An example of async form validation.

main.dart

import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Async Validator Example',
      debugShowCheckedModeBanner: false,
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: const _MyHomePage(title: 'Async Validator Example'),
    );
  }
}

class _MyHomePage extends StatefulWidget {
  const _MyHomePage({required this.title});

  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<_MyHomePage> {
  final TextEditingController _controller = TextEditingController();
  String? _valueCheckedForCollision;
  bool? _hasCollision;
    
  String? _validator(String? value) {
    if (_controller.value.text == '') {
      return 'Can\'t be empty!';
    }
    
    if (_controller.value.text != _valueCheckedForCollision) {
      _checkForCollision();
      return 'Checking for collision...';
    }
    
    return _hasCollision ?? false ? 'This already exists :(' : null;
  }
  
  // Check if the current value in the field is a collision, and update state with result.
  void _checkForCollision() async {
    final String valueCheckedForCollision = _controller.value.text;
    final bool hasCollision = await _fakeCollisionAPICall(valueCheckedForCollision);
    if (valueCheckedForCollision != _controller.value.text) {
      return;
    }
    setState(() {
      _valueCheckedForCollision = valueCheckedForCollision;
      _hasCollision = hasCollision;
    });
  }
  
  // Returns a future that resolves to false iff there is no collision on value.
  // Hardcoded to always return an error except for the value 'asdf', for demonstration.
  Future<bool> _fakeCollisionAPICall(String value) {
    return Future.delayed(const Duration(seconds: 4), () => value != 'asdf');
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            TextFormField(
              controller: _controller,
              autovalidateMode: AutovalidateMode.always,
              validator: _validator,
            ),
          ],
        ),
      ),
    );
  }
}