Question

我目前正在做一个颤振项目。我在底部做了一个平面按钮，并将函数 _submit() 作为 onPressed 方法。但似乎 _formKey.currentState.validate() 有错误。错误信息如下图所示

<块引用>

'validate' 方法在 null 上被调用。接收器：空尝试调用：validate()

我发现了与我类似的问题并试图解决它。但我找不到我的错误。

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

class AddTaskScreen extends StatefulWidget {
  AddTaskScreen({Key key}) : super(key: key);

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

class _AddTaskScreenState extends State<AddTaskScreen> {
  final _formKey = GlobalKey<FormState>();
  String _title = '';
  String _priority;
  DateTime _date = DateTime.now();
  TextEditingController _dateController = TextEditingController();

  final DateFormat _dateFormatter = DateFormat('MMM dd, yyy');
  final List<String> _priorities = ['Low', 'Medium', 'High'];

  _handleDatePicker() async {
    final DateTime date = await showDatePicker(
        context: context,
        initialDate: _date,
        firstDate: DateTime(2000),
        lastDate: DateTime(2100));
    if (date != null && date != _date) {
      setState(() {
        _date = date;
      });
      _dateController.text = _dateFormatter.format(date);
    }
  }

  _submit() {
    if (_formKey.currentState.validate()) {
      _formKey.currentState.save();
      print('$_title $_date $_priority');
      Navigator.pop(context);
    }
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: GestureDetector(
        onTap: () => FocusScope.of(context).unfocus(),
        child: SingleChildScrollView(
          child: Container(
            padding: EdgeInsets.symmetric(horizontal: 40.0, vertical: 80.0),
            child: Column(
              crossAxisAlignment: CrossAxisAlignment.start,
              children: <Widget>[
                GestureDetector(
                  onTap: () => Navigator.pop(context),
                  child: Icon(
                    Icons.arrow_back_ios,
                    size: 30.0,
                    color: Colors.black,
                  ),
                ),
                SizedBox(
                  height: 20.0,
                ),
                Text('Add Task',
                    style: TextStyle(
                        color: Colors.black,
                        fontSize: 40.0,
                        fontWeight: FontWeight.bold)),
                SizedBox(height: 10.0),
                Form(
                  key: _formKey,
                  child: Column(
                    children: <Widget>[
                      Padding(
                        padding: EdgeInsets.symmetric(vertical: 20.0),
                        child: TextFormField(
                            style: TextStyle(fontSize: 18.0),
                            decoration: InputDecoration(
                                labelText: 'Title',
                                labelStyle: TextStyle(fontSize: 18.0),
                                border: OutlineInputBorder(
                                    borderRadius: BorderRadius.circular(10.0))),
                            validator: (input) => input.trim().isEmpty
                                ? 'Please enter a task title'
                                : null,
                            onSaved: (input) => _title = input,
                            initialValue: _title),
                      ),
                      Padding(
                          padding: EdgeInsets.symmetric(vertical: 20.0),
                          child: TextFormField(
                            readOnly: true,
                            controller: _dateController,
                            style: TextStyle(fontSize: 18.0),
                            onTap: _handleDatePicker,
                            decoration: InputDecoration(
                                labelText: 'Date',
                                labelStyle: TextStyle(fontSize: 18.0),
                                border: OutlineInputBorder(
                                    borderRadius: BorderRadius.circular(10.0))),
                          )),
                      Padding(
                          padding: EdgeInsets.symmetric(vertical: 20.0),
                          child: DropdownButtonFormField(
                            icon: Icon(Icons.arrow_drop_down_circle),
                            iconSize: 22.0,
                            items: _priorities.map((String priority) {
                              return DropdownMenuItem(
                                value: priority,
                                child: Text(
                                  priority,
                                  style: TextStyle(
                                      color: Colors.black, fontSize: 18.0),
                                ),
                              );
                            }).toList(),
                            style: TextStyle(fontSize: 18.0),
                            decoration: InputDecoration(
                                labelText: 'Priority',
                                labelStyle: TextStyle(fontSize: 18.0),
                                border: OutlineInputBorder(
                                    borderRadius: BorderRadius.circular(10.0))),
                            validator: (input) => _priority == null
                                ? 'Please select a priority level'
                                : null,
                            onChanged: (value) {
                              setState(() {
                                _priority = value;
                              });
                            },
                            value: _priority,
                          )),
                      Container(
                        margin: EdgeInsets.symmetric(vertical: 20.0),
                        height: 60.0,
                        width: double.infinity,
                        decoration: BoxDecoration(
                            color: Colors.black,
                            borderRadius: BorderRadius.circular(30.0)),
                        child: FlatButton(
                            onPressed: _submit(),
                            child: Text(
                              'Add',
                              style: TextStyle(
                                  color: Colors.white, fontSize: 20.0),
                            )),
                      )
                    ],
                  ),
                )
              ],
            ),
          ),
        ),
      ),
    );
  }
}

Answer 1

您可以在下面复制粘贴运行完整代码
您可以将 _submit() 更改为 _submit
因为 _submit() 表示执行函数
代码片段更改自

FlatButton(
          onPressed: _submit(),

到

FlatButton(
          onPressed: _submit,

工作演示

完整代码

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

class AddTaskScreen extends StatefulWidget {
  AddTaskScreen({Key key}) : super(key: key);

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

class _AddTaskScreenState extends State<AddTaskScreen> {
  final _formKey = GlobalKey<FormState>();
  String _title = '';
  String _priority;
  DateTime _date = DateTime.now();
  TextEditingController _dateController = TextEditingController();

  final DateFormat _dateFormatter = DateFormat('MMM dd, yyy');
  final List<String> _priorities = ['Low', 'Medium', 'High'];

  _handleDatePicker() async {
    final DateTime date = await showDatePicker(
        context: context,
        initialDate: _date,
        firstDate: DateTime(2000),
        lastDate: DateTime(2100));
    if (date != null && date != _date) {
      setState(() {
        _date = date;
      });
      _dateController.text = _dateFormatter.format(date);
    }
  }

  _submit() {
    if (_formKey.currentState.validate()) {
      _formKey.currentState.save();
      print('$_title $_date $_priority');
      Navigator.pop(context);
    }
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: GestureDetector(
        onTap: () => FocusScope.of(context).unfocus(),
        child: SingleChildScrollView(
          child: Container(
            padding: EdgeInsets.symmetric(horizontal: 40.0, vertical: 80.0),
            child: Column(
              crossAxisAlignment: CrossAxisAlignment.start,
              children: <Widget>[
                GestureDetector(
                  onTap: () => Navigator.pop(context),
                  child: Icon(
                    Icons.arrow_back_ios,
                    size: 30.0,
                    color: Colors.black,
                  ),
                ),
                SizedBox(
                  height: 20.0,
                ),
                Text('Add Task',
                    style: TextStyle(
                        color: Colors.black,
                        fontSize: 40.0,
                        fontWeight: FontWeight.bold)),
                SizedBox(height: 10.0),
                Form(
                  key: _formKey,
                  child: Column(
                    children: <Widget>[
                      Padding(
                        padding: EdgeInsets.symmetric(vertical: 20.0),
                        child: TextFormField(
                            style: TextStyle(fontSize: 18.0),
                            decoration: InputDecoration(
                                labelText: 'Title',
                                labelStyle: TextStyle(fontSize: 18.0),
                                border: OutlineInputBorder(
                                    borderRadius: BorderRadius.circular(10.0))),
                            validator: (input) => input.trim().isEmpty
                                ? 'Please enter a task title'
                                : null,
                            onSaved: (input) => _title = input,
                            initialValue: _title),
                      ),
                      Padding(
                          padding: EdgeInsets.symmetric(vertical: 20.0),
                          child: TextFormField(
                            readOnly: true,
                            controller: _dateController,
                            style: TextStyle(fontSize: 18.0),
                            onTap: _handleDatePicker,
                            decoration: InputDecoration(
                                labelText: 'Date',
                                labelStyle: TextStyle(fontSize: 18.0),
                                border: OutlineInputBorder(
                                    borderRadius: BorderRadius.circular(10.0))),
                          )),
                      Padding(
                          padding: EdgeInsets.symmetric(vertical: 20.0),
                          child: DropdownButtonFormField(
                            icon: Icon(Icons.arrow_drop_down_circle),
                            iconSize: 22.0,
                            items: _priorities.map((String priority) {
                              return DropdownMenuItem(
                                value: priority,
                                child: Text(
                                  priority,
                                  style: TextStyle(
                                      color: Colors.black, fontSize: 18.0),
                                ),
                              );
                            }).toList(),
                            style: TextStyle(fontSize: 18.0),
                            decoration: InputDecoration(
                                labelText: 'Priority',
                                labelStyle: TextStyle(fontSize: 18.0),
                                border: OutlineInputBorder(
                                    borderRadius: BorderRadius.circular(10.0))),
                            validator: (input) => _priority == null
                                ? 'Please select a priority level'
                                : null,
                            onChanged: (value) {
                              setState(() {
                                _priority = value;
                              });
                            },
                            value: _priority,
                          )),
                      Container(
                        margin: EdgeInsets.symmetric(vertical: 20.0),
                        height: 60.0,
                        width: double.infinity,
                        decoration: BoxDecoration(
                            color: Colors.black,
                            borderRadius: BorderRadius.circular(30.0)),
                        child: FlatButton(
                            onPressed: _submit,
                            child: Text(
                              'Add',
                              style: TextStyle(
                                  color: Colors.white, fontSize: 20.0),
                            )),
                      )
                    ],
                  ),
                )
              ],
            ),
          ),
        ),
      ),
    );
  }
}

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: AddTaskScreen(),
    );
  }
}

Flutter 错误：在 null 上调用了方法“validate”。接收者：空尝试调用：validate()

1 个答案: