Add Doc Samples For CheckboxListTile, RadioListTile and SwitchListTile (#32703)

* Moved Radio documentation line to be above sample

* Added LabeledRadio sample

* Add LabeledCheckbox sample

* Add LabeledSwitch sample

* Added LinkedLabelRadio sample to RadioListTile

* Added LinkedLabelCheckbox sample to CheckboxListTile

* Added LinkedLabelSwitch sample to SwitchListTile

* Added reference to Semantics docs

* Improve simple SwitchListTile, RadioListTile and CheckboxListTile samples

* Added assets to all SwitchListTile, RadioListTile and CheckboxListTile samples

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

@@ -37,7 +37,9 @@ import 'theme_data.dart';
 /// To show the [CheckboxListTile] as disabled, pass null as the [onChanged]
 /// callback.
 ///
-/// {@tool sample}
+/// {@tool snippet --template=stateful_widget_scaffold}
+///
+/// ![CheckboxListTile sample](https://flutter.github.io/assets-for-api-docs/assets/material/checkbox_list_tile.png)
 ///
 /// This widget shows a checkbox that, when checked, slows down all animations
 /// (including the animation of the checkbox itself getting checked!).
@@ -45,15 +47,196 @@ import 'theme_data.dart';
 /// This sample requires that you also import 'package:flutter/scheduler.dart',
 /// so that you can reference [timeDilation].
 ///
+/// ```dart imports
+/// import 'package:flutter/scheduler.dart' show timeDilation;
+/// ```
 /// ```dart
-/// CheckboxListTile(
+/// @override
+/// Widget build(BuildContext context) {
+///   return Center(
+///     child: CheckboxListTile(
 ///       title: const Text('Animate Slowly'),
 ///       value: timeDilation != 1.0,
 ///       onChanged: (bool value) {
-///     setState(() { timeDilation = value ? 20.0 : 1.0; });
+///         setState(() { timeDilation = value ? 10.0 : 1.0; });
 ///       },
 ///       secondary: const Icon(Icons.hourglass_empty),
-/// )
+///     ),
+///   );
+/// }
+/// ```
+/// {@end-tool}
+///
+/// ## Semantics in CheckboxListTile
+///
+/// Since the entirety of the CheckboxListTile is interactive, it should represent
+/// itself as a single interactive entity.
+///
+/// To do so, a CheckboxListTile widget wraps its children with a [MergeSemantics]
+/// widget. [MergeSemantics] will attempt to merge its descendant [Semantics]
+/// nodes into one node in the semantics tree. Therefore, CheckboxListTile will
+/// throw an error if any of its children requires its own [Semantics] node.
+///
+/// For example, you cannot nest a [RichText] widget as a descendant of
+/// CheckboxListTile. [RichText] has an embedded gesture recognizer that
+/// requires its own [Semantics] node, which directly conflicts with
+/// CheckboxListTile's desire to merge all its descendants' semantic nodes
+/// into one. Therefore, it may be necessary to create a custom radio tile
+/// widget to accommodate similar use cases.
+///
+/// {@tool snippet --template=stateful_widget_scaffold}
+///
+/// ![Checkbox list tile semantics sample](https://flutter.github.io/assets-for-api-docs/assets/material/checkbox_list_tile_semantics.png)
+///
+/// Here is an example of a custom labeled checkbox widget, called
+/// LinkedLabelCheckbox, that includes an interactive [RichText] widget that
+/// handles tap gestures.
+///
+/// ```dart imports
+/// import 'package:flutter/gestures.dart';
+/// ```
+/// ```dart preamble
+/// class LinkedLabelCheckbox extends StatelessWidget {
+///   const LinkedLabelCheckbox({
+///     this.label,
+///     this.padding,
+///     this.value,
+///     this.onChanged,
+///   });
+///
+///   final String label;
+///   final EdgeInsets padding;
+///   final bool value;
+///   final Function onChanged;
+///
+///   @override
+///   Widget build(BuildContext context) {
+///     return Padding(
+///       padding: padding,
+///       child: Row(
+///         children: <Widget>[
+///           Expanded(
+///             child: RichText(
+///               text: TextSpan(
+///                 text: label,
+///                 style: TextStyle(
+///                   color: Colors.blueAccent,
+///                   decoration: TextDecoration.underline,
+///                 ),
+///                 recognizer: TapGestureRecognizer()
+///                   ..onTap = () {
+///                   print('Label has been tapped.');
+///                 },
+///               ),
+///             ),
+///           ),
+///           Checkbox(
+///             value: value,
+///             onChanged: (bool newValue) {
+///               onChanged(newValue);
+///             },
+///           ),
+///         ],
+///       ),
+///     );
+///   }
+/// }
+/// ```
+/// ```dart
+/// bool _isSelected = false;
+///
+/// @override
+/// Widget build(BuildContext context) {
+///   return Scaffold(
+///     body: Center(
+///       child: LinkedLabelCheckbox(
+///         label: 'Linked, tappable label text',
+///         padding: const EdgeInsets.symmetric(horizontal: 20.0),
+///         value: _isSelected,
+///         onChanged: (bool newValue) {
+///           setState(() {
+///             _isSelected = newValue;
+///           });
+///         },
+///       ),
+///     ),
+///   );
+/// }
+/// ```
+/// {@end-tool}
+///
+/// ## CheckboxListTile isn't exactly what I want
+///
+/// If the way CheckboxListTile pads and positions its elements isn't quite
+/// what you're looking for, you can create custom labeled checkbox widgets by
+/// combining [Checkbox] with other widgets, such as [Text], [Padding] and
+/// [InkWell].
+///
+/// {@tool snippet --template=stateful_widget_scaffold}
+///
+/// ![Custom checkbox list tile sample](https://flutter.github.io/assets-for-api-docs/assets/material/checkbox_list_tile_custom.png)
+///
+/// Here is an example of a custom LabeledCheckbox widget, but you can easily
+/// make your own configurable widget.
+///
+/// ```dart preamble
+/// class LabeledCheckbox extends StatelessWidget {
+///   const LabeledCheckbox({
+///     this.label,
+///     this.padding,
+///     this.value,
+///     this.onChanged,
+///   });
+///
+///   final String label;
+///   final EdgeInsets padding;
+///   final bool value;
+///   final Function onChanged;
+///
+///   @override
+///   Widget build(BuildContext context) {
+///     return InkWell(
+///       onTap: () {
+///         onChanged(!value);
+///       },
+///       child: Padding(
+///         padding: padding,
+///         child: Row(
+///           children: <Widget>[
+///             Expanded(child: Text(label)),
+///             Checkbox(
+///               value: value,
+///               onChanged: (bool newValue) {
+///                 onChanged(newValue);
+///               },
+///             ),
+///           ],
+///         ),
+///       ),
+///     );
+///   }
+/// }
+/// ```
+/// ```dart
+/// bool _isSelected = false;
+///
+/// @override
+/// Widget build(BuildContext context) {
+///   return Scaffold(
+///     body: Center(
+///       child: LabeledCheckbox(
+///         label: 'This is the label text',
+///         padding: const EdgeInsets.symmetric(horizontal: 20.0),
+///         value: _isSelected,
+///         onChanged: (bool newValue) {
+///           setState(() {
+///             _isSelected = newValue;
+///           });
+///         },
+///       ),
+///     ),
+///   );
+/// }
 /// ```
 /// {@end-tool}
 ///

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

@@ -42,6 +42,8 @@ const double _kInnerRadius = 4.5;
 /// This causes the buttons to rebuild with the updated `groupValue`, and
 /// therefore the selection of the second button.
 ///
+/// Requires one of its ancestors to be a [Material] widget.
+///
 /// ```dart preamble
 /// enum SingingCharacter { lafayette, jefferson }
 /// ```
@@ -80,8 +82,6 @@ const double _kInnerRadius = 4.5;
 /// ```
 /// {@end-tool}
 ///
-/// Requires one of its ancestors to be a [Material] widget.
-///
 /// See also:
 ///
 ///  * [RadioListTile], which combines this widget with a [ListTile] so that

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

@@ -40,20 +40,22 @@ import 'theme_data.dart';
 /// To show the [RadioListTile] as disabled, pass null as the [onChanged]
 /// callback.
 ///
-/// {@tool sample}
+/// {@tool snippet --template=stateful_widget_scaffold}
+///
+/// ![RadioListTile sample](https://flutter.github.io/assets-for-api-docs/assets/material/radio_list_tile.png)
 ///
 /// This widget shows a pair of radio buttons that control the `_character`
 /// field. The field is of the type `SingingCharacter`, an enum.
 ///
-/// ```dart
-/// // At the top level:
+/// ```dart preamble
 /// enum SingingCharacter { lafayette, jefferson }
-///
-/// // In the State of a stateful widget:
+/// ```
+/// ```dart
 /// SingingCharacter _character = SingingCharacter.lafayette;
 ///
-/// // In the build function of that State:
-/// Column(
+/// @override
+/// Widget build(BuildContext context) {
+///   return Column(
 ///     children: <Widget>[
 ///       RadioListTile<SingingCharacter>(
 ///         title: const Text('Lafayette'),
@@ -68,7 +70,216 @@ import 'theme_data.dart';
 ///         onChanged: (SingingCharacter value) { setState(() { _character = value; }); },
 ///       ),
 ///     ],
-/// )
+///   );
+/// }
+/// ```
+/// {@end-tool}
+///
+/// ## Semantics in RadioListTile
+///
+/// Since the entirety of the RadioListTile is interactive, it should represent
+/// itself as a single interactive entity.
+///
+/// To do so, a RadioListTile widget wraps its children with a [MergeSemantics]
+/// widget. [MergeSemantics] will attempt to merge its descendant [Semantics]
+/// nodes into one node in the semantics tree. Therefore, RadioListTile will
+/// throw an error if any of its children requires its own [Semantics] node.
+///
+/// For example, you cannot nest a [RichText] widget as a descendant of
+/// RadioListTile. [RichText] has an embedded gesture recognizer that
+/// requires its own [Semantics] node, which directly conflicts with
+/// RadioListTile's desire to merge all its descendants' semantic nodes
+/// into one. Therefore, it may be necessary to create a custom radio tile
+/// widget to accommodate similar use cases.
+///
+/// {@tool snippet --template=stateful_widget_scaffold}
+///
+/// ![Radio list tile semantics sample](https://flutter.github.io/assets-for-api-docs/assets/material/radio_list_tile_semantics.png)
+///
+/// Here is an example of a custom labeled radio widget, called
+/// LinkedLabelRadio, that includes an interactive [RichText] widget that
+/// handles tap gestures.
+///
+/// ```dart imports
+/// import 'package:flutter/gestures.dart';
+/// ```
+/// ```dart preamble
+/// class LinkedLabelRadio extends StatelessWidget {
+///   const LinkedLabelRadio({
+///     this.label,
+///     this.padding,
+///     this.groupValue,
+///     this.value,
+///     this.onChanged,
+///   });
+///
+///   final String label;
+///   final EdgeInsets padding;
+///   final bool groupValue;
+///   final bool value;
+///   final Function onChanged;
+///
+///   @override
+///   Widget build(BuildContext context) {
+///     return Padding(
+///       padding: padding,
+///       child: Row(
+///         children: <Widget>[
+///           Radio<bool>(
+///             groupValue: groupValue,
+///             value: value,
+///             onChanged: (bool newValue) {
+///               onChanged(newValue);
+///             }
+///           ),
+///           RichText(
+///             text: TextSpan(
+///               text: label,
+///               style: TextStyle(
+///                 color: Colors.blueAccent,
+///                 decoration: TextDecoration.underline,
+///               ),
+///               recognizer: TapGestureRecognizer()
+///                 ..onTap = () {
+///                 print('Label has been tapped.');
+///               },
+///             ),
+///           ),
+///         ],
+///       ),
+///     );
+///   }
+/// }
+/// ```
+/// ```dart
+/// bool _isRadioSelected = false;
+///
+/// @override
+/// Widget build(BuildContext context) {
+///   return Scaffold(
+///     body: Column(
+///       mainAxisAlignment: MainAxisAlignment.center,
+///       children: <Widget>[
+///         LinkedLabelRadio(
+///           label: 'First tappable label text',
+///           padding: EdgeInsets.symmetric(horizontal: 5.0),
+///           value: true,
+///           groupValue: _isRadioSelected,
+///           onChanged: (bool newValue) {
+///             setState(() {
+///               _isRadioSelected = newValue;
+///             });
+///           },
+///         ),
+///         LinkedLabelRadio(
+///           label: 'Second tappable label text',
+///           padding: EdgeInsets.symmetric(horizontal: 5.0),
+///           value: false,
+///           groupValue: _isRadioSelected,
+///           onChanged: (bool newValue) {
+///             setState(() {
+///               _isRadioSelected = newValue;
+///             });
+///           },
+///         ),
+///       ],
+///     ),
+///   );
+/// }
+/// ```
+/// {@end-tool}
+///
+/// ## RadioListTile isn't exactly what I want
+///
+/// If the way RadioListTile pads and positions its elements isn't quite what
+/// you're looking for, you can create custom labeled radio widgets by
+/// combining [Radio] with other widgets, such as [Text], [Padding] and
+/// [InkWell].
+///
+/// {@tool snippet --template=stateful_widget_scaffold}
+///
+/// ![Custom radio list tile sample](https://flutter.github.io/assets-for-api-docs/assets/material/radio_list_tile_custom.png)
+///
+/// Here is an example of a custom LabeledRadio widget, but you can easily
+/// make your own configurable widget.
+///
+/// ```dart preamble
+/// class LabeledRadio extends StatelessWidget {
+///   const LabeledRadio({
+///     this.label,
+///     this.padding,
+///     this.groupValue,
+///     this.value,
+///     this.onChanged,
+///   });
+///
+///   final String label;
+///   final EdgeInsets padding;
+///   final bool groupValue;
+///   final bool value;
+///   final Function onChanged;
+///
+///   @override
+///   Widget build(BuildContext context) {
+///     return InkWell(
+///       onTap: () {
+///         if (value != groupValue)
+///           onChanged(value);
+///       },
+///       child: Padding(
+///         padding: padding,
+///         child: Row(
+///           children: <Widget>[
+///             Radio<bool>(
+///               groupValue: groupValue,
+///               value: value,
+///               onChanged: (bool newValue) {
+///                 onChanged(newValue);
+///               },
+///             ),
+///             Text(label),
+///           ],
+///         ),
+///       ),
+///     );
+///   }
+/// }
+/// ```
+/// ```dart
+/// bool _isRadioSelected = false;
+///
+/// @override
+/// Widget build(BuildContext context) {
+///   return Scaffold(
+///     body: Column(
+///       mainAxisAlignment: MainAxisAlignment.center,
+///       children: <LabeledRadio>[
+///         LabeledRadio(
+///           label: 'This is the first label text',
+///           padding: const EdgeInsets.symmetric(horizontal: 5.0),
+///           value: true,
+///           groupValue: _isRadioSelected,
+///           onChanged: (bool newValue) {
+///             setState(() {
+///               _isRadioSelected = newValue;
+///             });
+///           },
+///         ),
+///         LabeledRadio(
+///           label: 'This is the second label text',
+///           padding: const EdgeInsets.symmetric(horizontal: 5.0),
+///           value: false,
+///           groupValue: _isRadioSelected,
+///           onChanged: (bool newValue) {
+///             setState(() {
+///               _isRadioSelected = newValue;
+///             });
+///           },
+///         ),
+///       ],
+///     ),
+///   );
+/// }
 /// ```
 /// {@end-tool}
 ///

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

@@ -46,7 +46,9 @@ enum _SwitchListTileType { material, adaptive }
 /// To show the [SwitchListTile] as disabled, pass null as the [onChanged]
 /// callback.
 ///
-/// {@tool sample --template=stateful_widget_scaffold}
+/// {@tool snippet --template=stateful_widget_scaffold}
+///
+/// ![SwitchListTile sample](https://flutter.github.io/assets-for-api-docs/assets/material/switch_list_tile.png)
 ///
 /// This widget shows a switch that, when toggled, changes the state of a [bool]
 /// member field called `_lights`.
@@ -54,6 +56,7 @@ enum _SwitchListTileType { material, adaptive }
 /// ```dart
 /// bool _lights = false;
 ///
+/// @override
 /// Widget build(BuildContext context) {
 ///   return Center(
 ///     child: SwitchListTile(
@@ -67,6 +70,181 @@ enum _SwitchListTileType { material, adaptive }
 /// ```
 /// {@end-tool}
 ///
+/// ## Semantics in SwitchListTile
+///
+/// Since the entirety of the SwitchListTile is interactive, it should represent
+/// itself as a single interactive entity.
+///
+/// To do so, a SwitchListTile widget wraps its children with a [MergeSemantics]
+/// widget. [MergeSemantics] will attempt to merge its descendant [Semantics]
+/// nodes into one node in the semantics tree. Therefore, SwitchListTile will
+/// throw an error if any of its children requires its own [Semantics] node.
+///
+/// For example, you cannot nest a [RichText] widget as a descendant of
+/// SwitchListTile. [RichText] has an embedded gesture recognizer that
+/// requires its own [Semantics] node, which directly conflicts with
+/// SwitchListTile's desire to merge all its descendants' semantic nodes
+/// into one. Therefore, it may be necessary to create a custom radio tile
+/// widget to accommodate similar use cases.
+///
+/// {@tool snippet --template=stateful_widget_scaffold}
+///
+/// ![Switch list tile semantics sample](https://flutter.github.io/assets-for-api-docs/assets/material/switch_list_tile_semantics.png)
+///
+/// Here is an example of a custom labeled radio widget, called
+/// LinkedLabelRadio, that includes an interactive [RichText] widget that
+/// handles tap gestures.
+///
+/// ```dart imports
+/// import 'package:flutter/gestures.dart';
+/// ```
+/// ```dart preamble
+/// class LinkedLabelSwitch extends StatelessWidget {
+///   const LinkedLabelSwitch({
+///     this.label,
+///     this.padding,
+///     this.value,
+///     this.onChanged,
+///   });
+///
+///   final String label;
+///   final EdgeInsets padding;
+///   final bool value;
+///   final Function onChanged;
+///
+///   @override
+///   Widget build(BuildContext context) {
+///     return Padding(
+///       padding: padding,
+///       child: Row(
+///         children: <Widget>[
+///           Expanded(
+///             child: RichText(
+///               text: TextSpan(
+///                 text: label,
+///                 style: TextStyle(
+///                   color: Colors.blueAccent,
+///                   decoration: TextDecoration.underline,
+///                 ),
+///                 recognizer: TapGestureRecognizer()
+///                   ..onTap = () {
+///                   print('Label has been tapped.');
+///                 },
+///               ),
+///             ),
+///           ),
+///           Switch(
+///             value: value,
+///             onChanged: (bool newValue) {
+///               onChanged(newValue);
+///             },
+///           ),
+///         ],
+///       ),
+///     );
+///   }
+/// }
+/// ```
+/// ```dart
+/// bool _isSelected = false;
+///
+/// @override
+/// Widget build(BuildContext context) {
+///   return Scaffold(
+///     body: Center(
+///       child: LinkedLabelSwitch(
+///         label: 'Linked, tappable label text',
+///         padding: const EdgeInsets.symmetric(horizontal: 20.0),
+///         value: _isSelected,
+///         onChanged: (bool newValue) {
+///           setState(() {
+///             _isSelected = newValue;
+///           });
+///         },
+///       ),
+///     ),
+///   );
+/// }
+/// ```
+/// {@end-tool}
+///
+/// ## SwitchListTile isn't exactly what I want
+///
+/// If the way SwitchListTile pads and positions its elements isn't quite what
+/// you're looking for, you can create custom labeled switch widgets by
+/// combining [Switch] with other widgets, such as [Text], [Padding] and
+/// [InkWell].
+///
+/// {@tool snippet --template=stateful_widget_scaffold}
+///
+/// ![Custom switch list tile sample](https://flutter.github.io/assets-for-api-docs/assets/material/switch_list_tile_custom.png)
+///
+/// Here is an example of a custom LabeledSwitch widget, but you can easily
+/// make your own configurable widget.
+///
+/// ```dart preamble
+/// class LabeledSwitch extends StatelessWidget {
+///   const LabeledSwitch({
+///     this.label,
+///     this.padding,
+///     this.groupValue,
+///     this.value,
+///     this.onChanged,
+///   });
+///
+///   final String label;
+///   final EdgeInsets padding;
+///   final bool groupValue;
+///   final bool value;
+///   final Function onChanged;
+///
+///   @override
+///   Widget build(BuildContext context) {
+///     return InkWell(
+///       onTap: () {
+///         onChanged(!value);
+///       },
+///       child: Padding(
+///         padding: padding,
+///         child: Row(
+///           children: <Widget>[
+///             Expanded(child: Text(label)),
+///             Switch(
+///               value: value,
+///               onChanged: (bool newValue) {
+///                 onChanged(newValue);
+///               },
+///             ),
+///           ],
+///         ),
+///       ),
+///     );
+///   }
+/// }
+/// ```
+/// ```dart
+/// bool _isSelected = false;
+///
+/// @override
+/// Widget build(BuildContext context) {
+///   return Scaffold(
+///     body: Center(
+///       child: LabeledSwitch(
+///         label: 'This is the label text',
+///         padding: const EdgeInsets.symmetric(horizontal: 20.0),
+///         value: _isSelected,
+///         onChanged: (bool newValue) {
+///           setState(() {
+///             _isSelected = newValue;
+///           });
+///         },
+///       ),
+///     ),
+///   );
+/// }
+/// ```
+/// {@end-tool}
+///
 /// See also:
 ///
 ///  * [ListTileTheme], which can be used to affect the style of list tiles,