[Form] Add is valid to FormState (#48948)

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

@@ -349,6 +349,16 @@ class FormFieldState<T> extends State<FormField<T>> {
   /// True if this field has any validation errors.
   bool get hasError => _errorText != null;
 
+  /// True if the current value is valid.
+  ///
+  /// This will not set [errorText] or [hasError] and it will not update
+  /// error display.
+  ///
+  /// See also:
+  ///
+  ///  * [validate], which may update [errorText] and [hasError].
+  bool get isValid => widget.validator?.call(_value) == null;
+
   /// Calls the [FormField]'s onSaved method with the current value.
   void save() {
     if (widget.onSaved != null)
@@ -365,6 +375,11 @@ class FormFieldState<T> extends State<FormField<T>> {
 
   /// Calls [FormField.validator] to set the [errorText]. Returns true if there
   /// were no errors.
+  ///
+  /// See also:
+  ///
+  ///  * [isValid], which passively gets the validity without setting
+  ///    [errorText] or [hasError].
   bool validate() {
     setState(() {
       _validate();

packages/flutter/test/widgets/form_test.dart

@@ -137,6 +137,100 @@ void main() {
     await checkErrorText('');
   });
 
+  testWidgets('isValid returns true when a field is valid', (WidgetTester tester) async {
+    final GlobalKey<FormFieldState<String>> fieldKey1 = GlobalKey<FormFieldState<String>>();
+    final GlobalKey<FormFieldState<String>> fieldKey2 = GlobalKey<FormFieldState<String>>();
+    const String validString = 'Valid string';
+    String validator(String s) => s == validString ? null : 'Error text';
+
+    Widget builder() {
+      return MaterialApp(
+        home: MediaQuery(
+          data: const MediaQueryData(devicePixelRatio: 1.0),
+          child: Directionality(
+            textDirection: TextDirection.ltr,
+            child: Center(
+              child: Material(
+                child: Form(
+                  child: ListView(
+                    children: <Widget>[
+                      TextFormField(
+                        key: fieldKey1,
+                        initialValue: validString,
+                        validator: validator,
+                        autovalidate: true
+                      ),
+                      TextFormField(
+                        key: fieldKey2,
+                        initialValue: validString,
+                        validator: validator,
+                        autovalidate: true
+                      ),
+                    ],
+                  ),
+                ),
+              ),
+            ),
+          ),
+        ),
+      );
+    }
+
+    await tester.pumpWidget(builder());
+
+    expect(fieldKey1.currentState.isValid, isTrue);
+    expect(fieldKey2.currentState.isValid, isTrue);
+  });
+
+  testWidgets(
+    'isValid returns false when the field is invalid and does not change error display',
+    (WidgetTester tester) async {
+      final GlobalKey<FormFieldState<String>> fieldKey1 = GlobalKey<FormFieldState<String>>();
+      final GlobalKey<FormFieldState<String>> fieldKey2 = GlobalKey<FormFieldState<String>>();
+      const String validString = 'Valid string';
+      String validator(String s) => s == validString ? null : 'Error text';
+
+      Widget builder() {
+        return MaterialApp(
+          home: MediaQuery(
+            data: const MediaQueryData(devicePixelRatio: 1.0),
+            child: Directionality(
+              textDirection: TextDirection.ltr,
+              child: Center(
+                child: Material(
+                  child: Form(
+                    child: ListView(
+                      children: <Widget>[
+                        TextFormField(
+                          key: fieldKey1,
+                          initialValue: validString,
+                          validator: validator,
+                          autovalidate: false,
+                        ),
+                        TextFormField(
+                          key: fieldKey2,
+                          initialValue: '',
+                          validator: validator,
+                          autovalidate: false,
+                        ),
+                      ],
+                    ),
+                  ),
+                ),
+              ),
+            ),
+          ),
+        );
+      }
+
+      await tester.pumpWidget(builder());
+
+      expect(fieldKey1.currentState.isValid, isTrue);
+      expect(fieldKey2.currentState.isValid, isFalse);
+      expect(fieldKey2.currentState.hasError, isFalse);
+    },
+  );
+
   testWidgets('Multiple TextFormFields communicate', (WidgetTester tester) async {
     final GlobalKey<FormState> formKey = GlobalKey<FormState>();
     final GlobalKey<FormFieldState<String>> fieldKey = GlobalKey<FormFieldState<String>>();