Add hasInteractedByUser getter in FormField (#131539)

Adds a getter to access the value of the private `RestorableBool _hasInteractedByUser`.

*List which issues are fixed by this PR. You must list at least one issue.*
Fixes #131538

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

@@ -423,6 +423,12 @@ class FormFieldState<T> extends State<FormField<T>> with RestorationMixin {
   /// True if this field has any validation errors.
   bool get hasError => _errorText.value != null;
 
+  /// Returns true if the user has modified the value of this field.
+  ///
+  /// This only updates to true once [didChange] has been called and resets to
+  /// false when [reset] is called.
+  bool get hasInteractedByUser => _hasInteractedByUser.value;
+
   /// True if the current value is valid.
   ///
   /// This will not set [errorText] or [hasError] and it will not update

packages/flutter/test/widgets/form_test.dart

@@ -854,4 +854,90 @@ void main() {
     expect(fieldValue, '123456');
     expect(formKey.currentState!.validate(), isFalse);
   });
+
+  testWidgets('hasInteractedByUser returns false when the input has not changed', (WidgetTester tester) async {
+    final GlobalKey<FormFieldState<String>> fieldKey = GlobalKey<FormFieldState<String>>();
+
+    final Widget widget = MaterialApp(
+      home: MediaQuery(
+        data: const MediaQueryData(),
+        child: Directionality(
+          textDirection: TextDirection.ltr,
+          child: Center(
+            child: Material(
+              child: TextFormField(
+                key: fieldKey,
+              ),
+            ),
+          ),
+        ),
+      ),
+    );
+
+    await tester.pumpWidget(widget);
+
+    expect(fieldKey.currentState!.hasInteractedByUser, isFalse);
+  });
+
+  testWidgets('hasInteractedByUser returns true after the input has changed', (WidgetTester tester) async {
+    final GlobalKey<FormFieldState<String>> fieldKey = GlobalKey<FormFieldState<String>>();
+
+    final Widget widget = MaterialApp(
+      home: MediaQuery(
+        data: const MediaQueryData(),
+        child: Directionality(
+          textDirection: TextDirection.ltr,
+          child: Center(
+            child: Material(
+              child: TextFormField(
+                key: fieldKey,
+              ),
+            ),
+          ),
+        ),
+      ),
+    );
+
+    await tester.pumpWidget(widget);
+
+    // initially, the field has not been interacted with
+    expect(fieldKey.currentState!.hasInteractedByUser, isFalse);
+
+    // after entering text, the field has been interacted with
+    await tester.enterText(find.byType(TextFormField), 'foo');
+    expect(fieldKey.currentState!.hasInteractedByUser, isTrue);
+  });
+
+  testWidgets('hasInteractedByUser returns false after the field is reset', (WidgetTester tester) async {
+    final GlobalKey<FormFieldState<String>> fieldKey = GlobalKey<FormFieldState<String>>();
+
+    final Widget widget = MaterialApp(
+      home: MediaQuery(
+        data: const MediaQueryData(),
+        child: Directionality(
+          textDirection: TextDirection.ltr,
+          child: Center(
+            child: Material(
+              child: TextFormField(
+                key: fieldKey,
+              ),
+            ),
+          ),
+        ),
+      ),
+    );
+
+    await tester.pumpWidget(widget);
+
+    // initially, the field has not been interacted with
+    expect(fieldKey.currentState!.hasInteractedByUser, isFalse);
+
+    // after entering text, the field has been interacted with
+    await tester.enterText(find.byType(TextFormField), 'foo');
+    expect(fieldKey.currentState!.hasInteractedByUser, isTrue);
+
+    // after resetting the field, it has not been interacted with again
+    fieldKey.currentState!.reset();
+    expect(fieldKey.currentState!.hasInteractedByUser, isFalse);
+  });
 }