Adapt FlexibleSpaceBar to iOS (#5037)

We should center the title of flexible space bars on iOS.

Related to #4962

packages/flutter/lib/src/animation/animations.dart

@@ -4,6 +4,8 @@
 
 import 'dart:ui' show VoidCallback;
 
+import 'package:flutter/foundation.dart';
+
 import 'animation.dart';
 import 'curves.dart';
 import 'listener_helpers.dart';
@@ -388,7 +390,19 @@ class CurvedAnimation extends Animation<double> with AnimationWithParentMixin<do
     if (activeCurve == null)
       return t;
     if (t == 0.0 || t == 1.0) {
-      assert(activeCurve.transform(t).round() == t);
+      assert(() {
+        final double transformedValue = activeCurve.transform(t);
+        final double roundedTransformedValue = transformedValue.round().toDouble();
+        if (roundedTransformedValue != t) {
+          throw new FlutterError(
+            'Invalided curve endpoint at $t.\n'
+            'Curves must map 0.0 to near zero and 1.0 to near one but '
+            '${activeCurve.runtimeType} mapped $t to $transformedValue, which '
+            'is near $roundedTransformedValue.'
+          );
+        }
+        return true;
+      });
       return t;
     }
     return activeCurve.transform(t);

packages/flutter/lib/src/animation/curves.dart

@@ -95,6 +95,8 @@ class Interval extends Curve {
     assert(end >= 0.0);
     assert(end <= 1.0);
     assert(end >= start);
+    if (t == 0.0 || t == 1.0)
+      return t;
     t = ((t - start) / (end - start)).clamp(0.0, 1.0);
     if (t == 0.0 || t == 1.0)
       return t;

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

@@ -31,7 +31,12 @@ class FlexibleSpaceBar extends StatefulWidget {
   ///
   /// Most commonly used in the [AppBar.flexibleSpace] field. Requires one of
   /// its ancestors to be a [Scaffold] widget.
-  FlexibleSpaceBar({ Key key, this.title, this.background }) : super(key: key);
+  FlexibleSpaceBar({
+    Key key,
+    this.title,
+    this.background,
+    this.centerTitle
+  }) : super(key: key);
 
   /// The primary contents of the flexible space bar when expanded.
   ///
@@ -43,6 +48,11 @@ class FlexibleSpaceBar extends StatefulWidget {
   /// Typically an [AssetImage] widget with [AssetImage.fit] set to [ImageFit.cover].
   final Widget background;
 
+  /// Whether the title should be centered.
+  ///
+  /// Defaults to being adapted to the current [TargetPlatform].
+  final bool centerTitle;
+
   @override
   _FlexibleSpaceBarState createState() => new _FlexibleSpaceBarState();
 }
@@ -63,6 +73,19 @@ class _FlexibleSpaceBarState extends State<FlexibleSpaceBar> {
     super.deactivate();
   }
 
+  bool _getEffectiveCenterTitle(ThemeData theme) {
+    if (config.centerTitle != null)
+      return config.centerTitle;
+    assert(theme.platform != null);
+    switch (theme.platform) {
+      case TargetPlatform.android:
+        return false;
+      case TargetPlatform.iOS:
+        return true;
+    }
+    return false;
+  }
+
   @override
   Widget build(BuildContext context) {
     assert(debugCheckHasScaffold(context));
@@ -101,6 +124,7 @@ class _FlexibleSpaceBarState extends State<FlexibleSpaceBar> {
 
     // title
     if (config.title != null) {
+      final ThemeData theme = Theme.of(context);
       final double fadeStart = (appBarHeight - toolBarHeight) / appBarHeight;
       final double fadeEnd = (appBarHeight - toolBarHeight / 2.0) / appBarHeight;
       final CurvedAnimation opacityCurve = new CurvedAnimation(
@@ -109,7 +133,7 @@ class _FlexibleSpaceBarState extends State<FlexibleSpaceBar> {
       );
       final int alpha = new Tween<double>(begin: 255.0, end: 0.0).evaluate(opacityCurve).toInt();
       if (alpha > 0) {
-        TextStyle titleStyle = Theme.of(context).primaryTextTheme.title;
+        TextStyle titleStyle = theme.primaryTextTheme.title;
         titleStyle = titleStyle.copyWith(
           color: titleStyle.color.withAlpha(alpha)
         );
@@ -120,18 +144,20 @@ class _FlexibleSpaceBarState extends State<FlexibleSpaceBar> {
           parent: _scaffoldAnimation,
           curve: new Interval(0.0, scaleAndAlignEnd)
         );
+        final bool effectiveCenterTitle = _getEffectiveCenterTitle(theme);
+        final FractionalOffset titleAlignment = effectiveCenterTitle ? FractionalOffset.bottomCenter : FractionalOffset.bottomLeft;
         children.add(new Padding(
-          padding: const EdgeInsets.only(left: 72.0, bottom: 14.0),
+          padding: new EdgeInsets.only(left: effectiveCenterTitle ? 0.0 : 72.0, bottom: 14.0),
           child: new Align(
             alignment: new Tween<FractionalOffset>(
               begin: new FractionalOffset(0.0, yAlignStart),
               end: new FractionalOffset(0.0, yAlignEnd)
             ).evaluate(scaleAndAlignCurve),
             child: new ScaleTransition(
-              alignment: FractionalOffset.bottomLeft,
+              alignment: titleAlignment,
               scale: new Tween<double>(begin: 1.5, end: 1.0).animate(scaleAndAlignCurve),
               child: new Align(
-                alignment: new FractionalOffset(0.0, 1.0),
+                alignment: titleAlignment,
                 child: new DefaultTextStyle(style: titleStyle, child: config.title)
               )
             )

packages/flutter/test/material/flexible_space_bar_test.dart

+// 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.
+
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('Flexible space back centers on iOS', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      new MaterialApp(
+        theme: new ThemeData(platform: TargetPlatform.android),
+        home: new Scaffold(
+          appBar: new AppBar(
+            flexibleSpace: new FlexibleSpaceBar(
+              title: new Text('X')
+            )
+          )
+        )
+      )
+    );
+
+    Finder title = find.text('X');
+    Point center = tester.getCenter(title);
+    Size size = tester.getSize(title);
+    expect(center.x, lessThan(400 - size.width / 2.0));
+
+    // Clear the widget tree to avoid animating between Android and iOS.
+    await tester.pumpWidget(new Container(key: new UniqueKey()));
+
+    await tester.pumpWidget(
+      new MaterialApp(
+        theme: new ThemeData(platform: TargetPlatform.iOS),
+        home: new Scaffold(
+          appBar: new AppBar(
+            flexibleSpace: new FlexibleSpaceBar(
+              title: new Text('X')
+            )
+          )
+        )
+      )
+    );
+
+    center = tester.getCenter(title);
+    size = tester.getSize(title);
+    expect(center.x, greaterThan(400 - size.width / 2.0));
+    expect(center.x, lessThan(400 + size.width / 2.0));
+  });
+}