Flutter 中的 TextFormField 验证

Question

我正在研究 Flutter TextFormField 。 我想在TextFormField下方显示一条错误消息，但是当我调试时出现此错误

在 null 上调用了“验证”方法。 接收方：null 尝试调用：validate()

    class HomePage extends StatefulWidget {
  @override
  _HomePageState createState() => _HomePageState();
}

class _HomePageState extends State<HomePage> {


  TextEditingController _titleController;
  TextEditingController _descriptionController;
  final _formKey = GlobalKey<FormState>();
 @override
  void initState() {
    super.initState();

    _titleController = new TextEditingController(text: widget.products.adTitle);
    _descriptionController = new TextEditingController(text: widget.products.description); @override
  Widget build(BuildContext context) {
  return Scaffold(
    appBar: AppBar(
      title: Text("Products")
    ),
    body: Container(
      margin: EdgeInsets.all(15.0),
      alignment:  Alignment.center,
      key: _formKey,
      child: Column(
        children: <Widget>[
          TextFormField(
            controller: _titleController,
            decoration: InputDecoration(labelText: 'Title'),
            validator: (text) {
                if (text == null || text.isEmpty) {
                  return 'Text is empty';
                }
                return null;
              },
         RaisedButton(
            child: (widget.products.id != null)? Text('Update') : Text('Add'),
            onPressed:(){ 
               if (_formKey.currentState.validate()) {
                 child: Text('Submit');
               }

Answer 1

为了使用验证 function，您的列应包装在表单中

Container(
   margin: EdgeInsets.all(15.0),
   alignment: Alignment.center,
   child: Form(
         key: _formKey,
         child: Column(children: <Widget>[
               TextFormField(
                  controller: _titleController,
                  decoration:InputDecoration(labelText: 'Title'),
                              validator: (text) {
                                if (text == null || text.isEmpty) {
                                  return 'Text is empty';
                                }
                                return null;
                              },
               )
              ]))),