Question

我在flutter应用中实现了日期选择。然后会显示选定的日期，但是一旦焦点改变，选定的DateTime对象就会变成null，而没有任何理由。

要存储DateTime和其他一些变量，我创建了一个对象，该对象作为最终变量存储在StatefulWidget类中。此对象还包括Strings，当DateTime为空时不会改变。

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;
      });
    }
  }
}

如上所示，DateTime被DatePicker选择，然后将结果存储在assignment对象中。但是，当您单击标题的TextField时，标题突然变为空。

gif示例：https://gph.is/g/aKn7yqq

Answer 1

在状态类中使用声明的变量，如下所示：

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),

}

Flutter中的变量突然为空

1 个答案: