title: Flutter - Delegate Textfield in separate Class
body: <p>I want to use a separate class with a text field in Flutter.
  The text field is already displayed and can be edited.
  How can I pass the variable <code>_eingegebenerWert</code> to the class in which an object of this class was created?</p>
  
  <p><strong>The code of the Text-Field Class:</strong></p>
  
  <pre><code>class eingabeTextbox extends StatelessWidget {
    final bool _nummerischeTastatur;
    final String _ueberschrift;
    final String _platzhalter;
    String _eingegebenerWert;
  
    eingabeTextbox(this._nummerischeTastatur, this._ueberschrift, this._platzhalter, this._eingegebenerWert);
  
    @override
    Widget build (BuildContext context){
      return new TextField(
          keyboardType: _nummerischeTastatur == true ? TextInputType.number : TextInputType.multiline,
          decoration: new InputDecoration(
              labelText: _ueberschrift,
              hintText: _platzhalter
          ),
          onChanged: (String str){
            _eingegebenerWert = str;
            print("neuer Wert:" + _eingegebenerWert);
          }
      );
    }
  }
  </code></pre>