Flutter button with animation access to onpressed method

class LoginPage extends StatefulWidget {
   @override
   _LoginPageState createState() => _LoginPageState();
}

class _LoginPageState extends State<LoginPage>
    with SingleTickerProviderStateMixin {
    static TextEditingController user = new TextEditingController();
   static TextEditingController pass = new TextEditingController();
   Session sesion = new Session();

@override
Widget build(BuildContext context) {
   return Scaffold(
   body: SafeArea(
      child: ListView(
      padding: EdgeInsets.symmetric(horizontal: 24.0),
      children: <Widget>[
      SizedBox(height: 80.0),
        Container(
          child: Row(children: [
            Image.asset(
              'assets/ucsm_plata.png',
              height: 159.0,
              width: 156.0,
              fit: BoxFit.cover,
            ),
            Column(
              mainAxisSize: MainAxisSize.max,
              children: <Widget>[
                Text('Universidad Catolica'),
                Text("de Santa Maria")
              ],
            )
          ]),
        ),
        SizedBox(height: 60.0),
        PrimaryColorOverride(
          color: color_icono,
          child: TextField(
            controller: user,
            maxLength: 8,
            decoration: InputDecoration(
                hintText: 'Username',
                icon: new Icon(
                  Icons.portrait,
                  color: Colors.grey,
                )),
          ),
        ),
        SizedBox(height: 12.0),
        PrimaryColorOverride(
          color: color_icono,
          child: TextField(
            controller: pass,
            maxLength: 32,
            decoration: InputDecoration(
                hintText: 'Password',
                icon: new Icon(
                  Icons.lock,
                  color: Colors.grey,
                )),
            obscureText: true,
          ),
        ),
        SizedBox(height: 20.0),
        Column(
          children: <Widget>[
            Container(
              margin: EdgeInsets.all(16.0),
              child: ProgressButton(),
            ),
            FlatButton(
              child: Text('Olvide mi clave'),
              shape: BeveledRectangleBorder(
                borderRadius: BorderRadius.all(Radius.circular(7.0)),
              ),
              onPressed: () {
                user.clear();
                pass.clear();
              },
            ),
          ],
        ),
      ],
    ),
  ),
);

} }

I use this code for an animated flutter button I have a login which uses this button, as this button is defined in another file to be able to reuse it I need to be able to access its onpresedd method to send it a function and when the function finishes finishing the animation

This is the function is a post that is defined in another file

class Session {
   String url = "http://10.0.2.2/Class_php/Coneccion.php";
   postData(String user, String pass) async {
   try {
   http.Response response =
      await http.post(url, body: {"CNRODNI": user, "CCLAVE": pass});
   var data = json.decode(response.body);
   String results = data.toString();
   if (response.statusCode == 200) {
       print(results);
   } else {
       print("Failed http call.");
   }
   } catch (exception) {
     print(exception.toString());
   }
  }