Let translucent Cupertino bars have its scaffold children automatically pad their heights (#13194)

* Let lists automatically add sliver padding from media query. Translucent nav and tab bars leave behind media query paddings in scaffolds.

* tests

* const lint

* Rename base abstract class to generalized ObstructingPreferredSizeWidget

examples/flutter_gallery/lib/demo/cupertino/cupertino_navigation_demo.dart

@@ -131,17 +131,21 @@ class CupertinoDemoTab1 extends StatelessWidget {
             largeTitle: const Text('Colors'),
             trailing: const ExitButton(),
           ),
-          new SliverList(
-            delegate: new SliverChildBuilderDelegate(
-              (BuildContext context, int index) {
-                return new Tab1RowItem(
-                  index: index,
-                  lastItem: index == 49,
-                  color: colorItems[index],
-                  colorName: colorNameItems[index],
-                );
-              },
-              childCount: 50,
+          new SliverPadding(
+            // Top media query padding already consumed by CupertinoSliverNavigationBar.
+            padding: MediaQuery.of(context).removePadding(removeTop: true).padding,
+            sliver: new SliverList(
+              delegate: new SliverChildBuilderDelegate(
+                (BuildContext context, int index) {
+                  return new Tab1RowItem(
+                    index: index,
+                    lastItem: index == 49,
+                    color: colorItems[index],
+                    colorName: colorNameItems[index],
+                  );
+                },
+                childCount: 50,
+              ),
             ),
           ),
         ],
@@ -271,7 +275,7 @@ class Tab1ItemPageState extends State<Tab1ItemPage> {
       ),
       child: new ListView(
         children: <Widget>[
-          const Padding(padding: const EdgeInsets.only(top: 80.0)),
+          const Padding(padding: const EdgeInsets.only(top: 16.0)),
           new Padding(
             padding: const EdgeInsets.symmetric(horizontal: 16.0),
             child: new Row(
@@ -396,7 +400,6 @@ class CupertinoDemoTab2 extends StatelessWidget {
       ),
       child: new ListView(
         children: <Widget>[
-          const Padding(padding: const EdgeInsets.only(top: 60.0)),
           new Tab2Header(),
         ]..addAll(buildTab2Conversation()),
       ),
@@ -686,7 +689,6 @@ List<Widget> buildTab2Conversation() {
         ),
       ],
     ),
-    const Padding(padding: const EdgeInsets.only(bottom: 80.0)),
   ];
 }
 
@@ -702,7 +704,7 @@ class CupertinoDemoTab3 extends StatelessWidget {
         decoration: const BoxDecoration(color: const Color(0xFFEFEFF4)),
         child: new ListView(
           children: <Widget>[
-            const Padding(padding: const EdgeInsets.only(top: 100.0)),
+            const Padding(padding: const EdgeInsets.only(top: 32.0)),
             new GestureDetector(
               onTap: () {
                 Navigator.of(context, rootNavigator: true).push(

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

@@ -11,6 +11,7 @@ import 'package:flutter/widgets.dart';
 import 'button.dart';
 import 'colors.dart';
 import 'icons.dart';
+import 'page_scaffold.dart';
 
 /// Standard iOS navigation bar height without the status bar.
 const double _kNavBarPersistentHeight = 44.0;
@@ -68,7 +69,7 @@ const TextStyle _kLargeTitleTextStyle = const TextStyle(
 ///    [CupertinoNavigationBar].
 ///  * [CupertinoSliverNavigationBar] for a navigation bar to be placed in a
 ///    scrolling list and that supports iOS-11-style large titles.
-class CupertinoNavigationBar extends StatelessWidget implements PreferredSizeWidget {
+class CupertinoNavigationBar extends StatelessWidget implements ObstructingPreferredSizeWidget {
   /// Creates a navigation bar in the iOS style.
   const CupertinoNavigationBar({
     Key key,
@@ -116,11 +117,12 @@ class CupertinoNavigationBar extends StatelessWidget implements PreferredSizeWid
   final Color actionsForegroundColor;
 
   /// True if the navigation bar's background color has no transparency.
-  bool get opaque => backgroundColor.alpha == 0xFF;
+  @override
+  bool get fullObstruction => backgroundColor.alpha == 0xFF;
 
   @override
   Size get preferredSize {
-    return opaque ? const Size.fromHeight(_kNavBarPersistentHeight) : Size.zero;
+    return const Size.fromHeight(_kNavBarPersistentHeight);
   }
 
   @override

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

@@ -6,7 +6,6 @@ import 'package:flutter/foundation.dart';
 import 'package:flutter/widgets.dart';
 
 import 'colors.dart';
-import 'nav_bar.dart';
 
 /// Implements a single iOS application page's layout.
 ///
@@ -32,41 +31,52 @@ class CupertinoPageScaffold extends StatelessWidget {
   ///
   /// If translucent, the main content may slide behind it.
   /// Otherwise, the main content's top margin will be offset by its height.
+  ///
+  /// The scaffold assumes the nav bar will consume the [MediaQuery] top padding.
   // TODO(xster): document its page transition animation when ready
-  final PreferredSizeWidget navigationBar;
+  final ObstructingPreferredSizeWidget navigationBar;
 
   /// Widget to show in the main content area.
   ///
-  /// Content can slide under the [navigationBar] when they're translucent.
+  /// Content can slide under the [navigationBar] when they're translucent with
+  /// a [MediaQuery] padding hinting the top obstructed area.
   final Widget child;
 
   @override
   Widget build(BuildContext context) {
     final List<Widget> stacked = <Widget>[];
-    Widget childWithMediaQuery = child;
 
-    double topPadding = 0.0;
+    Widget paddedContent = child;
     if (navigationBar != null) {
-      topPadding += navigationBar.preferredSize.height;
-      // If the navigation bar has a preferred size, pad it and the OS status
-      // bar as well. Otherwise, let the content extend to the complete top
-      // of the page.
-      if (topPadding > 0.0) {
-        final EdgeInsets mediaQueryPadding = MediaQuery.of(context).padding;
-        topPadding += mediaQueryPadding.top;
-        childWithMediaQuery = new MediaQuery.removePadding(
-          context: context,
-          removeTop: true,
+      final MediaQueryData existingMediaQuery = MediaQuery.of(context);
+
+      // TODO(https://github.com/flutter/flutter/issues/12912):
+      // Use real size after partial layout instead of preferred size.
+      final double topPadding = navigationBar.preferredSize.height
+          + existingMediaQuery.padding.top;
+
+      // If nav bar is opaquely obstructing, directly shift the main content
+      // down. If translucent, let main content draw behind nav bar but hint the
+      // obstructed area.
+      if (navigationBar.fullObstruction) {
+        paddedContent = new Padding(
+          padding: new EdgeInsets.only(top: topPadding),
+          child: child,
+        );
+      } else {
+        paddedContent = new MediaQuery(
+          data: existingMediaQuery.copyWith(
+            padding: existingMediaQuery.padding.copyWith(
+              top: topPadding,
+            ),
+          ),
           child: child,
         );
       }
     }
 
     // The main content being at the bottom is added to the stack first.
-    stacked.add(new Padding(
-      padding: new EdgeInsets.only(top: topPadding),
-      child: childWithMediaQuery,
-    ));
+    stacked.add(paddedContent);
 
     if (navigationBar != null) {
       stacked.add(new Positioned(
@@ -84,4 +94,14 @@ class CupertinoPageScaffold extends StatelessWidget {
       ),
     );
   }
-}
\ No newline at end of file
+}
+
+/// Widget that has a preferred size and reports whether it fully obstructs
+/// widgets behind it.
+abstract class ObstructingPreferredSizeWidget extends PreferredSizeWidget {
+  /// If true, this widget fully obstructs widgets behind it by the specified
+  /// size.
+  ///
+  /// If false, this widget partially obstructs.
+  bool get fullObstruction;
+}

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

@@ -117,7 +117,8 @@ class CupertinoTabScaffold extends StatefulWidget {
   /// When the tab becomes inactive, its content is still cached in the widget
   /// tree [Offstage] and its animations disabled.
   ///
-  /// Content can slide under the [tabBar] when it's translucent.
+  /// Content can slide under the [tabBar] when it's translucent with a
+  /// [MediaQuery] padding hinting the bottom obstructed area.
   final IndexedWidgetBuilder tabBuilder;
 
   @override
@@ -131,18 +132,43 @@ class _CupertinoTabScaffoldState extends State<CupertinoTabScaffold> {
   Widget build(BuildContext context) {
     final List<Widget> stacked = <Widget>[];
 
-    // The main content being at the bottom is added to the stack first.
-    stacked.add(
-      new Padding(
-        padding: new EdgeInsets.only(bottom: widget.tabBar.opaque ? widget.tabBar.preferredSize.height : 0.0),
-        child: new _TabView(
-          currentTabIndex: _currentPage,
-          tabNumber: widget.tabBar.items.length,
-          tabBuilder: widget.tabBuilder,
-        )
-      ),
+    Widget content = new _TabView(
+      currentTabIndex: _currentPage,
+      tabNumber: widget.tabBar.items.length,
+      tabBuilder: widget.tabBuilder,
     );
 
+    if (widget.tabBar != null) {
+      final MediaQueryData existingMediaQuery = MediaQuery.of(context);
+
+      // TODO(https://github.com/flutter/flutter/issues/12912):
+      // Use real size after partial layout instead of preferred size.
+      final double bottomPadding = widget.tabBar.preferredSize.height
+          + existingMediaQuery.padding.bottom;
+
+      // If tab bar opaque, directly stop the main content higher. If
+      // translucent, let main content draw behind the tab bar but hint the
+      // obstructed area.
+      if (widget.tabBar.opaque) {
+        content = new Padding(
+          padding: new EdgeInsets.only(bottom: bottomPadding),
+          child: content,
+        );
+      } else {
+        content = new MediaQuery(
+          data: existingMediaQuery.copyWith(
+            padding: existingMediaQuery.padding.copyWith(
+              bottom: bottomPadding,
+            ),
+          ),
+          child: content,
+        );
+      }
+    }
+
+    // The main content being at the bottom is added to the stack first.
+    stacked.add(content);
+
     if (widget.tabBar != null) {
       stacked.add(new Align(
         alignment: Alignment.bottomCenter,

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

@@ -7,6 +7,7 @@ import 'package:flutter/rendering.dart';
 
 import 'basic.dart';
 import 'framework.dart';
+import 'media_query.dart';
 import 'primary_scroll_controller.dart';
 import 'scroll_controller.dart';
 import 'scroll_physics.dart';
@@ -372,8 +373,33 @@ abstract class BoxScrollView extends ScrollView {
   @override
   List<Widget> buildSlivers(BuildContext context) {
     Widget sliver = buildChildLayout(context);
-    if (padding != null)
-      sliver = new SliverPadding(padding: padding, sliver: sliver);
+    EdgeInsetsGeometry effectivePadding = padding;
+    if (padding == null) {
+      final MediaQueryData mediaQuery = MediaQuery.of(context, nullOk: true);
+      if (mediaQuery != null) {
+        // Automatically pad sliver with padding from MediaQuery.
+        final EdgeInsets mediaQueryHorizontalPadding =
+            mediaQuery.padding.copyWith(top: 0.0, bottom: 0.0);
+        final EdgeInsets mediaQueryVerticalPadding =
+            mediaQuery.padding.copyWith(left: 0.0, right: 0.0);
+        // Consume the main axis padding with SliverPadding.
+        effectivePadding = scrollDirection == Axis.vertical
+            ? mediaQueryVerticalPadding
+            : mediaQueryHorizontalPadding;
+        // Leave behind the cross axis padding.
+        sliver = new MediaQuery(
+          data: mediaQuery.copyWith(
+            padding: scrollDirection == Axis.vertical
+                ? mediaQueryHorizontalPadding
+                : mediaQueryVerticalPadding,
+          ),
+          child: sliver,
+        );
+      }
+    }
+
+    if (effectivePadding != null)
+      sliver = new SliverPadding(padding: effectivePadding, sliver: sliver);
     return <Widget>[ sliver ];
   }
 

packages/flutter/test/cupertino/scaffold_test.dart

@@ -78,6 +78,66 @@ void main() {
     expect(tester.getSize(find.byWidget(page1Center)).height, 600.0 - 44.0 - 50.0);
   });
 
+  testWidgets('Contents have automatic sliver padding between translucent bars', (WidgetTester tester) async {
+    final Container content = new Container(height: 600.0, width: 600.0);
+
+    await tester.pumpWidget(
+      new WidgetsApp(
+        color: const Color(0xFFFFFFFF),
+        onGenerateRoute: (RouteSettings settings) {
+          return new CupertinoPageRoute<Null>(
+            settings: settings,
+            builder: (BuildContext context) {
+              return new MediaQuery(
+                data: const MediaQueryData(
+                  padding: const EdgeInsets.symmetric(vertical: 20.0),
+                ),
+                child: new CupertinoTabScaffold(
+                  tabBar: 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'),
+                      ),
+                    ],
+                  ),
+                  tabBuilder: (BuildContext context, int index) {
+                    return index == 0
+                        ? new CupertinoPageScaffold(
+                          navigationBar: const CupertinoNavigationBar(
+                            middle: const Text('Title'),
+                          ),
+                          child: new ListView(
+                            children: <Widget>[
+                              content,
+                            ],
+                          ),
+                        )
+                        : new Stack();
+                  }
+                ),
+              );
+            },
+          );
+        },
+      ),
+    );
+
+    // List content automatically padded by nav bar and top media query padding.
+    expect(tester.getTopLeft(find.byWidget(content)).dy, 20.0 + 44.0);
+
+    // Overscroll to the bottom.
+    await tester.drag(find.byWidget(content), const Offset(0.0, -400.0));
+    await tester.pump(const Duration(seconds: 1));
+
+    // List content automatically padded by tab bar and bottom media query padding.
+    expect(tester.getBottomLeft(find.byWidget(content)).dy, 600 - 20.0 - 50.0);
+  });
+
   testWidgets('iOS independent tab navigation', (WidgetTester tester) async {
     // A full on iOS information architecture app with 2 tabs, and 2 pages
     // in each with independent navigation states.

packages/flutter/test/widgets/list_view_test.dart

@@ -265,4 +265,32 @@ void main() {
     expect(delegate.log, equals(<String>['didFinishLayout firstIndex=2 lastIndex=5']));
     delegate.log.clear();
   });
+
+  testWidgets('ListView automatically pad MediaQuery on axis', (WidgetTester tester) async {
+    EdgeInsets innerMediaQueryPadding;
+
+    await tester.pumpWidget(
+      new Directionality(
+        textDirection: TextDirection.ltr,
+        child: new MediaQuery(
+          data: const MediaQueryData(
+            padding: const EdgeInsets.all(30.0),
+          ),
+          child: new ListView(
+            children: <Widget>[
+              const Text('top', textDirection: TextDirection.ltr),
+              new Builder(builder: (BuildContext context) {
+                innerMediaQueryPadding = MediaQuery.of(context).padding;
+                return new Container();
+              }),
+            ],
+          ),
+        ),
+      ),
+    );
+    // Automatically apply the top/bottom padding into sliver.
+    expect(tester.getTopLeft(find.text('top')).dy, 30.0);
+    // Leave left/right padding as is for children.
+    expect(innerMediaQueryPadding, const EdgeInsets.symmetric(horizontal: 30.0));
+  });
 }