validate-email-stream.dart

import 'dart:html';
import 'dart:async';

void main() {
  final InputElement input = querySelector('input');
  final DivElement div = querySelector('div');
  
  
  final validator = new StreamTransformer.fromHandlers(
    handleData: (inputValue, sink) {
      if (inputValue.contains('@')) {
        sink.add(inputValue);
      } else {
        sink.addError('Enter a valid email');
      }
    }
  );
  
  input.onInput
    .map((dynamic event) => event.target.value)
    .transform(validator)
    .listen(
      (event) {
        div.innerHtml = '';
        print('That email looks valid');
      },
      onError: (error) => div.innerHtml = error
    );
}