Stream Example

index.html

<h4>Enter your Email</h4>
<input />
<div style="color:red"></div>

main.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(
  		(inputValue) => div.innerHtml = "",
    onError: (err) => div.innerHtml = err
  );
}