Fixes initial validation with AutovalidateMode.always on first build (#156708)

Fixes #142701
This PR fixes an issue where on the first build
`AutovalidateMode.always` was not called.

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there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [ ] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

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[Discord]:
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[Data Driven Fixes]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Data-driven-Fixes.md

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

@@ -12,6 +12,7 @@ import 'package:flutter/foundation.dart';
 import 'package:flutter/rendering.dart';
 
 import 'basic.dart';
+import 'binding.dart';
 import 'focus_manager.dart';
 import 'focus_scope.dart';
 import 'framework.dart';
@@ -250,22 +251,10 @@ class FormState extends State<Form> {
 
   void _register(FormFieldState<dynamic> field) {
     _fields.add(field);
-    if (widget.autovalidateMode == AutovalidateMode.onUnfocus) {
-      field._focusNode.addListener(() => _updateField(field));
-    }
   }
 
   void _unregister(FormFieldState<dynamic> field) {
     _fields.remove(field);
-    if (widget.autovalidateMode == AutovalidateMode.onUnfocus) {
-      field._focusNode.removeListener(()=> _updateField(field));
-    }
-  }
-
-  void _updateField(FormFieldState<dynamic> field) {
-    if (!field._focusNode.hasFocus) {
-      _validate();
-    }
   }
 
   @protected
@@ -703,6 +692,25 @@ class FormFieldState<T> extends State<FormField<T>> with RestorationMixin {
   }
 
   @protected
+  @override
+  void didChangeDependencies() {
+    super.didChangeDependencies();
+    switch (Form.maybeOf(context)?.widget.autovalidateMode) {
+      case AutovalidateMode.always:
+        WidgetsBinding.instance.addPostFrameCallback((_) {
+          // If the form is already validated, don't validate again.
+          if (widget.enabled && !hasError && !isValid) {
+            validate();
+          }
+        });
+      case AutovalidateMode.onUnfocus:
+      case AutovalidateMode.onUserInteraction:
+      case AutovalidateMode.disabled:
+      case null:
+        break;
+    }
+  }
+
   @override
   void dispose() {
     _errorText.dispose();
@@ -749,7 +757,6 @@ class FormFieldState<T> extends State<FormField<T>> with RestorationMixin {
 
     return widget.builder(this);
   }
-
 }
 
 /// Used to configure the auto validation of [FormField] and [Form] widgets.

packages/flutter/test/material/time_picker_test.dart

@@ -2074,7 +2074,6 @@ void main() {
     // Enter invalid hour.
     await tester.enterText(find.byType(TextField).first, 'AB');
     await tester.tap(find.text(okString));
-    await tester.pumpAndSettle();
 
     expect(tester.getSize(findBorderPainter().first), const Size(96.0, 70.0));
   });
@@ -2093,7 +2092,6 @@ void main() {
     // Enter invalid hour.
     await tester.enterText(find.byType(TextField).first, 'AB');
     await tester.tap(find.text(okString));
-    await tester.pumpAndSettle();
 
     expect(tester.getSize(findBorderPainter().first), const Size(96.0, 70.0));
   });

packages/flutter/test/material/time_picker_theme_test.dart

@@ -872,7 +872,6 @@ void main() {
     // Enter invalid hour.
     await tester.enterText(find.byType(TextField).first, 'AB');
     await tester.tap(find.text('OK'));
-    await tester.pumpAndSettle();
 
     expect(tester.getSize(findBorderPainter().first), const Size(96.0, 72.0));
   });
@@ -890,7 +889,6 @@ void main() {
     // Enter invalid hour.
     await tester.enterText(find.byType(TextField).first, 'AB');
     await tester.tap(find.text('OK'));
-    await tester.pumpAndSettle();
 
     expect(tester.getSize(findBorderPainter().first), const Size(96.0, 70.0));
   });

packages/flutter/test/widgets/form_test.dart

@@ -1457,4 +1457,44 @@ void main() {
 
     expect(focusNode2.hasFocus, isTrue);
   });
+
+  testWidgets('AutovalidateMode.always should validate on second build', (WidgetTester tester) async {
+    String errorText(String? value) => '$value/error';
+
+    await tester.pumpWidget(
+      MaterialApp(
+        theme: ThemeData(),
+        home: Center(
+          child: Form(
+            autovalidateMode: AutovalidateMode.always,
+            child: Material(
+              child: Column(
+                children: <Widget>[
+                  TextFormField(
+                    initialValue: 'foo',
+                    validator: errorText,
+                  ),
+                  TextFormField(
+                    initialValue: 'bar',
+                    validator: errorText,
+                  ),
+                ],
+              ),
+            ),
+          ),
+        ),
+      ),
+    );
+
+    // The validation happens in a post frame callback, so the error
+    // doesn't show up until the second frame.
+    expect(find.text(errorText('foo')), findsNothing);
+    expect(find.text(errorText('bar')), findsNothing);
+
+    await tester.pump();
+
+    // The error shows up on the second frame.
+    expect(find.text(errorText('foo')), findsOneWidget);
+    expect(find.text(errorText('bar')), findsOneWidget);
+  });
 }