Reimplement the theme transition animation by actually animating the Theme.

As part of this:
 - A lot of classes got new lerp functions, including e.g. TextStyle.
 - Theme's constructor story got overhauled. You can now configure
   everything if you really want to, and we're better about defaults.
 - Material no longer automatically animates its background color.
   (It still does for its shadow.)
 - Tabs try to get the indicator color from the theme.
 - The fields in ThemeData got reordered for sanity.
 - Theme.== and Theme.hashCode got fixed.
 - Typography got a bit of a spring cleaning.

Fixes #613.

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

@@ -8,6 +8,12 @@ class IconThemeData {
   const IconThemeData({ this.color });
   final IconThemeColor color;
 
+  static IconThemeData lerp(IconThemeData begin, IconThemeData end, double t) {
+    return new IconThemeData(
+      color: t < 0.5 ? begin.color : end.color
+    );
+  }
+
   bool operator ==(dynamic other) {
     if (other is! IconThemeData)
       return false;

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

@@ -160,12 +160,18 @@ class _MaterialState extends State<Material> {
         curve: Curves.ease,
         duration: kThemeChangeDuration,
         decoration: new BoxDecoration(
-          backgroundColor: backgroundColor,
           borderRadius: kMaterialEdges[config.type],
           boxShadow: config.elevation == 0 ? null : elevationToShadow[config.elevation],
           shape: config.type == MaterialType.circle ? BoxShape.circle : BoxShape.rectangle
         ),
-        child: contents
+        child: new Container(
+          decoration: new BoxDecoration(
+            borderRadius: kMaterialEdges[config.type],
+            backgroundColor: backgroundColor,
+            shape: config.type == MaterialType.circle ? BoxShape.circle : BoxShape.rectangle
+          ),
+          child: contents
+        )
       );
     }
     return contents;

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

@@ -141,8 +141,9 @@ class _MaterialAppState extends State<MaterialApp> implements BindingObserver {
       data: new MediaQueryData(size: _size),
       child: new LocaleQuery(
         data: _localeData,
-        child: new Theme(
+        child: new AnimatedTheme(
           data: theme,
+          duration: kThemeAnimationDuration,
           child: new DefaultTextStyle(
             style: _errorTextStyle,
             child: new DefaultAssetBundle(

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

@@ -717,9 +717,17 @@ class _TabBarState<T> extends ScrollableState<TabBar<T>> implements TabBarSelect
 
     ThemeData themeData = Theme.of(context);
     Color backgroundColor = Material.of(context).color;
-    Color indicatorColor = themeData.accentColor;
-    if (indicatorColor == backgroundColor)
+    Color indicatorColor = themeData.indicatorColor;
+    if (indicatorColor == backgroundColor) {
+      // ThemeData tries to avoid this by having indicatorColor avoid being the
+      // primaryColor. However, it's possible that the tab strip is on a
+      // Material that isn't the primaryColor. In that case, if the indicator
+      // color ends up clashing, then this overrides it. When that happens,
+      // automatic transitions of the theme will likely look ugly as the
+      // indicator color suddenly snaps to white at one end, but it's not clear
+      // how to avoid that any further.
       indicatorColor = Colors.white;
+    }
 
     TextStyle textStyle = themeData.primaryTextTheme.body1;
     IconThemeData iconTheme = themeData.primaryIconTheme;

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

@@ -2,12 +2,15 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'package:flutter/animation.dart';
 import 'package:flutter/widgets.dart';
 
 import 'theme_data.dart';
 
 export 'theme_data.dart' show ThemeData, ThemeBrightness;
 
+const kThemeAnimationDuration = const Duration(milliseconds: 200);
+
 class Theme extends InheritedWidget {
   Theme({
     Key key,
@@ -37,3 +40,55 @@ class Theme extends InheritedWidget {
     description.add('$data');
   }
 }
+
+/// An animated value that interpolates [BoxConstraint]s.
+class AnimatedThemeDataValue extends AnimatedValue<ThemeData> {
+  AnimatedThemeDataValue(ThemeData begin, { ThemeData end, Curve curve, Curve reverseCurve })
+    : super(begin, end: end, curve: curve, reverseCurve: reverseCurve);
+
+  ThemeData lerp(double t) => ThemeData.lerp(begin, end, t);
+}
+
+/// Animated version of [Theme] which automatically transitions the colours,
+/// etc, over a given duration whenever the given theme changes.
+class AnimatedTheme extends AnimatedWidgetBase {
+  AnimatedTheme({
+    Key key,
+    this.data,
+    Curve curve: Curves.linear,
+    Duration duration,
+    this.child
+  }) : super(key: key, curve: curve, duration: duration) {
+    assert(child != null);
+    assert(data != null);
+  }
+
+  final ThemeData data;
+
+  final Widget child;
+
+  _AnimatedThemeState createState() => new _AnimatedThemeState();
+}
+
+class _AnimatedThemeState extends AnimatedWidgetBaseState<AnimatedTheme> {
+  AnimatedThemeDataValue _data;
+
+  void forEachVariable(VariableVisitor visitor) {
+    // TODO(ianh): Use constructor tear-offs when it becomes possible
+    _data = visitor(_data, config.data, (dynamic value) => new AnimatedThemeDataValue(value));
+    assert(_data != null);
+  }
+
+  Widget build(BuildContext context) {
+    return new Theme(
+      child: config.child,
+      data: _data.value
+    );
+  }
+
+  void debugFillDescription(List<String> description) {
+    super.debugFillDescription(description);
+    if (_data != null)
+      description.add('$_data');
+  }
+}

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

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-import 'dart:ui' show Color, hashValues;
+import 'dart:ui' show Color, hashValues, hashList, lerpDouble;
 
 import 'colors.dart';
 import 'icon_theme_data.dart';
@@ -29,41 +29,109 @@ const Color _kDarkThemeSplashColor = const Color(0x40CCCCCC);
 
 class ThemeData {
 
-  ThemeData({
+  ThemeData.raw({
+    this.brightness,
+    this.primarySwatch,
+    this.primaryColor,
+    this.primaryColorBrightness,
+    this.canvasColor,
+    this.cardColor,
+    this.dividerColor,
+    this.highlightColor,
+    this.splashColor,
+    this.unselectedColor,
+    this.disabledColor,
+    this.accentColor,
+    this.accentColorBrightness,
+    this.indicatorColor,
+    this.hintColor,
+    this.hintOpacity,
+    this.text,
+    this.primaryTextTheme,
+    this.primaryIconTheme
+  }) {
+    assert(brightness != null);
+    // primarySwatch can be null; TODO(ianh): see https://github.com/flutter/flutter/issues/1277
+    assert(primaryColor != null);
+    assert(primaryColorBrightness != null);
+    assert(canvasColor != null);
+    assert(cardColor != null);
+    assert(dividerColor != null);
+    assert(highlightColor != null);
+    assert(splashColor != null);
+    assert(unselectedColor != null);
+    assert(disabledColor != null);
+    assert(accentColor != null);
+    assert(accentColorBrightness != null);
+    assert(indicatorColor != null);
+    assert(hintColor != null);
+    assert(hintOpacity != null);
+    assert(text != null);
+    assert(primaryTextTheme != null);
+    assert(primaryIconTheme != null);
+  }
+
+  factory ThemeData({
     ThemeBrightness brightness: ThemeBrightness.light,
     Map<int, Color> primarySwatch,
+    Color primaryColor,
+    ThemeBrightness primaryColorBrightness,
+    Color canvasColor,
+    Color cardColor,
+    Color dividerColor,
+    Color highlightColor,
+    Color splashColor,
+    Color unselectedColor,
+    Color disabledColor,
     Color accentColor,
-    this.accentColorBrightness: ThemeBrightness.dark,
-    TextTheme text
-  }): this.brightness = brightness,
-      this.primarySwatch = primarySwatch,
-      primaryColorBrightness = primarySwatch == null ? brightness : ThemeBrightness.dark,
-      canvasColor = brightness == ThemeBrightness.dark ? Colors.grey[850] : Colors.grey[50],
-      cardColor = brightness == ThemeBrightness.dark ? Colors.grey[800] : Colors.white,
-      dividerColor = brightness == ThemeBrightness.dark ? const Color(0x1FFFFFFF) : const Color(0x1F000000),
-      // Some users want the pre-multiplied color, others just want the opacity.
-      hintColor = brightness == ThemeBrightness.dark ? const Color(0x42FFFFFF) : const Color(0x4C000000),
-      hintOpacity = brightness == ThemeBrightness.dark ? 0.26 : 0.30,
-      highlightColor = brightness == ThemeBrightness.dark ? _kDarkThemeHighlightColor : _kLightThemeHighlightColor,
-      splashColor = brightness == ThemeBrightness.dark ? _kDarkThemeSplashColor : _kLightThemeSplashColor,
-      text = brightness == ThemeBrightness.dark ? Typography.white : Typography.black {
-    assert(brightness != null);
-
-    if (primarySwatch == null) {
-      if (brightness == ThemeBrightness.dark) {
-        _primaryColor = Colors.grey[900];
-      } else {
-        _primaryColor = Colors.grey[100];
-      }
-    } else {
-      _primaryColor = primarySwatch[500];
-    }
-
-    if (accentColor == null) {
-      _accentColor = primarySwatch == null ? Colors.blue[500] : primarySwatch[500];
-    } else {
-      _accentColor = accentColor;
-    }
+    ThemeBrightness accentColorBrightness: ThemeBrightness.dark,
+    Color indicatorColor,
+    Color hintColor,
+    double hintOpacity,
+    TextTheme text,
+    TextTheme primaryTextTheme,
+    IconThemeData primaryIconTheme
+  }) {
+    // brightness default is in the arguments list
+    bool isDark = brightness == ThemeBrightness.dark;
+    primaryColor ??= primarySwatch == null ? isDark ? Colors.grey[900] : Colors.grey[100] : primarySwatch[500];
+    primaryColorBrightness ??= primarySwatch == null ? brightness : ThemeBrightness.dark /* swatch[500] is always dark */;
+    canvasColor ??= isDark ? Colors.grey[850] : Colors.grey[50];
+    cardColor ??= isDark ? Colors.grey[800] : Colors.white;
+    dividerColor ??= isDark ? const Color(0x1FFFFFFF) : const Color(0x1F000000);
+    highlightColor ??= isDark ? _kDarkThemeHighlightColor : _kLightThemeHighlightColor;
+    splashColor ??= isDark ? _kDarkThemeSplashColor : _kLightThemeSplashColor;
+    unselectedColor ??= isDark ? Colors.white70 : Colors.black54;
+    disabledColor ??= isDark ? Colors.white30 : Colors.black26;
+    accentColor ??= primarySwatch == null ? Colors.blue[500] : primarySwatch[500];
+    // accentColorBrightness default is in the arguments list
+    indicatorColor ??= accentColor == primaryColor ? Colors.white : accentColor;
+    hintColor ??= isDark ? const Color(0x42FFFFFF) : const Color(0x4C000000);
+    hintOpacity ??= hintColor != null ? hintColor.alpha / 0xFF : isDark ? 0.26 : 0.30;
+    text ??= isDark ? Typography.white : Typography.black;
+    primaryTextTheme ??= primaryColorBrightness == ThemeBrightness.dark ? Typography.white : Typography.black;
+    primaryIconTheme ??= primaryColorBrightness == ThemeBrightness.dark ? const IconThemeData(color: IconThemeColor.white) : const IconThemeData(color: IconThemeColor.black);
+    return new ThemeData.raw(
+      brightness: brightness,
+      primarySwatch: primarySwatch,
+      primaryColor: primaryColor,
+      primaryColorBrightness: primaryColorBrightness,
+      canvasColor: canvasColor,
+      cardColor: cardColor,
+      dividerColor: dividerColor,
+      highlightColor: highlightColor,
+      splashColor: splashColor,
+      unselectedColor: unselectedColor,
+      disabledColor: disabledColor,
+      accentColor: accentColor,
+      accentColorBrightness: accentColorBrightness,
+      indicatorColor: indicatorColor,
+      hintColor: hintColor,
+      hintOpacity: hintOpacity,
+      text: text,
+      primaryTextTheme: primaryTextTheme,
+      primaryIconTheme: primaryIconTheme
+    );
   }
 
   factory ThemeData.light() => new ThemeData(primarySwatch: Colors.blue, brightness: ThemeBrightness.light);
@@ -78,93 +146,129 @@ class ThemeData {
   /// all dark. When the ThemeBrightness is light, the canvas and card colors
   /// are bright, and the primary color's darkness varies as described by
   /// primaryColorBrightness. The primaryColor does not contrast well with the
-  /// card and canvas colors when the brightness is dask; when the birghtness is
+  /// card and canvas colors when the brightness is dark; when the brightness is
   /// dark, use Colors.white or the accentColor for a contrasting color.
   final ThemeBrightness brightness;
 
   final Map<int, Color> primarySwatch;
-  final Color canvasColor;
-  final Color cardColor;
-  final Color dividerColor;
-  final Color hintColor;
-  final Color highlightColor;
-  final Color splashColor;
-  final double hintOpacity;
-
-  /// Text with a color that contrasts with the card and canvas colors.
-  final TextTheme text;
 
   /// The background colour for major parts of the app (toolbars, tab bars, etc)
-  Color get primaryColor => _primaryColor;
-  Color _primaryColor;
+  final Color primaryColor;
 
   /// The brightness of the primaryColor. Used to determine the colour of text and
   /// icons placed on top of the primary color (e.g. toolbar text).
   final ThemeBrightness primaryColorBrightness;
 
-  /// A text theme that contrasts with the primary color.
-  TextTheme get primaryTextTheme {
-    if (primaryColorBrightness == ThemeBrightness.dark)
-      return Typography.white;
-    return Typography.black;
-  }
-
-  IconThemeData get primaryIconTheme {
-    if (primaryColorBrightness == ThemeBrightness.dark)
-      return const IconThemeData(color: IconThemeColor.white);
-    return const IconThemeData(color: IconThemeColor.black);
-  }
-
-  Color get unselectedColor {
-    if (brightness == ThemeBrightness.dark)
-      return Colors.white70;
-    return Colors.black54;
-  }
-
-  Color get disabledColor {
-    if (brightness == ThemeBrightness.dark)
-      return Colors.white30;
-    return Colors.black26;
-  }
+  final Color canvasColor;
+  final Color cardColor;
+  final Color dividerColor;
+  final Color highlightColor;
+  final Color splashColor;
+  final Color unselectedColor;
+  final Color disabledColor;
 
   /// The foreground color for widgets (knobs, text, etc)
-  Color get accentColor => _accentColor;
-  Color _accentColor;
+  final Color accentColor;
 
   /// The brightness of the accentColor. Used to determine the colour of text
   /// and icons placed on top of the accent color (e.g. the icons on a floating
   /// action button).
   final ThemeBrightness accentColorBrightness;
 
+  /// The color of the selected tab indicator in a tab strip.
+  final Color indicatorColor;
+
+  // Some users want the pre-multiplied color, others just want the opacity.
+  final Color hintColor;
+  final double hintOpacity;
+
+  /// Text with a color that contrasts with the card and canvas colors.
+  final TextTheme text;
+
+  /// A text theme that contrasts with the primary color.
+  final TextTheme primaryTextTheme;
+
+  final IconThemeData primaryIconTheme;
+
+  static ThemeData lerp(ThemeData begin, ThemeData end, double t) {
+    Map<int, Color> primarySwatch;
+    if (begin.primarySwatch != null && end.primarySwatch != null) {
+      primarySwatch = <int, Color>{};
+      for (int index in begin.primarySwatch.keys) {
+        if (!end.primarySwatch.containsKey(index))
+          continue;
+        primarySwatch[index] = Color.lerp(begin.primarySwatch[index], end.primarySwatch[index], t);
+      }
+    }
+    return new ThemeData.raw(
+      brightness: t < 0.5 ? begin.brightness : end.brightness,
+      primarySwatch: primarySwatch,
+      primaryColor: Color.lerp(begin.primaryColor, end.primaryColor, t),
+      primaryColorBrightness: t < 0.5 ? begin.primaryColorBrightness : end.primaryColorBrightness,
+      canvasColor: Color.lerp(begin.canvasColor, end.canvasColor, t),
+      cardColor: Color.lerp(begin.cardColor, end.cardColor, t),
+      dividerColor: Color.lerp(begin.dividerColor, end.dividerColor, t),
+      highlightColor: Color.lerp(begin.highlightColor, end.highlightColor, t),
+      splashColor: Color.lerp(begin.splashColor, end.splashColor, t),
+      unselectedColor: Color.lerp(begin.unselectedColor, end.unselectedColor, t),
+      disabledColor: Color.lerp(begin.disabledColor, end.disabledColor, t),
+      accentColor: Color.lerp(begin.accentColor, end.accentColor, t),
+      accentColorBrightness: t < 0.5 ? begin.accentColorBrightness : end.accentColorBrightness,
+      indicatorColor: Color.lerp(begin.indicatorColor, end.indicatorColor, t),
+      hintColor: Color.lerp(begin.hintColor, end.hintColor, t),
+      hintOpacity: lerpDouble(begin.hintOpacity, end.hintOpacity, t),
+      text: TextTheme.lerp(begin.text, end.text, t),
+      primaryTextTheme: TextTheme.lerp(begin.primaryTextTheme, end.primaryTextTheme, t),
+      primaryIconTheme: IconThemeData.lerp(begin.primaryIconTheme, end.primaryIconTheme, t)
+    );
+  }
+
   bool operator==(Object other) {
     if (other.runtimeType != runtimeType)
       return false;
     ThemeData otherData = other;
     return (otherData.brightness == brightness) &&
            (otherData.primarySwatch == primarySwatch) &&
+           (otherData.primaryColor == primaryColor) &&
+           (otherData.primaryColorBrightness == primaryColorBrightness) &&
            (otherData.canvasColor == canvasColor) &&
            (otherData.cardColor == cardColor) &&
            (otherData.dividerColor == dividerColor) &&
-           (otherData.hintColor == hintColor) &&
            (otherData.highlightColor == highlightColor) &&
+           (otherData.splashColor == splashColor) &&
+           (otherData.unselectedColor == unselectedColor) &&
+           (otherData.disabledColor == disabledColor) &&
+           (otherData.accentColor == accentColor) &&
+           (otherData.accentColorBrightness == accentColorBrightness) &&
+           (otherData.indicatorColor == indicatorColor) &&
+           (otherData.hintColor == hintColor) &&
            (otherData.hintOpacity == hintOpacity) &&
            (otherData.text == text) &&
-           (otherData.primaryColorBrightness == primaryColorBrightness) &&
-           (otherData.accentColorBrightness == accentColorBrightness);
+           (otherData.primaryTextTheme == primaryTextTheme) &&
+           (otherData.primaryIconTheme == primaryIconTheme);
   }
   int get hashCode {
     return hashValues(
       brightness,
-      primarySwatch,
+      hashList(primarySwatch.keys),
+      hashList(primarySwatch.values),
+      primaryColor,
+      primaryColorBrightness,
       canvasColor,
       cardColor,
       dividerColor,
-      hintColor,
       highlightColor,
+      splashColor,
+      unselectedColor,
+      disabledColor,
+      accentColor,
+      accentColorBrightness,
+      indicatorColor,
+      hintColor,
       hintOpacity,
       text,
-      primaryColorBrightness,
-      accentColorBrightness
+      primaryTextTheme,
+      primaryIconTheme
     );
   }
 

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

@@ -4,8 +4,6 @@
 
 // See http://www.google.com/design/spec/style/typography.html
 
-import 'dart:ui' show Color;
-
 import 'package:flutter/painting.dart';
 
 import 'colors.dart';
@@ -13,8 +11,11 @@ import 'colors.dart';
 // TODO(eseidel): Font weights are supposed to be language relative!
 // TODO(jackson): Baseline should be language relative!
 // These values are for English-like text.
+// TODO(ianh): There's no font-family specified here.
 class TextTheme {
 
+  const TextTheme._(this.display4, this.display3, this.display2, this.display1, this.headline, this.title, this.subhead, this.body2, this.body1, this.caption, this.button);
+
   const TextTheme._black()
     : display4 = const TextStyle(inherit: false, fontSize: 112.0, fontWeight: FontWeight.w100, color: Colors.black54, textBaseline: TextBaseline.alphabetic),
       display3 = const TextStyle(inherit: false, fontSize:  56.0, fontWeight: FontWeight.w400, color: Colors.black54, textBaseline: TextBaseline.alphabetic),
@@ -53,26 +54,26 @@ class TextTheme {
   final TextStyle caption;
   final TextStyle button;
 
+  static TextTheme lerp(TextTheme begin, TextTheme end, double t) {
+    return new TextTheme._(
+      TextStyle.lerp(begin.display4, end.display4, t),
+      TextStyle.lerp(begin.display3, end.display3, t),
+      TextStyle.lerp(begin.display2, end.display2, t),
+      TextStyle.lerp(begin.display1, end.display1, t),
+      TextStyle.lerp(begin.headline, end.headline, t),
+      TextStyle.lerp(begin.title, end.title, t),
+      TextStyle.lerp(begin.subhead, end.subhead, t),
+      TextStyle.lerp(begin.body2, end.body2, t),
+      TextStyle.lerp(begin.body1, end.body1, t),
+      TextStyle.lerp(begin.caption, end.caption, t),
+      TextStyle.lerp(begin.button, end.button, t)
+    );
+  }
+
 }
 
 class Typography {
   Typography._();
-
   static const TextTheme black = const TextTheme._black();
   static const TextTheme white = const TextTheme._white();
-
-  // TODO(abarth): Maybe this should be hard-coded in Scaffold?
-  static const String typeface = 'font-family: sans-serif';
-
-  // TODO(ianh): Remove this when we remove fn2, now that it's hard-coded in App.
-  static const TextStyle error = const TextStyle(
-    color: const Color(0xD0FF0000),
-    fontFamily: 'monospace',
-    fontSize: 48.0,
-    fontWeight: FontWeight.w900,
-    textAlign: TextAlign.right,
-    decoration: TextDecoration.underline,
-    decorationColor: const Color(0xFFFF00),
-    decorationStyle: TextDecorationStyle.double
-  );
 }

packages/flutter/lib/src/painting/text_style.dart

@@ -127,6 +127,31 @@ class TextStyle {
     );
   }
 
+  /// Interpolate between two text styles.
+  ///
+  /// This will not work well if the styles don't set the same fields.
+  static TextStyle lerp(TextStyle begin, TextStyle end, double t) {
+    assert(begin.inherit == end.inherit);
+    return new TextStyle(
+      inherit: end.inherit,
+      color: Color.lerp(begin.color, end.color, t),
+      fontFamily: t < 0.5 ? begin.fontFamily : end.fontFamily,
+      fontSize: ui.lerpDouble(begin.fontSize ?? end.fontSize, end.fontSize ?? begin.fontSize, t),
+      // TODO(ianh): Replace next line with "fontWeight: FontWeight.lerp(begin.fontWeight, end.fontWeight, t)," once engine is revved
+      fontWeight: FontWeight.values[ui.lerpDouble(begin?.fontWeight.index ?? FontWeight.normal.index, end?.fontWeight.index ?? FontWeight.normal.index, t.clamp(0.0, 1.0)).round()],
+      fontStyle: t < 0.5 ? begin.fontStyle : end.fontStyle,
+      letterSpacing: ui.lerpDouble(begin.letterSpacing ?? end.letterSpacing, end.letterSpacing ?? begin.letterSpacing, t),
+      wordSpacing: ui.lerpDouble(begin.wordSpacing ?? end.wordSpacing, end.wordSpacing ?? begin.wordSpacing, t),
+      textAlign: t < 0.5 ? begin.textAlign : end.textAlign,
+      textBaseline: t < 0.5 ? begin.textBaseline : end.textBaseline,
+      height: ui.lerpDouble(begin.height ?? end.height, end.height ?? begin.height, t),
+      decoration: t < 0.5 ? begin.decoration : end.decoration,
+      decorationColor: Color.lerp(begin.decorationColor, end.decorationColor, t),
+      decorationStyle: t < 0.5 ? begin.decorationStyle : end.decorationStyle
+    );
+  }
+  
+
   ui.TextStyle get textStyle {
     return new ui.TextStyle(
       color: color,

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

@@ -317,7 +317,7 @@ class _AnimatedContainerState extends AnimatedWidgetBaseState<AnimatedContainer>
 }
 
 /// Animated version of [Positioned] which automatically transitions the child's
-/// position over a given duration whenever the given positon changes.
+/// position over a given duration whenever the given position changes.
 ///
 /// Only works if it's the child of a [Stack].
 class AnimatedPositioned extends AnimatedWidgetBase {