In a Flutter widget test, how can I verify a field validation error message?

I recreated your case and was able to validate the inline error message. The idea is to add a delay of 1 second before we could test assertion. Here's what I did:

main.dart code to display textformfield and validate Email upon tapping Send button:

TextFormField(            decoration: new InputDecoration(hintText: 'Email ID'),            keyboardType: TextInputType.emailAddress,            maxLength: 32,            validator: validateEmail,            onSaved: (String val) {              email = val;            }),

validateEmail method snippet:

if (value.length == 0) {      return "Email is Required";    } else if(!regExp.hasMatch(value)){      return "Invalid Email";    }else {      return null;    }

Widget test to validate the inline error message for email field:

void main() {  testWidgets('validate email inline error message',          (WidgetTester tester) async {        await tester.pumpWidget(MyApp());        final buttonFinder = find.text('Send');        final emailErrorFinder = find.text('Email is Required');        await tester.tap(buttonFinder);        print('button tapped');        await tester.pump(const Duration(milliseconds: 100)); // add delay            expect(emailErrorFinder, findsOneWidget);        print('validated email inline error');      });}

Test result:

Hope this answers your question.