
/// See also:
///
///  * [Scaffold], which provides standard app elements like an [AppBar] and a [Drawer].
///  * [Navigator], which is used to manage the app's stack of pages.
///  * [MaterialPageRoute], which defines an app page that transitions in a material-specific way.
///  * [WidgetsApp], which defines the basic app elements but does not depend on the material library.
class MaterialApp extends StatefulWidget {
  /// Creates a MaterialApp.
  ///
  /// At least one of [home], [routes], [onGenerateRoute], or [builder] must be
  /// non-null. If only [routes] is given, it must include an entry for the
  /// [Navigator.defaultRouteName] (`/`), since that is the route used when the
  /// application is launched with an intent that specifies an otherwise
  /// unsupported route.
  ///
  /// This class creates an instance of [WidgetsApp].
  ///
  /// The boolean arguments, [routes], and [navigatorObservers], must not be null.
  MaterialApp({ // can't be const because the asserts use methods on Map :-(
    Key key,
    this.navigatorKey,
    this.home,
    this.routes: const <String, WidgetBuilder>{},
    this.initialRoute,
    this.onGenerateRoute,
    this.onUnknownRoute,
    this.navigatorObservers: const <NavigatorObserver>[],
    this.builder,
    this.title: '',
    this.onGenerateTitle,
    this.color,
    this.theme,
    this.locale,
    this.localizationsDelegates,
    this.localeResolutionCallback,
    this.supportedLocales: const <Locale>[const Locale('en', 'US')],
    this.debugShowMaterialGrid: false,
    this.showPerformanceOverlay: false,
    this.checkerboardRasterCacheImages: false,
    this.checkerboardOffscreenLayers: false,
    this.showSemanticsDebugger: false,
    this.debugShowCheckedModeBanner: true,
  }) : assert(routes != null),
       assert(navigatorObservers != null),
       assert(
         home == null ||
         !routes.containsKey(Navigator.defaultRouteName),
         'If the home property is specified, the routes table '
         'cannot include an entry for "/", since it would be redundant.'
       ),
       assert(
         builder != null ||
         home != null ||
         routes.containsKey(Navigator.defaultRouteName) ||
         onGenerateRoute != null ||
         onUnknownRoute != null,
         'Either the home property must be specified, '
         'or the routes table must include an entry for "/", '
         'or there must be on onGenerateRoute callback specified, '
         'or there must be an onUnknownRoute callback specified, '
         'or the builder property must be specified, '
         'because otherwise there is nothing to fall back on if the '
         'app is started with an intent that specifies an unknown route.'
       ),
       assert(
         (home != null ||
          routes.isNotEmpty ||
          onGenerateRoute != null ||
          onUnknownRoute != null)
         ||
         (builder != null &&
          navigatorKey == null &&
          initialRoute == null &&
          navigatorObservers.isEmpty),
         'If no route is provided using '
         'home, routes, onGenerateRoute, or onUnknownRoute, '
         'a non-null callback for the builder property must be provided, '
         'and the other navigator-related properties, '
         'navigatorKey, initialRoute, and navigatorObservers, '
         'must have their initial values '
         '(null, null, and the empty list, respectively).'
       ),
       assert(title != null),
       assert(debugShowMaterialGrid != null),
       assert(showPerformanceOverlay != null),
       assert(checkerboardRasterCacheImages != null),
       assert(checkerboardOffscreenLayers != null),
       assert(showSemanticsDebugger != null),
       assert(debugShowCheckedModeBanner != null),
       super(key: key);

  /// A key to use when building the [Navigator].
  ///
  /// If a [navigatorKey] is specified, the [Navigator] can be directly
  /// manipulated without first obtaining it from a [BuildContext] via
  /// [Navigator.of]: from the [navigatorKey], use the [GlobalKey.currentState]
  /// getter.
  ///
  /// If this is changed, a new [Navigator] will be created, losing all the
  /// application state in the process; in that case, the [navigatorObservers]
  /// must also be changed, since the previous observers will be attached to the
  /// previous navigator.
  ///
  /// The [Navigator] is only built if routes are provided (either via [home],
  /// [routes], [onGenerateRoute], or [onUnknownRoute]); if they are not,
  /// [navigatorKey] must be null and [builder] must not be null.
  final GlobalKey<NavigatorState> navigatorKey;

  /// The widget for the default route of the app ([Navigator.defaultRouteName],
  /// which is `/`).
  ///
  /// This is the route that is displayed first when the application is started
  /// normally, unless [initialRoute] is specified. It's also the route that's
  /// displayed if the [initialRoute] can't be displayed.
  ///
  /// To be able to directly call [Theme.of], [MediaQuery.of], etc, in the code
  /// that sets the [home] argument in the constructor, you can use a [Builder]
  /// widget to get a [BuildContext].
  ///
  /// If [home] is specified, then [routes] must not include an entry for `/`,
  /// as [home] takes its place.
  ///
  /// The [Navigator] is only built if routes are provided (either via [home],
  /// [routes], [onGenerateRoute], or [onUnknownRoute]); if they are not,
  /// [builder] must not be null.
  ///
  /// The difference between using [home] and using [builder] is that the [home]
  /// subtree is inserted into the application below a [Navigator] (and thus
  /// below an [Overlay], which [Navigator] uses). With [home], therefore,
  /// dialog boxes will work automatically, [Tooltip]s will work, the [routes]
  /// table will be used, and APIs such as [Navigator.push] and [Navigator.pop]
  /// will work as expected. In contrast, the widget returned from [builder] is
  /// inserted _above_ the [MaterialApp]'s [Navigator] (if any).
  final Widget home;

  /// The application's top-level routing table.
  ///
  /// When a named route is pushed with [Navigator.pushNamed], the route name is
  /// looked up in this map. If the name is present, the associated
  /// [WidgetBuilder] is used to construct a [MaterialPageRoute] that performs
  /// an appropriate transition, including [Hero] animations, to the new route.
  ///
  /// If the app only has one page, then you can specify it using [home] instead.
  ///
  /// If [home] is specified, then it implies an entry in this table for the
  /// [Navigator.defaultRouteName] route (`/`), and it is an error to
  /// redundantly provide such a route in the [routes] table.
  ///
  /// If a route is requested that is not specified in this table (or by
  /// [home]), then the [onGenerateRoute] callback is called to build the page
  /// instead.
  ///
  /// The [Navigator] is only built if routes are provided (either via [home],
  /// [routes], [onGenerateRoute], or [onUnknownRoute]); if they are not,
  /// [builder] must not be null.
  final Map<String, WidgetBuilder> routes;

  /// The name of the first route to show, if a [Navigator] is built.
  ///
  /// Defaults to [Window.defaultRouteName], which may be overridden by the code
  /// that launched the application.
  ///
  /// If the route contains slashes, then it is treated as a "deep link", and
  /// before this route is pushed, the routes leading to this one are pushed
  /// also. For example, if the route was `/a/b/c`, then the app would start
  /// with the three routes `/a`, `/a/b`, and `/a/b/c` loaded, in that order.
  ///
  /// If any part of this process fails to generate routes, then the
  /// [initialRoute] is ignored and [Navigator.defaultRouteName] is used instead
  /// (`/`). This can happen if the app is started with an intent that specifies
  /// a non-existent route.
  ///
  /// The [Navigator] is only built if routes are provided (either via [home],
  /// [routes], [onGenerateRoute], or [onUnknownRoute]); if they are not,
  /// [initialRoute] must be null and [builder] must not be null.
  ///
  /// See also:
  ///
  ///  * [Navigator.initialRoute], which is used to implement this property.
  ///  * [Navigator.push], for pushing additional routes.
  ///  * [Navigator.pop], for removing a route from the stack.
  final String initialRoute;

  /// The route generator callback used when the app is navigated to a
  /// named route.
  ///
  /// This is used if [routes] does not contain the requested route.
  ///
  /// If this returns null when building the routes to handle the specified
  /// [initialRoute], then all the routes are discarded and
  /// [Navigator.defaultRouteName] is used instead (`/`). See [initialRoute].
  ///
  /// During normal app operation, the [onGenerateRoute] callback will only be
  /// applied to route names pushed by the application, and so should never
  /// return null.
  ///
  /// The [Navigator] is only built if routes are provided (either via [home],
  /// [routes], [onGenerateRoute], or [onUnknownRoute]); if they are not,
  /// [builder] must not be null.
  final RouteFactory onGenerateRoute;

  /// Called when [onGenerateRoute] fails to generate a route, except for the
  /// [initialRoute].
  ///
  /// This callback is typically used for error handling. For example, this
  /// callback might always generate a "not found" page that describes the route
  /// that wasn't found.
  ///
  /// The default implementation pushes a route that displays an ugly error
  /// message.
  ///
  /// The [Navigator] is only built if routes are provided (either via [home],
  /// [routes], [onGenerateRoute], or [onUnknownRoute]); if they are not,
  /// [builder] must not be null.
  final RouteFactory onUnknownRoute;

  /// The list of observers for the [Navigator] created for this app.
  ///
  /// This list must be replaced by a list of newly-created observers if the
  /// [navigatorKey] is changed.
  ///
  /// The [Navigator] is only built if routes are provided (either via [home],
  /// [routes], [onGenerateRoute], or [onUnknownRoute]); if they are not,
  /// [navigatorObservers] must be the empty list and [builder] must not be null.
  final List<NavigatorObserver> navigatorObservers;

  /// A builder for inserting widgets above the [Navigator] but below the other
  /// widgets created by the [MaterialApp] widget, or for replacing the
  /// [Navigator] entirely.
  ///
  /// For example, from the [BuildContext] passed to this method, the
  /// [Directionality], [Localizations], [DefaultTextStyle], [MediaQuery], etc,
  /// are all available. They can also be overridden in a way that impacts all
  /// the routes in the [Navigator].
  ///
  /// This is rarely useful, but can be used in applications that wish to
  /// override those defaults, e.g. to force the application into right-to-left
  /// mode despite being in English, or to override the [MediaQuery] metrics
  /// (e.g. to leave a gap for advertisements shown by a plugin from OEM code).
  ///
  /// The [builder] callback is passed two arguments, the [BuildContext] (as
  /// `context`) and a [Navigator] widget (as `child`).
  ///
  /// If no routes are provided using [home], [routes], [onGenerateRoute], or
  /// [onUnknownRoute], the `child` will be null, and it is the responsibility
  /// of the [builder] to provide the application's routing machinery.
  ///
  /// If routes _are_ provided using one or more of those properties, then
  /// `child` is not null, and the returned value should include the `child` in
  /// the widget subtree; if it does not, then the application will have no
  /// navigator and the [navigatorKey], [home], [routes], [onGenerateRoute],
  /// [onUnknownRoute], [initialRoute], and [navigatorObservers] properties will
  /// have no effect.
  ///
  /// If [builder] is null, it is as if a builder was specified that returned
  /// the `child` directly. If it is null, routes must be provided using one of
  /// the other properties listed above.
  ///
  /// Unless a [Navigator] is provided, either implicitly from [builder] being
  /// null, or by a [builder] including its `child` argument, or by a [builder]
  /// explicitly providing a [Navigator] of its own, features such as
  /// [showDialog] and [showMenu], widgets such as [Tooltip], [PopupMenuButton],
  /// or [Hero], and APIs such as [Navigator.push] and [Navigator.pop], will not
  /// function.
  ///
  /// For specifically overriding the [title] with a value based on the
  /// [Localizations], consider [onGenerateTitle] instead.
  final TransitionBuilder builder;

  /// A one-line description used by the device to identify the app for the user.
  ///
  /// On Android the titles appear above the task manager's app snapshots which are
  /// displayed when the user presses the "recent apps" button. Similarly, on
  /// iOS the titles appear in the App Switcher when the user double presses the
  /// home button.
  ///
  /// To provide a localized title instead, use [onGenerateTitle].
  ///
  /// This value is passed unmodified to [WidgetsApp.title].
  final String title;

  /// If non-null this callback is called to produce the app's
  /// title string, otherwise [title] is used.
  ///
  /// The [onGenerateTitle] `context` parameter includes the [WidgetsApp]'s
  /// [Localizations] widget so that this callback can be used to produce a
  /// localized title.
  ///
  /// This callback function must not return null.
  ///
  /// The [onGenerateTitle] callback is called each time the [MaterialApp]
  /// rebuilds.
  ///
  /// This value is passed unmodified to [WidgetsApp.onGenerateTitle].
  final GenerateAppTitle onGenerateTitle;

  /// The colors to use for the application's widgets.
  final ThemeData theme;

  /// The primary color to use for the application in the operating system
  /// interface.
  ///
  /// For example, on Android this is the color used for the application in the
  /// application switcher.
  final Color color;

  /// The initial locale for this app's [Localizations] widget.
  ///
  /// If the `locale` is null the system's locale value is used.
  final Locale locale;

  /// The delegates for this app's [Localizations] widget.
  ///
  /// The delegates collectively define all of the localized resources
  /// for this application's [Localizations] widget.
  ///
  /// Delegates that produce [WidgetsLocalizations] and [MaterialLocalizations]
  /// are included automatically. Apps can provide their own versions of these
  /// localizations by creating implementations of
  /// [LocalizationsDelegate<WidgetsLocalizations>] or
  /// [LocalizationsDelegate<MaterialLocalizations>] whose load methods return
  /// custom versions of [WidgetsLocalizations] or [MaterialLocalizations].
  ///
  /// For example: to add support to [MaterialLocalizations] for a
  /// locale it doesn't already support, say `const Locale('foo', 'BR')`,
  /// one could just extend [DefaultMaterialLocalizations]:
  ///
  /// ```dart
  /// class FooLocalizations extends DefaultMaterialLocalizations {
  ///   FooLocalizations(Locale locale) : super(locale);
  ///   @override
  ///   String get okButtonLabel {
  ///     if (locale == const Locale('foo', 'BR'))
  ///       return 'foo';
  ///     return super.okButtonLabel;
  ///   }
  /// }
  ///
  /// ```
  ///
  /// A `FooLocalizationsDelegate` is essentially just a method that constructs
  /// a `FooLocalizations` object. We return a [SynchronousFuture] here because
  /// no asynchronous work takes place upon "loading" the localizations object.
  ///
  /// ```dart
  /// class FooLocalizationsDelegate extends LocalizationsDelegate<MaterialLocalizations> {
  ///   const FooLocalizationsDelegate();
  ///   @override
  ///   Future<FooLocalizations> load(Locale locale) {
  ///     return new SynchronousFuture(new FooLocalizations(locale));
  ///   }
  ///   @override
  ///   bool shouldReload(FooLocalizationsDelegate old) => false;
  /// }
  /// ```
  ///
  /// Constructing a [MaterialApp] with a `FooLocalizationsDelegate` overrides
  /// the automatically included delegate for [MaterialLocalizations] because
  /// only the first delegate of each [LocalizationsDelegate.type] is used and
  /// the automatically included delegates are added to the end of the app's
  /// [localizationsDelegates] list.
  ///
  /// ```dart
  /// new MaterialApp(
  ///   localizationsDelegates: [
  ///     const FooLocalizationsDelegate(),
  ///   ],
  ///   // ...
  /// )
  /// ```
  final Iterable<LocalizationsDelegate<dynamic>> localizationsDelegates;

  /// This callback is responsible for choosing the app's locale
  /// when the app is started, and when the user changes the
  /// device's locale.
  ///
  /// The returned value becomes the locale of this app's [Localizations]
  /// widget. The callback's `locale` parameter is the device's locale when
  /// the app started, or the device locale the user selected after the app was
  /// started. The callback's `supportedLocales` parameter is just the value
  /// [supportedLocales].
  ///
  /// An app could use this callback to substitute locales based on the app's
  /// intended audience. If the device's OS provides a prioritized
  /// list of locales, this callback could be used to defer to it.
  ///
  /// If the callback is null then the resolved locale is:
  /// - The callback's `locale` parameter if it's equal to a supported locale.
  /// - The first supported locale with the same [Locale.languageCode] as the
  ///   callback's `locale` parameter.
  /// - The first supported locale.
  ///
  /// This callback is passed along to the [WidgetsApp] built by this widget.
  final LocaleResolutionCallback localeResolutionCallback;

  /// The list of locales that this app has been localized for.
  ///
  /// By default only the American English locale is supported. Apps should
  /// configure this list to match the locales they support.
  ///
  /// This list must not null. It's default value is just
  /// `[const Locale('en', 'US')]`. It is passed along unmodified to the
  /// [WidgetsApp] built by this widget.
  ///
  /// The order of the list matters. By default, if the device's locale doesn't
  /// exactly match a locale in [supportedLocales] then the first locale in
  /// [supportedLocales] with a matching [Locale.languageCode] is used. If that
  /// fails then the first locale in [supportedLocales] is used. The default
  /// locale resolution algorithm can be overridden with [localeResolutionCallback].
  ///
  /// The material widgets include translations for locales with the following
  /// language codes:
  /// ```
  /// ar - Arabic
  /// de - German
  /// en - English
  /// es - Spanish
  /// fa - Farsi (Persian)
  /// fr - French
  /// he - Hebrew
  /// it - Italian
  /// ja - Japanese
  /// ps - Pashto
  /// pt - Portugese
  /// ro - Romanian
  /// ru - Russian
  /// sd - Sindhi
  /// ur - Urdu
  /// zh - Chinese (simplified)
  /// ```
  final Iterable<Locale> supportedLocales;

  /// Turns on a performance overlay.
  ///
  /// See also:
  ///
  ///  * <https://flutter.io/debugging/#performanceoverlay>
  final bool showPerformanceOverlay;

  /// Turns on checkerboarding of raster cache images.
  final bool checkerboardRasterCacheImages;

  /// Turns on checkerboarding of layers rendered to offscreen bitmaps.
  final bool checkerboardOffscreenLayers;

  /// Turns on an overlay that shows the accessibility information
  /// reported by the framework.
  final bool showSemanticsDebugger;

  /// Turns on a little "DEBUG" banner in checked mode to indicate
  /// that the app is in checked mode. This is on by default (in
  /// checked mode), to turn it off, set the constructor argument to
  /// false. In release mode this has no effect.
  ///
  /// To get this banner in your application if you're not using
  /// WidgetsApp, include a [CheckedModeBanner] widget in your app.
  ///
  /// This banner is intended to deter people from complaining that your
  /// app is slow when it's in checked mode. In checked mode, Flutter
  /// enables a large number of expensive diagnostics to aid in
  /// development, and so performance in checked mode is not
  /// representative of what will happen in release mode.
  final bool debugShowCheckedModeBanner;

  /// Turns on a [GridPaper] overlay that paints a baseline grid
  /// Material apps.
  ///
  /// Only available in checked mode.
  ///
  /// See also:
  ///
  ///  * <https://material.google.com/layout/metrics-keylines.html>
  final bool debugShowMaterialGrid;

  @override
  _MaterialAppState createState() => new _MaterialAppState();
}

class _MaterialScrollBehavior extends ScrollBehavior {
  @override
  TargetPlatform getPlatform(BuildContext context) {
    return Theme.of(context).platform;
  }

  @override
  Widget buildViewportChrome(BuildContext context, Widget child, AxisDirection axisDirection) {
    // When modifying this function, consider modifying the implementation in
    // the base class as well.
    switch (getPlatform(context)) {
      case TargetPlatform.iOS:
        return child;
      case TargetPlatform.android:
      case TargetPlatform.fuchsia:
        return new GlowingOverscrollIndicator(
          child: child,
          axisDirection: axisDirection,
          color: Theme.of(context).accentColor,
        );
    }
    return null;
  }
}

class _MaterialAppState extends State<MaterialApp> {
  HeroController _heroController;

  @override
  void initState() {
    super.initState();
    _heroController = new HeroController(createRectTween: _createRectTween);
    _updateNavigator();
  }

  @override
  void didUpdateWidget(MaterialApp oldWidget) {
    super.didUpdateWidget(oldWidget);
    if (widget.navigatorKey != oldWidget.navigatorKey) {
      // If the Navigator changes, we have to create a new observer, because the
      // old Navigator won't be disposed (and thus won't unregister with its
      // observers) until after the new one has been created (because the
      // Navigator has a GlobalKey).
      _heroController = new HeroController(createRectTween: _createRectTween);
    }
    _updateNavigator();
  }

  bool _haveNavigator;
  List<NavigatorObserver> _navigatorObservers;

  void _updateNavigator() {
    _haveNavigator = widget.home != null ||
                     widget.routes.isNotEmpty ||
                     widget.onGenerateRoute != null ||
                     widget.onUnknownRoute != null;
    _navigatorObservers = new List<NavigatorObserver>.from(widget.navigatorObservers)
      ..add(_heroController);
  }

  RectTween _createRectTween(Rect begin, Rect end) {
    return new MaterialRectArcTween(begin: begin, end: end);
  }

  Route<dynamic> _onGenerateRoute(RouteSettings settings) {
    final String name = settings.name;
    WidgetBuilder builder;
    if (name == Navigator.defaultRouteName && widget.home != null) {
      builder = (BuildContext context) => widget.home;
    } else {
      builder = widget.routes[name];
    }
    if (builder != null) {
      return new MaterialPageRoute<dynamic>(
        builder: builder,
        settings: settings,
      );
    }
    if (widget.onGenerateRoute != null)
      return widget.onGenerateRoute(settings);
    return null;
  }

  Route<dynamic> _onUnknownRoute(RouteSettings settings) {
    assert(() {
      if (widget.onUnknownRoute == null) {
        throw new FlutterError(
          'Could not find a generator for route $settings in the $runtimeType.\n'
          'Generators for routes are searched for in the following order:\n'
          ' 1. For the "/" route, the "home" property, if non-null, is used.\n'
          ' 2. Otherwise, the "routes" table is used, if it has an entry for '
          'the route.\n'
          ' 3. Otherwise, onGenerateRoute is called. It should return a '
          'non-null value for any valid route not handled by "home" and "routes".\n'
          ' 4. Finally if all else fails onUnknownRoute is called.\n'
          'Unfortunately, onUnknownRoute was not set.'
        );
      }
      return true;
    }());
    final Route<dynamic> result = widget.onUnknownRoute(settings);
    assert(() {
      if (result == null) {
        throw new FlutterError(
          'The onUnknownRoute callback returned null.\n'
          'When the $runtimeType requested the route $settings from its '
          'onUnknownRoute callback, the callback returned null. Such callbacks '
          'must never return null.'
        );
      }
      return true;
    }());
    return result;
  }

  // Combine the Localizations for Material with the ones contributed
  // by the localizationsDelegates parameter, if any. Only the first delegate
  // of a particular LocalizationsDelegate.type is loaded so the
  // localizationsDelegate parameter can be used to override
  // _MaterialLocalizationsDelegate.
  Iterable<LocalizationsDelegate<dynamic>> get _localizationsDelegates sync* {
    if (widget.localizationsDelegates != null)
      yield* widget.localizationsDelegates;
    yield DefaultMaterialLocalizations.delegate;
  }

  @override
  Widget build(BuildContext context) {
    final ThemeData theme = widget.theme ?? new ThemeData.fallback();
    Widget result = new AnimatedTheme(
      data: theme,
      isMaterialAppTheme: true,
      child: new WidgetsApp(
        key: new GlobalObjectKey(this),
        navigatorKey: widget.navigatorKey,
        navigatorObservers: _haveNavigator ? _navigatorObservers : null,
        initialRoute: widget.initialRoute,
        onGenerateRoute: _haveNavigator ? _onGenerateRoute : null,
        onUnknownRoute: _haveNavigator ? _onUnknownRoute : null,
        builder: widget.builder,
        title: widget.title,
        onGenerateTitle: widget.onGenerateTitle,
        textStyle: _errorTextStyle,
        // blue is the primary color of the default theme
        color: widget.color ?? theme?.primaryColor ?? Colors.blue,
        locale: widget.locale,
        localizationsDelegates: _localizationsDelegates,
        localeResolutionCallback: widget.localeResolutionCallback,
        supportedLocales: widget.supportedLocales,
        showPerformanceOverlay: widget.showPerformanceOverlay,
        checkerboardRasterCacheImages: widget.checkerboardRasterCacheImages,
        checkerboardOffscreenLayers: widget.checkerboardOffscreenLayers,
        showSemanticsDebugger: widget.showSemanticsDebugger,
        debugShowCheckedModeBanner: widget.debugShowCheckedModeBanner,
        inspectorSelectButtonBuilder: (BuildContext context, VoidCallback onPressed) {
          return new FloatingActionButton(
            child: const Icon(Icons.search),
            onPressed: onPressed,
            mini: true,
          );
        },
      )
    );

    assert(() {
      if (widget.debugShowMaterialGrid) {
        result = new GridPaper(
          color: const Color(0xE0F9BBE0),
          interval: 8.0,
          divisions: 2,
          subdivisions: 1,
          child: result,
        );
      }
      return true;
    }());

    return new ScrollConfiguration(
      behavior: new _MaterialScrollBehavior(),
      child: result,
    );
  }
}
