Variables are suddenly null in Flutter

Question

I've implemented a date selection in my flutter app. The selected date then gets displayed, but as soon as the focus changes, the selected DateTime object becomes null without any reason.

To store the DateTime and some other variables I've created an object which is stored as a final variable in the StatefulWidget class. This object also includes Strings which don't change when the DateTime becomes null.

import 'package:flutter/material.dart';
import 'package:smartyne/core/assignment.dart';

class AssignmentPage extends StatefulWidget {

  final bool editMode;
  final Assignment assignment;

  AssignmentPage(this.assignment, this.editMode);

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

class _AssignmentPageState extends State<AssignmentPage> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
        ...
        body: Padding(
          padding: EdgeInsets.all(16.0),
          child: Column(
            children: <Widget>[
              TextField(
                decoration: InputDecoration(
                    border: OutlineInputBorder(), labelText: "Title"),
                onChanged: (value) {
                  widget.assignment.title = value;
                  setState(() {});
                },
              ),

              ...

              SizedBox(
                height: 16.0,
              ),
              Row(
                mainAxisAlignment: MainAxisAlignment.spaceBetween,
                children: <Widget>[
                  MaterialButton(
                    child: Text(widget.assignment.deadlineAsString),
                    onPressed: () {
                      _showDatePicker();
                    },
                  ),

                  ...

                ],
              )
            ],
          ),
        ));
  }

  void _showDatePicker() async {
    final DateTime newDeadline = await showDatePicker(
        context: context,
        initialDate: widget.assignment.deadline == null ? DateTime.now() : widget.assignment.deadline,
        firstDate: DateTime(1900),
        lastDate: DateTime(2099)
    );

    if (newDeadline != null && newDeadline != widget.assignment.deadline) {
      setState(() {
        widget.assignment.deadline = newDeadline;
      });
    }
  }
}

As shown above, the DateTime gets selected by a DatePicker and the result is then stored in the assignment object. But when you click on the TextField for the title, it suddenly turns null.

Example gif: https://gph.is/g/aKn7yqq

score -2 · Accepted Answer · answered May 30 '19 at 10:33

-2

Use the declared variable inside state class like below

import 'package:flutter/material.dart';
import 'package:smartyne/core/assignment.dart';

class AssignmentPage extends StatefulWidget {

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

class _AssignmentPageState extends State<AssignmentPage> {
  final bool editMode;
  final Assignment assignment;
 @override
  Widget build(BuildContext context) {
return new Container();

//access it like 
//Text(assignment.deadlineAsString),
//instead of
//Text(widget.assignment.deadlineAsString),

}

answered May 30 '19 at 10:33

Aravindh Kumar

1,213
11
22

In that case, what is the benefit of keeping `AssignmentPage` a `StatefulWidget`? – CopsOnRoad May 30 '19 at 10:45
this is a wrong approach: https://stackoverflow.com/questions/50818770/passing-data-to-a-stateful-widget/50818870 – Rubens Melo May 30 '19 at 11:22
This is working now, but what would be a right and working solution? – eli2003 May 30 '19 at 12:03

Variables are suddenly null in Flutter

1 Answers1