PopScope example improvements (#142163)

Attempting to help users understand how to build a confirmation dialog when exiting a route.

examples/api/lib/widgets/form/form.1.dart

@@ -62,8 +62,13 @@ class _SaveableFormState extends State<_SaveableForm> {
     });
   }
 
-  Future<void> _showDialog() async {
-    final bool? shouldDiscard = await showDialog<bool>(
+  /// Shows a dialog and resolves to true when the user has indicated that they
+  /// want to pop.
+  ///
+  /// A return value of null indicates a desire not to pop, such as when the
+  /// user has dismissed the modal without tapping a button.
+  Future<bool?> _showDialog() {
+    return showDialog<bool>(
       context: context,
       builder: (BuildContext context) {
         return AlertDialog(
@@ -86,19 +91,13 @@ class _SaveableFormState extends State<_SaveableForm> {
         );
       },
     );
-
-    if (shouldDiscard ?? false) {
-      // Since this is the root route, quit the app where possible by invoking
-      // the SystemNavigator. If this wasn't the root route, then
-      // Navigator.maybePop could be used instead.
-      // See https://github.com/flutter/flutter/issues/11490
-      SystemNavigator.pop();
-    }
   }
 
   void _save(String? value) {
+    final String nextSavedValue = value ?? '';
     setState(() {
-      _savedValue = value ?? '';
+      _savedValue = nextSavedValue;
+      _isDirty = nextSavedValue != _controller.text;
     });
   }
 
@@ -112,11 +111,18 @@ class _SaveableFormState extends State<_SaveableForm> {
           const SizedBox(height: 20.0),
           Form(
             canPop: !_isDirty,
-            onPopInvoked: (bool didPop) {
+            onPopInvoked: (bool didPop) async {
               if (didPop) {
                 return;
               }
-              _showDialog();
+              final bool shouldPop = await _showDialog() ?? false;
+              if (shouldPop) {
+                // Since this is the root route, quit the app where possible by
+                // invoking the SystemNavigator. If this wasn't the root route,
+                // then Navigator.maybePop could be used instead.
+                // See https://github.com/flutter/flutter/issues/11490
+                SystemNavigator.pop();
+              }
             },
             autovalidateMode: AutovalidateMode.always,
             child: Column(
@@ -146,9 +152,9 @@ class _SaveableFormState extends State<_SaveableForm> {
             ),
           ),
           TextButton(
-            onPressed: () {
-              if (_isDirty) {
-                _showDialog();
+            onPressed: () async {
+              final bool shouldPop = !_isDirty || (await _showDialog() ?? false);
+              if (!shouldPop) {
                 return;
               }
               // Since this is the root route, quit the app where possible by

examples/api/lib/widgets/pop_scope/pop_scope.0.dart

@@ -62,8 +62,13 @@ class _PageTwo extends StatefulWidget {
 }
 
 class _PageTwoState extends State<_PageTwo> {
-  void _showBackDialog() {
-    showDialog<void>(
+  /// Shows a dialog and resolves to true when the user has indicated that they
+  /// want to pop.
+  ///
+  /// A return value of null indicates a desire not to pop, such as when the
+  /// user has dismissed the modal without tapping a button.
+  Future<bool?> _showBackDialog() {
+    return showDialog<bool>(
       context: context,
       builder: (BuildContext context) {
         return AlertDialog(
@@ -78,7 +83,7 @@ class _PageTwoState extends State<_PageTwo> {
               ),
               child: const Text('Nevermind'),
               onPressed: () {
-                Navigator.pop(context);
+                Navigator.pop(context, false);
               },
             ),
             TextButton(
@@ -87,8 +92,7 @@ class _PageTwoState extends State<_PageTwo> {
               ),
               child: const Text('Leave'),
               onPressed: () {
-                Navigator.pop(context);
-                Navigator.pop(context);
+                Navigator.pop(context, true);
               },
             ),
           ],
@@ -107,15 +111,21 @@ class _PageTwoState extends State<_PageTwo> {
             const Text('Page Two'),
             PopScope(
               canPop: false,
-              onPopInvoked: (bool didPop) {
+              onPopInvoked: (bool didPop) async {
                 if (didPop) {
                   return;
                 }
-                _showBackDialog();
+                final bool shouldPop = await _showBackDialog() ?? false;
+                if (context.mounted && shouldPop) {
+                  Navigator.pop(context);
+                }
               },
               child: TextButton(
-                onPressed: () {
-                  _showBackDialog();
+                onPressed: () async {
+                  final bool shouldPop = await _showBackDialog() ?? false;
+                  if (context.mounted && shouldPop) {
+                    Navigator.pop(context);
+                  }
                 },
                 child: const Text('Go back'),
               ),

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

@@ -26,8 +26,8 @@ import 'routes.dart';
 /// [PopScope], in which case it will be `false`.
 ///
 /// {@tool dartpad}
-/// This sample demonstrates how to use this widget to handle nested navigation
-/// in a bottom navigation bar.
+/// This sample demonstrates showing a confirmation dialog before navigating
+/// away from a page.
 ///
 /// ** See code in examples/api/lib/widgets/pop_scope/pop_scope.0.dart **
 /// {@end-tool}