如何将 Firestore 中的数据设置为 Flutter 中 TextField 的初始值？

import 'package:flutter/material.dart';
import 'package:cloud_firestore/cloud_firestore.dart';
import 'package:firebase_auth/firebase_auth.dart';

class ProfileSettings extends StatefulWidget {

 @override
 _ProfileSettingsState createState() => _ProfileSettingsState();
 
}

class _ProfileSettingsState extends State<ProfileSettings> {


 final firestoreInstance = Firestore.instance;

 void _setData() async{
   var firebaseUser = await FirebaseAuth.instance.currentUser();
   firestoreInstance.collection("tryingdata").document(firebaseUser.uid).setData(
       {
         "name" : "john",
         "age" : 20,
         "email" : "[email protected]",
         "address" : {
           "street" : "street 24",
           "city" : "new york"
         }
       }).then((_){
     print("success!");
   });
 }



 void _updateData()async{
   var firebaseUser = await FirebaseAuth.instance.currentUser();
   firestoreInstance
       .collection("tryingdata")
       .document(firebaseUser.uid)
       .updateData({"age": _age.text}).then((_) {
     print("success!");
     print(_age);
   });
 }

 final _auth = FirebaseAuth.instance;
 String userAge;
 TextEditingController _age = TextEditingController();

 void _retrieveData() async{
   var firebaseUser = await FirebaseAuth.instance.currentUser();
   firestoreInstance.collection("tryingdata").document(firebaseUser.uid).get().then((value)
     => userAge = value.data["age"]);
   print(userAge);
 }

 @override

 Widget build(BuildContext context) {
   _retrieveData();
   return Scaffold(
     appBar: AppBar(
       title: Text(
         "Profilepage",
         style: TextStyle(
           fontFamily: "tepeno",
           fontWeight: FontWeight.w600,
         ),
       ),
       actions: <Widget>[

       ],
     ),
         body: Column(
         children: <Widget>[
           TextField(
             controller: _age,
             initialvalue: //here I need help,
             textAlign: TextAlign.left,
             decoration: InputDecoration(
               border: InputBorder.none,
               hintText: 'PLEASE ENTER YOUR AGE',
               hintStyle: TextStyle(color: Colors.grey),
             ),
           ),
             RaisedButton(
             onPressed: () {
                     _setData();
             },
               child: const Text('SetData', style: TextStyle(fontSize: 12)),
            ),
           RaisedButton(
             onPressed: () {
              _updateData();
             },
             child: const Text('UpdateData', style: TextStyle(fontSize: 12)),
           ),
           RaisedButton(
             onPressed: () {
               _retrieveData();
             },
             child: const Text('RetrieveData', style: TextStyle(fontSize: 12)),
           ),
         ]
     ),
   );
 }

}

_setData() 和 _updateData() 函数工作正常。我想在 TextField 中显示初始值。当我点击 _RetrieveData() 按钮时，它会打印出值，例如：20 然后我将数据更新为 21，当我再次打印出来时，它仍然是 20。有时它也会打印出空值。我也在小部件构建中运行 _RetrieveData() 。所以我需要从 firestore 获取数据并将其设置为 TextField 中的初始值，然后我可以更新它。我该如何解决这个问题？我是 Flutter 的新手。谢谢！

我得到了一些帮助，这是解决方案：

Future<dynamic>_retrieveData() async{
    var firebaseUser = await FirebaseAuth.instance.currentUser();
    await Firestore.instance.collection("tryingdata").document(firebaseUser.uid).snapshots().listen((DocumentSnapshot doc) async{
      Map<String, dynamic> document =doc.data;
      setState((){
        userAge=document['age'];
      });

    });
  }
  
  @override
  void initState() {
    // TODO: implement initState
    super.initState();
    _retrieveData();
  }