Create a CupertinoScaffold (#10543)

* works using pageview
it’s a bit heavy and scrolls
no documentation or test yet

* docs and tests

* more docs tweak

* revert drive by page view changes

* swap out the indexed stack for a custom thing

* layout everything on the cheap
tests pass

* add more tests

* move back to stack and add docs

* review notes

* build all the tabs once built

packages/flutter/lib/cupertino.dart

@@ -14,6 +14,7 @@ export 'src/cupertino/colors.dart';
 export 'src/cupertino/dialog.dart';
 export 'src/cupertino/nav_bar.dart';
 export 'src/cupertino/page.dart';
+export 'src/cupertino/scaffold.dart';
 export 'src/cupertino/slider.dart';
 export 'src/cupertino/switch.dart';
 export 'src/cupertino/thumb_painter.dart';

packages/flutter/lib/src/cupertino/bottom_tab_bar.dart

@@ -31,7 +31,7 @@ const Color _kDefaultTabBarBorderColor = const Color(0x4C000000);
 /// default), it will produce a blurring effect to the content behind it.
 //
 // TODO(xster): document using with a CupertinoScaffold.
-class CupertinoTabBar extends StatelessWidget {
+class CupertinoTabBar extends StatelessWidget implements PreferredSizeWidget {
   /// Creates a tab bar in the iOS style.
   CupertinoTabBar({
     Key key,
@@ -81,10 +81,14 @@ class CupertinoTabBar extends StatelessWidget {
   /// should configure itself to match the icon theme's size and color.
   final double iconSize;
 
+  /// True if the tab bar's background color has no transparency.
+  bool get opaque => backgroundColor.alpha == 0xFF;
+
+  @override
+  Size get preferredSize => const Size.fromHeight(_kTabBarHeight);
+
   @override
   Widget build(BuildContext context) {
-    final bool addBlur = backgroundColor.alpha != 0xFF;
-
     Widget result = new DecoratedBox(
       decoration: new BoxDecoration(
         border: const Border(
@@ -120,7 +124,7 @@ class CupertinoTabBar extends StatelessWidget {
       ),
     );
 
-    if (addBlur) {
+    if (!opaque) {
       // For non-opaque backgrounds, apply a blur effect.
       result = new ClipRect(
         child: new BackdropFilter(
@@ -141,6 +145,7 @@ class CupertinoTabBar extends StatelessWidget {
         _wrapActiveItem(
           new Expanded(
             child: new GestureDetector(
+              behavior: HitTestBehavior.opaque,
               onTap: onTap == null ? null : () { onTap(index); },
               child: new Padding(
                 padding: const EdgeInsets.only(bottom: 4.0),
@@ -175,4 +180,28 @@ class CupertinoTabBar extends StatelessWidget {
       ),
     );
   }
+
+  /// Create a clone of the current [CupertinoTabBar] but with provided
+  /// parameters overriden.
+  CupertinoTabBar copyWith({
+    Key key,
+    List<BottomNavigationBarItem> items,
+    Color backgroundColor,
+    Color activeColor,
+    Color inactiveColor,
+    Size iconSize,
+    int currentIndex,
+    ValueChanged<int> onTap,
+  }) {
+    return new CupertinoTabBar(
+       key: key ?? this.key,
+       items: items ?? this.items,
+       backgroundColor: backgroundColor ?? this.backgroundColor,
+       activeColor: activeColor ?? this.activeColor,
+       inactiveColor: inactiveColor ?? this.inactiveColor,
+       iconSize: iconSize ?? this.iconSize,
+       currentIndex: currentIndex ?? this.currentIndex,
+       onTap: onTap ?? this.onTap,
+    );
+  }
 }

packages/flutter/lib/src/cupertino/nav_bar.dart

@@ -69,13 +69,14 @@ class CupertinoNavigationBar extends StatelessWidget implements PreferredSizeWid
   /// The [title] remains black if it's a text as per iOS standard design.
   final Color actionsForegroundColor;
 
+  /// True if the nav bar's background color has no transparency.
+  bool get opaque => backgroundColor.alpha == 0xFF;
+
   @override
   Size get preferredSize => const Size.fromHeight(_kNavBarHeight);
 
   @override
   Widget build(BuildContext context) {
-    final bool addBlur = backgroundColor.alpha != 0xFF;
-
     Widget styledMiddle = middle;
     if (styledMiddle.runtimeType == Text || styledMiddle.runtimeType == DefaultTextStyle) {
       // Let the middle be black rather than `actionsForegroundColor` in case
@@ -132,7 +133,7 @@ class CupertinoNavigationBar extends StatelessWidget implements PreferredSizeWid
       ),
     );
 
-    if (addBlur) {
+    if (!opaque) {
       // For non-opaque backgrounds, apply a blur effect.
       result = new ClipRect(
         child: new BackdropFilter(

packages/flutter/lib/src/cupertino/scaffold.dart

@@ -0,0 +1,229 @@
+// Copyright 2017 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/foundation.dart';
+import 'package:flutter/widgets.dart';
+import 'bottom_tab_bar.dart';
+import 'nav_bar.dart';
+
+/// Implements a basic iOS application's layout and behavior structure.
+///
+/// The scaffold lays out the navigation bar on top, the tab bar at the bottom
+/// and tabbed or untabbed content between or behind the bars.
+///
+/// For tabbed scaffolds, the tab's active item and the actively showing tab
+/// in the content area are automatically connected.
+// TODO(xster): describe navigator handlings.
+// TODO(xster): add an example.
+class CupertinoScaffold extends StatefulWidget {
+  /// Construct a [CupertinoScaffold] without tabs.
+  ///
+  /// The [tabBar] and [rootTabPageBuilder] fields are not used in a [CupertinoScaffold]
+  /// without tabs.
+  // TODO(xster): document that page transitions will happen behind the navigation
+  // bar.
+  const CupertinoScaffold({
+    Key key,
+    this.navigationBar,
+    @required this.child,
+  }) : assert(child != null),
+       tabBar = null,
+       rootTabPageBuilder = null,
+       super(key: key);
+
+  /// Construct a [CupertinoScaffold] with tabs.
+  ///
+  /// A [tabBar] and a [rootTabPageBuilder] are required. The [CupertinoScaffold]
+  /// will automatically listen to the provided [CupertinoTabBar]'s tap callbacks
+  /// to change the active tab.
+  ///
+  /// Tabs' contents are built with the provided [rootTabPageBuilder] at the active
+  /// tab index. [rootTabPageBuilder] must be able to build the same number of
+  /// pages as the [tabBar.items.length]. Inactive tabs will be moved [Offstage]
+  /// and its animations disabled.
+  ///
+  /// The [child] field is not used in a [CupertinoScaffold] with tabs.
+  const CupertinoScaffold.tabbed({
+    Key key,
+    this.navigationBar,
+    @required this.tabBar,
+    @required this.rootTabPageBuilder,
+  }) : assert(tabBar != null),
+       assert(rootTabPageBuilder != null),
+       child = null,
+       super(key: key);
+
+  /// The [navigationBar], typically a [CupertinoNavigationBar], is drawn at the
+  /// top of the screen.
+  ///
+  /// If translucent, the main content may slide behind it.
+  /// Otherwise, the main content's top margin will be offset by its height.
+  // TODO(xster): document its page transition animation when ready
+  final PreferredSizeWidget navigationBar;
+
+  /// The [tabBar] is a [CupertinoTabBar] drawn at the bottom of the screen
+  /// that lets the user switch between different tabs in the main content area
+  /// when present.
+  ///
+  /// This parameter is required and must be non-null when the [new CupertinoScaffold.tabbed]
+  /// constructor is used.
+  ///
+  /// When provided, [CupertinoTabBar.currentIndex] will be ignored and will
+  /// be managed by the [CupertinoScaffold] to show the currently selected page
+  /// as the active item index. If [CupertinoTabBar.onTap] is provided, it will
+  /// still be called. [CupertinoScaffold] automatically also listen to the
+  /// [CupertinoTabBar]'s `onTap` to change the [CupertinoTabBar]'s `currentIndex`
+  /// and change the actively displayed tab in [CupertinoScaffold]'s own
+  /// main content area.
+  ///
+  /// If translucent, the main content may slide behind it.
+  /// Otherwise, the main content's bottom margin will be offset by its height.
+  final CupertinoTabBar tabBar;
+
+  /// An [IndexedWidgetBuilder] that's called when tabs become active.
+  ///
+  /// Used when a tabbed scaffold is constructed via the [new CupertinoScaffold.tabbed]
+  /// constructor and must be non-null.
+  ///
+  /// When the tab becomes inactive, its content is still cached in the widget
+  /// tree [Offstage] and its animations disabled.
+  ///
+  /// Content can slide under the [navigationBar] or the [tabBar] when they're
+  /// translucent.
+  final IndexedWidgetBuilder rootTabPageBuilder;
+
+  /// Widget to show in the main content area when the scaffold is used without
+  /// tabs.
+  ///
+  /// Used when the default [new CupertinoScaffold] constructor is used and must
+  /// be non-null.
+  ///
+  /// Content can slide under the [navigationBar] or the [tabBar] when they're
+  /// translucent.
+  final Widget child;
+
+  @override
+  _CupertinoScaffoldState createState() => new _CupertinoScaffoldState();
+}
+
+class _CupertinoScaffoldState extends State<CupertinoScaffold> {
+  int _currentPage = 0;
+
+  /// Pad the given middle widget with or without top and bottom offsets depending
+  /// on whether the middle widget should slide behind translucent bars.
+  Widget _padMiddle(Widget middle) {
+    double topPadding = MediaQuery.of(context).padding.top;
+    if (widget.navigationBar is CupertinoNavigationBar) {
+      final CupertinoNavigationBar top = widget.navigationBar;
+      if (top.opaque)
+        topPadding += top.preferredSize.height;
+    }
+
+    double bottomPadding = 0.0;
+    if (widget.tabBar?.opaque ?? false)
+      bottomPadding = widget.tabBar.preferredSize.height;
+
+    return new Padding(
+      padding: new EdgeInsets.only(top: topPadding, bottom: bottomPadding),
+      child: middle,
+    );
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    final List<Widget> stacked = <Widget>[];
+
+    // The main content being at the bottom is added to the stack first.
+    if (widget.child != null) {
+      stacked.add(_padMiddle(widget.child));
+    } else if (widget.rootTabPageBuilder != null) {
+      stacked.add(_padMiddle(new _TabView(
+        currentTabIndex: _currentPage,
+        tabNumber: widget.tabBar.items.length,
+        rootTabPageBuilder: widget.rootTabPageBuilder,
+      )));
+    }
+
+    if (widget.navigationBar != null) {
+      stacked.add(new Align(
+        alignment: FractionalOffset.topCenter,
+        child: widget.navigationBar,
+      ));
+    }
+
+    if (widget.tabBar != null) {
+      stacked.add(new Align(
+        alignment: FractionalOffset.bottomCenter,
+        // Override the tab bar's currentIndex to the current tab and hook in
+        // our own listener to update the _currentPage on top of a possibly user
+        // provided callback.
+        child: widget.tabBar.copyWith(
+            currentIndex: _currentPage,
+            onTap: (int newIndex) {
+              setState(() {
+                _currentPage = newIndex;
+              });
+              // Chain the user's original callback.
+              if (widget.tabBar.onTap != null)
+                widget.tabBar.onTap(newIndex);
+            }
+        ),
+      ));
+    }
+
+    return new Stack(
+      children: stacked,
+    );
+  }
+}
+
+/// An widget laying out multiple tabs with only one active tab being built
+/// at a time and on stage. Off stage tabs' animations are stopped.
+class _TabView extends StatefulWidget {
+  _TabView({
+    @required this.currentTabIndex,
+    @required this.tabNumber,
+    @required this.rootTabPageBuilder,
+  }) : assert(currentTabIndex != null),
+       assert(tabNumber != null && tabNumber > 0),
+       assert(rootTabPageBuilder != null);
+
+  final int currentTabIndex;
+  final int tabNumber;
+  final IndexedWidgetBuilder rootTabPageBuilder;
+
+  @override
+  _TabViewState createState() => new _TabViewState();
+}
+
+class _TabViewState extends State<_TabView> {
+  List<Widget> tabs;
+
+  @override
+  void initState() {
+    super.initState();
+    tabs = new List<Widget>(widget.tabNumber);
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return new Stack(
+      children: new List<Widget>.generate(widget.tabNumber, (int index) {
+        final bool active = index == widget.currentTabIndex;
+
+        // TODO(xster): lazily replace empty tabs with Navigators instead.
+        if (active || tabs[index] != null)
+          tabs[index] = widget.rootTabPageBuilder(context, index);
+
+        return new Offstage(
+          offstage: !active,
+          child: new TickerMode(
+            enabled: active,
+            child: tabs[index] ?? new Container(),
+          ),
+        );
+      }),
+    );
+  }
+}

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

@@ -1914,12 +1914,13 @@ class ListBody extends MultiChildRenderObjectWidget {
 /// placed relative to the stack according to their top, right, bottom, and left
 /// properties.
 ///
-/// The stack paints its children in order. If you want to change the order in
-/// which the children paint, you can rebuild the stack with the children in
-/// the new order. If you reorder the children in this way, consider giving the
-/// children non-null keys. These keys will cause the framework to move the
-/// underlying objects for the children to their new locations rather than
-/// recreate them at their new location.
+/// The stack paints its children in order with the first child being at the
+/// bottom. If you want to change the order in which the children paint, you
+/// can rebuild the stack with the children in the new order. If you reorder
+/// the children in this way, consider giving the children non-null keys.
+/// These keys will cause the framework to move the underlying objects for
+/// the children to their new locations rather than recreate them at their
+/// new location.
 ///
 /// For more details about the stack layout algorithm, see [RenderStack].
 ///

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

@@ -83,7 +83,7 @@ class _ToolbarLayout extends MultiChildLayoutDelegate {
   // If false the middle widget should be left justified within the space
   // between the leading and trailing widgets.
   // If true the middle widget is centered within the toolbar (not within the horizontal
-  // space bewteen the leading and trailing widgets).
+  // space between the leading and trailing widgets).
   // TODO(xster): document RTL once supported.
   final bool centerMiddle;
 

packages/flutter/test/cupertino/scaffold_test.dart

@@ -0,0 +1,203 @@
+// Copyright 2017 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/cupertino.dart';
+import 'package:flutter/widgets.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+import '../rendering/rendering_tester.dart';
+import '../services/mocks_for_image_cache.dart';
+
+List<int> selectedTabs;
+
+void main() {
+  setUp(() {
+    selectedTabs = <int>[];
+  });
+
+  testWidgets('Contents are behind translucent bar', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      new WidgetsApp(
+        color: const Color(0xFFFFFFFF),
+        onGenerateRoute: (RouteSettings settings) {
+          // TODO(xster): change to a CupertinoPageRoute.
+          return new PageRouteBuilder<Null>(
+            settings: settings,
+            pageBuilder: (BuildContext context, Animation<double> animation, Animation<double> secondaryAnimation) {
+              return const CupertinoScaffold(
+                // Default nav bar is translucent.
+                navigationBar: const CupertinoNavigationBar(
+                  middle: const Text('Title'),
+                ),
+                child: const Center(),
+              );
+            },
+          );
+        },
+      ),
+    );
+
+    expect(tester.getTopLeft(find.byType(Center)), const Offset(0.0, 0.0));
+  });
+
+  testWidgets('Contents are between opaque bars', (WidgetTester tester) async {
+    final Center page1Center = const Center();
+
+    await tester.pumpWidget(
+      new WidgetsApp(
+        color: const Color(0xFFFFFFFF),
+        onGenerateRoute: (RouteSettings settings) {
+          // TODO(xster): change to a CupertinoPageRoute.
+          return new PageRouteBuilder<Null>(
+            settings: settings,
+            pageBuilder: (BuildContext context, Animation<double> animation, Animation<double> secondaryAnimation) {
+              return new CupertinoScaffold.tabbed(
+                navigationBar: const CupertinoNavigationBar(
+                  backgroundColor: CupertinoColors.white,
+                  middle: const Text('Title'),
+                ),
+                tabBar: _buildTabBar(),
+                rootTabPageBuilder: (BuildContext context, int index) {
+                   return index == 0 ? page1Center : new Stack();
+                }
+              );
+            },
+          );
+        },
+      ),
+    );
+
+    expect(tester.getSize(find.byWidget(page1Center)).height, 600.0 - 44.0 - 50.0);
+  });
+
+  testWidgets('Tab switching', (WidgetTester tester) async {
+    final List<int> tabsPainted = <int>[];
+
+    await tester.pumpWidget(
+      new WidgetsApp(
+        color: const Color(0xFFFFFFFF),
+        onGenerateRoute: (RouteSettings settings) {
+          // TODO(xster): change to a CupertinoPageRoute.
+          return new PageRouteBuilder<Null>(
+            settings: settings,
+            pageBuilder: (BuildContext context, Animation<double> animation, Animation<double> secondaryAnimation) {
+              return new CupertinoScaffold.tabbed(
+                navigationBar: const CupertinoNavigationBar(
+                  backgroundColor: CupertinoColors.white,
+                  middle: const Text('Title'),
+                ),
+                tabBar: _buildTabBar(),
+                rootTabPageBuilder: (BuildContext context, int index) {
+                  return new CustomPaint(
+                    child: new Text('Page ${index + 1}'),
+                    painter: new TestCallbackPainter(
+                      onPaint: () { tabsPainted.add(index); }
+                    )
+                  );
+                }
+              );
+            },
+          );
+        },
+      ),
+    );
+
+    expect(tabsPainted, <int>[0]);
+    RichText tab1 = tester.widget(find.descendant(
+      of: find.text('Tab 1'),
+      matching: find.byType(RichText),
+    ));
+    expect(tab1.text.style.color, CupertinoColors.activeBlue);
+    RichText tab2 = tester.widget(find.descendant(
+      of: find.text('Tab 2'),
+      matching: find.byType(RichText),
+    ));
+    expect(tab2.text.style.color, CupertinoColors.inactiveGray);
+
+    await tester.tap(find.text('Tab 2'));
+    await tester.pump();
+
+    expect(tabsPainted, <int>[0, 1]);
+    tab1 = tester.widget(find.descendant(
+      of: find.text('Tab 1'),
+      matching: find.byType(RichText),
+    ));
+    expect(tab1.text.style.color, CupertinoColors.inactiveGray);
+    tab2 = tester.widget(find.descendant(
+      of: find.text('Tab 2'),
+      matching: find.byType(RichText),
+    ));
+    expect(tab2.text.style.color, CupertinoColors.activeBlue);
+
+    await tester.tap(find.text('Tab 1'));
+    await tester.pump();
+
+    expect(tabsPainted, <int>[0, 1, 0]);
+  });
+
+  testWidgets('Tabs are lazy built and moved offstage when inactive', (WidgetTester tester) async {
+    final List<int> tabsBuilt = <int>[];
+
+    await tester.pumpWidget(
+      new WidgetsApp(
+        color: const Color(0xFFFFFFFF),
+        onGenerateRoute: (RouteSettings settings) {
+          // TODO(xster): change to a CupertinoPageRoute.
+          return new PageRouteBuilder<Null>(
+            settings: settings,
+            pageBuilder: (BuildContext context, Animation<double> animation, Animation<double> secondaryAnimation) {
+              return new CupertinoScaffold.tabbed(
+                  navigationBar: const CupertinoNavigationBar(
+                    backgroundColor: CupertinoColors.white,
+                    middle: const Text('Title'),
+                  ),
+                  tabBar: _buildTabBar(),
+                  rootTabPageBuilder: (BuildContext context, int index) {
+                    tabsBuilt.add(index);
+                    return new Text('Page ${index + 1}');
+                  }
+              );
+            },
+          );
+        },
+      ),
+    );
+
+    expect(tabsBuilt, <int>[0]);
+    expect(find.text('Page 1'), findsOneWidget);
+    expect(find.text('Page 2'), findsNothing);
+
+    await tester.tap(find.text('Tab 2'));
+    await tester.pump();
+
+    // Both tabs are built but only one is onstage.
+    expect(tabsBuilt, <int>[0, 0, 1]);
+    expect(find.text('Page 1', skipOffstage: false), isOffstage);
+    expect(find.text('Page 2'), findsOneWidget);
+
+    await tester.tap(find.text('Tab 1'));
+    await tester.pump();
+
+    expect(tabsBuilt, <int>[0, 0, 1, 0, 1]);
+    expect(find.text('Page 1'), findsOneWidget);
+    expect(find.text('Page 2', skipOffstage: false), isOffstage);
+  });
+}
+
+CupertinoTabBar _buildTabBar() {
+  return new CupertinoTabBar(
+    items: <BottomNavigationBarItem>[
+      const BottomNavigationBarItem(
+        icon: const ImageIcon(const TestImageProvider(24, 24)),
+        title: const Text('Tab 1'),
+      ),
+      const BottomNavigationBarItem(
+        icon: const ImageIcon(const TestImageProvider(24, 24)),
+        title: const Text('Tab 2'),
+      ),
+    ],
+    backgroundColor: CupertinoColors.white,
+    onTap: (int newTab) => selectedTabs.add(newTab),
+  );
+}