在flutter中获取用户数据到firestore？

如果用户输入正确的数据，那么数据已经在firestore中，那么下一页将会出现，如果不正确，则会给出错误 . 我怎么能这样做？

import'package：flutter / material.dart'; import'dart：async';

import'package：cloud_firestore / cloud_firestore.dart';

void main（）=> runApp（new MaterialApp（title：'Registration'，home：new Signup（），））;

class Signup扩展了StatelessWidget {

字符串一;字符串二;

var用户;

Future onValidate() async {
  //var user;
  DocumentSnapshot snapshot = await

  Firestore.instance.collection("organization").document('').get().then((user));

      String code_A = snapshot['code_A'];
      String code_B = snapshot['code_B'];

}

@override小部件构建（BuildContext context）{return Scaffold（appBar：AppBar（title：Text（“Sign up”），），body：new Container（padding：new EdgeInsets.all（20.0），child：new Form（// key：this._formKey，child：new ListView（children：[new TextFormField（keyboardType：TextInputType.number，onSaved：（String value），validator：_validateEmail，decoration：new InputDecoration（labelText：'one' ）），

new TextFormField(
                keyboardType: TextInputType.number,
                onSaved: (String value) {
                  two = value;
                },
                validator: _validateEmail,
                decoration: new InputDecoration(
                    labelText: 'two'
                )
            ),

            /*new TextFormField(
                keyboardType: TextInputType.text,
                // Use email input type for emails.
                decoration: new InputDecoration(

                    labelText: 'Mobile number'
                )
            ),*/
            new Container(
              //width: screenSize.width,
              child: new RaisedButton(
                child: new Text(
                  'Sign Up',
                  style: new TextStyle(
                      color: Colors.white
                  ),
                ),

                onPressed: (){
                  if
                  }


                //color: Colors.blue,
              ),
              margin: new EdgeInsets.only(
                  top: 20.0
              ),
            )
          ],
        ),
      )
  ),
);

}}