Flutter-从SharedPreferences传递的数据显示为null

Question

So I have 2 screens. On screen 1, I retrieved an encoded JSON from SharedPreferences, and I have a button that navigates to Screen 2 and passes the retreived data. But on Screen 2, the data keeps showing as null. Isn't the widget meant to update when the state of the former widget updates ?

SCREEN 1

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

import 'screen2.dart';

class ScreenA extends StatefulWidget {
  @override
  _ProfileState createState() => new _ScreenAState();
}

class _ScreenAState extends State<Profile> {
    Map _user;

    @override
    void initState() {
      _getUser();

      super.initState();
    }

    Future<bool> _getUser() async {
      SharedPreferences prefs = await SharedPreferences.getInstance(); 
      String result = prefs.getString("user") ?? "";
      if(result != ""){
        setState(() {
          _user = json.decode(result);  
        });
      }
      return true;
    }

    @override
    Widget build(BuildContext context) {
      return new Scaffold(
        body: Container(
           child: RaisedButton(
             child: Text("Go to Screen 2"),
             onPressed: (){
                Navigator.push(
                  context, 
                  MaterialPageRoute(
                    builder: (context)=> Screen2(user: user)
                  )
                );
             }
          )
        )
      )
    }
}

SCREEN 2

import 'package:flutter/material.dart';

class Screen2 extends StatefulWidget {
  Screen2({this.user});
  final Map user;
  @override
  _Screen2State createState() => new _Screen2State();
}

class _Screen2State extends State<Screen2> {
  Map _user;
  @override
    void initState() {
      _user = widget.user;
      super.initState();
    }
  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      body: Text(_user["name"])
    );
  }
}

But when I Click the button in ScreenA this error is shown.

So, how do I pass the data from shared Prefrences to another Screen ?

Answer 1

Your code does not check if the _user variable is not null.

One thing you could do is to enable the button only if _user is not null. Thus, you are sure you are only calling the second page with a non null user.

     onPressed: _user != null ? (){
        Navigator.push(
          context, 
          MaterialPageRoute(
            builder: (context)=> Screen2(user: _user)
          )
        );
     } : null,