[NNBD] Migrate sample code pt 5 (#72845)

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

@@ -48,7 +48,7 @@ class _ActiveItem implements Comparable<_ActiveItem> {
 ///
 /// {@youtube 560 315 https://www.youtube.com/watch?v=ZtfItHwFlZ8}
 ///
-/// {@tool dartpad --template=freeform_no_null_safety}
+/// {@tool dartpad --template=freeform}
 /// This sample application uses an [AnimatedList] to create an effect when
 /// items are removed or added to the list.
 ///
@@ -65,9 +65,9 @@ class _ActiveItem implements Comparable<_ActiveItem> {
 ///
 /// class _AnimatedListSampleState extends State<AnimatedListSample> {
 ///   final GlobalKey<AnimatedListState> _listKey = GlobalKey<AnimatedListState>();
-///   ListModel<int> _list;
-///   int _selectedItem;
-///   int _nextItem; // The next item inserted when the user presses the '+' button.
+///   late ListModel<int> _list;
+///   int? _selectedItem;
+///   late int _nextItem; // The next item inserted when the user presses the '+' button.
 ///
 ///   @override
 ///   void initState() {
@@ -111,14 +111,14 @@ class _ActiveItem implements Comparable<_ActiveItem> {
 ///
 ///   // Insert the "next item" into the list model.
 ///   void _insert() {
-///     final int index = _selectedItem == null ? _list.length : _list.indexOf(_selectedItem);
+///     final int index = _selectedItem == null ? _list.length : _list.indexOf(_selectedItem!);
 ///     _list.insert(index, _nextItem++);
 ///   }
 ///
 ///   // Remove the selected item from the list model.
 ///   void _remove() {
 ///     if (_selectedItem != null) {
-///       _list.removeAt(_list.indexOf(_selectedItem));
+///       _list.removeAt(_list.indexOf(_selectedItem!));
 ///       setState(() {
 ///         _selectedItem = null;
 ///       });
@@ -168,30 +168,30 @@ class _ActiveItem implements Comparable<_ActiveItem> {
 /// /// of [AnimatedListState.insertItem] and [AnimatedList.removeItem].
 /// class ListModel<E> {
 ///   ListModel({
-///     @required this.listKey,
-///     @required this.removedItemBuilder,
-///     Iterable<E> initialItems,
-///   }) : assert(listKey != null),
-///       assert(removedItemBuilder != null),
-///       _items = List<E>.from(initialItems ?? <E>[]);
+///     required this.listKey,
+///     required this.removedItemBuilder,
+///     Iterable<E>? initialItems,
+///   }) : _items = List<E>.from(initialItems ?? <E>[]);
 ///
 ///   final GlobalKey<AnimatedListState> listKey;
 ///   final dynamic removedItemBuilder;
 ///   final List<E> _items;
 ///
-///   AnimatedListState get _animatedList => listKey.currentState;
+///   AnimatedListState? get _animatedList => listKey.currentState;
 ///
 ///   void insert(int index, E item) {
 ///     _items.insert(index, item);
-///     _animatedList.insertItem(index);
+///     _animatedList!.insertItem(index);
 ///   }
 ///
 ///   E removeAt(int index) {
 ///     final E removedItem = _items.removeAt(index);
 ///     if (removedItem != null) {
-///       _animatedList.removeItem(
+///       _animatedList!.removeItem(
 ///         index,
-///           (BuildContext context, Animation<double> animation) => removedItemBuilder(removedItem, context, animation),
+///         (BuildContext context, Animation<double> animation) {
+///           return removedItemBuilder(removedItem, context, animation);
+///         },
 ///       );
 ///     }
 ///     return removedItem;
@@ -212,24 +212,22 @@ class _ActiveItem implements Comparable<_ActiveItem> {
 /// /// varies from 0 to 128 as the animation varies from 0.0 to 1.0.
 /// class CardItem extends StatelessWidget {
 ///   const CardItem({
-///     Key key,
-///     @required this.animation,
+///     Key? key,
 ///     this.onTap,
-///     @required this.item,
-///     this.selected: false
-///   }) : assert(animation != null),
-///        assert(item != null && item >= 0),
-///        assert(selected != null),
+///     this.selected = false,
+///     required this.animation,
+///     required this.item,
+///   }) : assert(item >= 0),
 ///        super(key: key);
 ///
 ///   final Animation<double> animation;
-///   final VoidCallback onTap;
+///   final VoidCallback? onTap;
 ///   final int item;
 ///   final bool selected;
 ///
 ///   @override
 ///   Widget build(BuildContext context) {
-///     TextStyle textStyle = Theme.of(context).textTheme.headline4;
+///     TextStyle textStyle = Theme.of(context).textTheme.headline4!;
 ///     if (selected)
 ///       textStyle = textStyle.copyWith(color: Colors.lightGreenAccent[400]);
 ///     return Padding(
@@ -517,7 +515,7 @@ class AnimatedListState extends State<AnimatedList> with TickerProviderStateMixi
 /// [GlobalKey] or use the static [SliverAnimatedList.of] method from an item's
 /// input callback.
 ///
-/// {@tool dartpad --template=freeform_no_null_safety}
+/// {@tool dartpad --template=freeform}
 /// This sample application uses a [SliverAnimatedList] to create an animated
 /// effect when items are removed or added to the list.
 ///
@@ -538,9 +536,9 @@ class AnimatedListState extends State<AnimatedList> with TickerProviderStateMixi
 ///   final GlobalKey<SliverAnimatedListState> _listKey = GlobalKey<SliverAnimatedListState>();
 ///   final GlobalKey<ScaffoldState> _scaffoldKey = GlobalKey<ScaffoldState>();
 ///   final GlobalKey<ScaffoldMessengerState> _scaffoldMessengerKey = GlobalKey<ScaffoldMessengerState>();
-///   ListModel<int> _list;
-///   int _selectedItem;
-///   int _nextItem; // The next item inserted when the user presses the '+' button.
+///   late ListModel<int> _list;
+///   int? _selectedItem;
+///   late int _nextItem; // The next item inserted when the user presses the '+' button.
 ///
 ///   @override
 ///   void initState() {
@@ -583,19 +581,19 @@ class AnimatedListState extends State<AnimatedList> with TickerProviderStateMixi
 ///
 ///   // Insert the "next item" into the list model.
 ///   void _insert() {
-///     final int index = _selectedItem == null ? _list.length : _list.indexOf(_selectedItem);
+///     final int index = _selectedItem == null ? _list.length : _list.indexOf(_selectedItem!);
 ///     _list.insert(index, _nextItem++);
 ///   }
 ///
 ///   // Remove the selected item from the list model.
 ///   void _remove() {
 ///     if (_selectedItem != null) {
-///       _list.removeAt(_list.indexOf(_selectedItem));
+///       _list.removeAt(_list.indexOf(_selectedItem!));
 ///       setState(() {
 ///         _selectedItem = null;
 ///       });
 ///     } else {
-///       _scaffoldMessengerKey.currentState.showSnackBar(SnackBar(
+///       _scaffoldMessengerKey.currentState!.showSnackBar(SnackBar(
 ///         content: Text(
 ///           'Select an item to remove from the list.',
 ///           style: TextStyle(fontSize: 20),
@@ -658,18 +656,16 @@ class AnimatedListState extends State<AnimatedList> with TickerProviderStateMixi
 /// // of [AnimatedListState.insertItem] and [AnimatedList.removeItem].
 /// class ListModel<E> {
 ///   ListModel({
-///     @required this.listKey,
-///     @required this.removedItemBuilder,
-///     Iterable<E> initialItems,
-///   }) : assert(listKey != null),
-///        assert(removedItemBuilder != null),
-///        _items = List<E>.from(initialItems ?? <E>[]);
+///     required this.listKey,
+///     required this.removedItemBuilder,
+///     Iterable<E>? initialItems,
+///   }) : _items = List<E>.from(initialItems ?? <E>[]);
 ///
 ///   final GlobalKey<SliverAnimatedListState> listKey;
 ///   final dynamic removedItemBuilder;
 ///   final List<E> _items;
 ///
-///   SliverAnimatedListState get _animatedList => listKey.currentState;
+///   SliverAnimatedListState get _animatedList => listKey.currentState!;
 ///
 ///   void insert(int index, E item) {
 ///     _items.insert(index, item);
@@ -702,18 +698,16 @@ class AnimatedListState extends State<AnimatedList> with TickerProviderStateMixi
 /// // transitions from 0.0 to 1.0.
 /// class CardItem extends StatelessWidget {
 ///   const CardItem({
-///     Key key,
-///     @required this.animation,
-///     @required this.item,
+///     Key? key,
 ///     this.onTap,
 ///     this.selected = false,
-///   }) : assert(animation != null),
-///        assert(item != null && item >= 0),
-///        assert(selected != null),
+///     required this.animation,
+///     required this.item,
+///   }) : assert(item >= 0),
 ///        super(key: key);
 ///
 ///   final Animation<double> animation;
-///   final VoidCallback onTap;
+///   final VoidCallback? onTap;
 ///   final int item;
 ///   final bool selected;
 ///

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

@@ -11,7 +11,7 @@ import 'framework.dart';
 /// Animated widget that automatically transitions its size over a given
 /// duration whenever the given child's size changes.
 ///
-/// {@tool dartpad --template=stateful_widget_scaffold_center_freeform_state_no_null_safety}
+/// {@tool dartpad --template=stateful_widget_scaffold_center_freeform_state}
 /// This example makes a [Container] react to being touched, causing the child
 /// of the [AnimatedSize] widget, here a [FlutterLogo], to animate.
 ///

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

@@ -51,7 +51,7 @@ enum AutofillContextAction {
 /// autofillable input fields in an [AutofillGroup], so the user input of the
 /// [Form] can be saved for future autofill by the platform.
 ///
-/// {@tool dartpad --template=stateful_widget_scaffold_no_null_safety}
+/// {@tool dartpad --template=stateful_widget_scaffold}
 ///
 /// An example form with autofillable fields grouped into different
 /// `AutofillGroup`s.
@@ -92,8 +92,10 @@ enum AutofillContextAction {
 ///        const Text('Billing address'),
 ///        Checkbox(
 ///          value: isSameAddress,
-///          onChanged: (bool newValue) {
+///          onChanged: (bool? newValue) {
+///            if (newValue != null) {
 ///              setState(() { isSameAddress = newValue; });
+///            }
 ///          },
 ///        ),
 ///        // Again the address fields are grouped together for the same reason.

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

@@ -65,7 +65,7 @@ enum DismissDirection {
 ///
 /// {@youtube 560 315 https://www.youtube.com/watch?v=iEMgjrfuc58}
 ///
-/// {@tool dartpad --template=stateful_widget_scaffold_no_null_safety}
+/// {@tool dartpad --template=stateful_widget_scaffold}
 ///
 /// This sample shows how you can use the [Dismissible] widget to
 /// remove list items using swipe gestures. Swipe any of the list

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

@@ -48,7 +48,7 @@ import 'inherited_notifier.dart';
 /// the focus traversal order, call `Focus.of(context).nextFocus()`. To unfocus
 /// a widget, call `Focus.of(context).unfocus()`.
 ///
-/// {@tool dartpad --template=stateful_widget_scaffold_no_null_safety}
+/// {@tool dartpad --template=stateful_widget_scaffold}
 /// This example shows how to manage focus using the [Focus] and [FocusScope]
 /// widgets. See [FocusNode] for a similar example that doesn't use [Focus] or
 /// [FocusScope].
@@ -93,7 +93,7 @@ import 'inherited_notifier.dart';
 ///     debugLabel: 'Scope',
 ///     autofocus: true,
 ///     child: DefaultTextStyle(
-///       style: textTheme.headline4,
+///       style: textTheme.headline4!,
 ///       child: Focus(
 ///         onKey: _handleKeyPress,
 ///         debugLabel: 'Button',
@@ -128,7 +128,7 @@ import 'inherited_notifier.dart';
 /// ```
 /// {@end-tool}
 ///
-/// {@tool dartpad --template=stateless_widget_material_no_null_safety}
+/// {@tool dartpad --template=stateless_widget_material}
 /// This example shows how to wrap another widget in a [Focus] widget to make it
 /// focusable. It wraps a [Container], and changes the container's color when it
 /// is set as the [FocusManager.primaryFocus].
@@ -139,7 +139,10 @@ import 'inherited_notifier.dart';
 ///
 /// ```dart preamble
 /// class FocusableText extends StatelessWidget {
-///   const FocusableText(this.data, {Key key, this.autofocus}) : super(key: key);
+///   const FocusableText(this.data, {
+///     Key? key,
+///     required this.autofocus,
+///   }) : super(key: key);
 ///
 ///   /// The string to display as the text for this widget.
 ///   final String data;
@@ -186,7 +189,7 @@ import 'inherited_notifier.dart';
 /// ```
 /// {@end-tool}
 ///
-/// {@tool dartpad --template=stateful_widget_material_no_null_safety}
+/// {@tool dartpad --template=stateful_widget_material}
 /// This example shows how to focus a newly-created widget immediately after it
 /// is created.
 ///
@@ -738,7 +741,7 @@ class _FocusState extends State<Focus> {
 /// the focus traversal order, call `Focus.of(context).nextFocus()`. To unfocus
 /// a widget, call `Focus.of(context).unfocus()`.
 ///
-/// {@tool dartpad --template=stateful_widget_material_no_null_safety}
+/// {@tool dartpad --template=stateful_widget_material}
 /// This example demonstrates using a [FocusScope] to restrict focus to a particular
 /// portion of the app. In this case, restricting focus to the visible part of a
 /// Stack.
@@ -749,19 +752,19 @@ class _FocusState extends State<Focus> {
 /// /// This is just a separate widget to simplify the example.
 /// class Pane extends StatelessWidget {
 ///   const Pane({
-///     Key key,
-///     this.focusNode,
+///     Key? key,
+///     required this.focusNode,
 ///     this.onPressed,
+///     required this.backgroundColor,
+///     required this.icon,
 ///     this.child,
-///     this.backgroundColor,
-///     this.icon,
 ///   }) : super(key: key);
 ///
 ///   final FocusNode focusNode;
-///   final VoidCallback onPressed;
-///   final Widget child;
+///   final VoidCallback? onPressed;
 ///   final Color backgroundColor;
 ///   final Widget icon;
+///   final Widget? child;
 ///
 ///   @override
 ///   Widget build(BuildContext context) {
@@ -835,7 +838,7 @@ class _FocusState extends State<Focus> {
 ///                   child: Text('ANOTHER BUTTON TO FOCUS'),
 ///                 ),
 ///                 DefaultTextStyle(
-///                     style: Theme.of(context).textTheme.headline2,
+///                     style: Theme.of(context).textTheme.headline2!,
 ///                     child: Text('BACKDROP')),
 ///               ],
 ///             ),
@@ -864,7 +867,7 @@ class _FocusState extends State<Focus> {
 ///                 ? null
 ///                 : () => setState(() => backdropIsVisible = true),
 ///             child: DefaultTextStyle(
-///                 style: Theme.of(context).textTheme.headline2,
+///                 style: Theme.of(context).textTheme.headline2!,
 ///                 child: Text('FOREGROUND')),
 ///           ),
 ///         ),

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

@@ -44,7 +44,7 @@ enum OverflowBarAlignment {
 /// If the layout overflows, then children's order within their
 /// column is specified by [overflowDirection] instead.
 ///
-/// {@tool dartpad --template=stateless_widget_scaffold_center_no_null_safety}
+/// {@tool dartpad --template=stateless_widget_scaffold_center}
 ///
 /// This example defines a simple approximation of a dialog
 /// layout, where the layout of the dialog's action buttons are

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

@@ -203,7 +203,7 @@ class _RenderSliverFractionalPadding extends RenderSliverEdgeInsetsPadding {
 /// of space that has been scrolled beforehand has not exceeded the main axis
 /// extent of the viewport.
 ///
-/// {@tool dartpad --template=stateless_widget_scaffold_no_null_safety}
+/// {@tool dartpad --template=stateless_widget_scaffold}
 ///
 /// In this sample the [SliverFillRemaining] sizes its [child] to fill the
 /// remaining extent of the viewport in both axes. The icon is centered in the
@@ -239,7 +239,7 @@ class _RenderSliverFractionalPadding extends RenderSliverEdgeInsetsPadding {
 /// [SliverFillRemaining] will defer to the size of its [child] if the
 /// child's size exceeds the remaining space in the viewport.
 ///
-/// {@tool dartpad --template=stateless_widget_scaffold_no_null_safety}
+/// {@tool dartpad --template=stateless_widget_scaffold}
 ///
 /// In this sample the [SliverFillRemaining] defers to the size of its [child]
 /// because the child's extent exceeds that of the remaining extent of the
@@ -281,7 +281,7 @@ class _RenderSliverFractionalPadding extends RenderSliverEdgeInsetsPadding {
 /// [SliverFillRemaining] will defer to the size of its [child] if the
 /// [SliverConstraints.precedingScrollExtent] exceeded the length of the viewport's main axis.
 ///
-/// {@tool dartpad --template=stateless_widget_scaffold_no_null_safety}
+/// {@tool dartpad --template=stateless_widget_scaffold}
 ///
 /// In this sample the [SliverFillRemaining] defers to the size of its [child]
 /// because the [SliverConstraints.precedingScrollExtent] has gone
@@ -330,7 +330,7 @@ class _RenderSliverFractionalPadding extends RenderSliverEdgeInsetsPadding {
 ///
 /// {@animation 250 500 https://flutter.github.io/assets-for-api-docs/assets/widgets/sliver_fill_remaining_fill_overscroll.mp4}
 ///
-/// {@tool sample --template=stateless_widget_scaffold_no_null_safety}
+/// {@tool sample --template=stateless_widget_scaffold}
 ///
 /// In this sample the [SliverFillRemaining]'s child stretches to fill the
 /// overscroll area when [fillOverscroll] is true. This sample also features a