How to write the Validators in seperate Dart file?

Question

I have the validators for a couple of TextFormFields, but is i write the validators in the same class I can retreive it but if the write in a seperate class I just cannot initialize it as its not showing it.

Thing I have done for now :

class _RegistrationPageState extends State<RegistrationPage> {
    @override
  Widget build(BuildContext context) {
    return Scaffold(
      backgroundColor: LightColors.kAqua,
      body: SingleChildScrollView(
        child: Padding(
          padding: const EdgeInsets.only(top: 32.0),
          child: Column(
            crossAxisAlignment: CrossAxisAlignment.start,
            children: <Widget>[
               TextFormField(
                     validator: validateEmail,
                     controller: emailController,
                     onChanged: (val){
                     setState(() => email = val);
                     },
                     decoration: InputDecoration(
                          prefixIcon: Icon(Icons.email),
                          hintText: "Email",
                          hintStyle: TextStyle(color: Colors.grey),
                          border: InputBorder.none),
                     ),
            ]
          )
        ) 
      )
    );
  }

String validateEmail(String value){
    String pattern = r'(^[a-z,A-Z ]*$)';
    RegExp regExp = new RegExp(pattern);
    if (!regExp.hasMatch(value)) {
      return "Name must be a-z and A-Z";
    }
    return null;
  }
}

By using this I can get the validateEmail in the validator field but if i just write the same validateEmail in a seperate class I'm unable to fetch it.

class Validators {

  String validateEmail(String value){
    String pattern = r'(^[a-z,A-Z ]*$)';
    RegExp regExp = new RegExp(pattern);
    if (!regExp.hasMatch(value)) {
      return "Name must be a-z and A-Z";
    }
    return null;
  }
}

Answer 1

If you want to use your validator function all over the project. Just paste it outside the class and use it any where Or you can also create new file and paste the function there and use them on other file On this approach don't forget to import the file. (use ctrl + . on the function name to import the file, compiler automatically find the file and import it)

These type of function called Top-level Function.

class _RegistrationPageState extends State<RegistrationPage> {
    @override
  Widget build(BuildContext context) {
    return Scaffold(
      backgroundColor: LightColors.kAqua,
      body: SingleChildScrollView(
        child: Padding(
          padding: const EdgeInsets.only(top: 32.0),
          child: Column(
            crossAxisAlignment: CrossAxisAlignment.start,
            children: <Widget>[
               TextFormField(
                     validator: validateEmail,
                     controller: emailController,
                     onChanged: (val){
                     setState(() => email = val);
                     },
                     decoration: InputDecoration(
                          prefixIcon: Icon(Icons.email),
                          hintText: "Email",
                          hintStyle: TextStyle(color: Colors.grey),
                          border: InputBorder.none),
                     ),
            ]
          )
        ) 
      )
    );
  }
}

String validateEmail(String value){
    String pattern = r'(^[a-z,A-Z ]*$)';
    RegExp regExp = new RegExp(pattern);
    if (!regExp.hasMatch(value)) {
      return "Name must be a-z and A-Z";
    }
    return null;
  }

or If you want to use these function inside a class. then first create an Instance and call the function.

Answer 2

You can copy paste run full code below
You can use validator: Validators().validateEmail
code snippet

TextFormField(
             validator: Validators().validateEmail,

working demo

full code

import 'package:flutter/material.dart';

class RegistrationPage extends StatefulWidget {
  @override
  _RegistrationPageState createState() => _RegistrationPageState();
}

class _RegistrationPageState extends State<RegistrationPage> {
  TextEditingController emailController = TextEditingController();
  String email;
  final _formKey = GlobalKey<FormState>();

  @override
  Widget build(BuildContext context) {
    return Scaffold(
        //backgroundColor: LightColors.kAqua,
        body: SingleChildScrollView(
            child: Padding(
                padding: const EdgeInsets.only(top: 32.0),
                child: Form(
                  key: _formKey,
                  child: Column(
                      crossAxisAlignment: CrossAxisAlignment.start,
                      children: <Widget>[
                        TextFormField(
                          validator: Validators().validateEmail,
                          controller: emailController,
                          onChanged: (val) {
                            setState(() => email = val);
                          },
                          decoration: InputDecoration(
                              prefixIcon: Icon(Icons.email),
                              hintText: "Email",
                              hintStyle: TextStyle(color: Colors.grey),
                              border: InputBorder.none),
                        ),
                        ElevatedButton(
                          onPressed: () {
                            if (_formKey.currentState.validate()) {}
                          },
                          child: Text('Submit'),
                        ),
                      ]),
                ))));
  }
}

class Validators {
  String validateEmail(String value) {
    print("value $value");

    String pattern = r'(^[a-z,A-Z ]*$)';
    RegExp regExp = new RegExp(pattern);
    if (!regExp.hasMatch(value)) {
      return "Name must be a-z and A-Z";
    }
    return null;
  }
}

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
        visualDensity: VisualDensity.adaptivePlatformDensity,
      ),
      home: RegistrationPage(),
    );
  }
}