在Flutter中显示TextField的错误文本

Question

我正在构建具有2个输入的登录屏幕。按下按钮后，我想验证电子邮件地址。我的猜测是我必须让视图知道该值已更改。

按钮

   RaisedButton(
    color: Colors.blue,
    textColor: Colors.white,
    child: Text('Login'),
    shape: RoundedRectangleBorder(
        borderRadius: new BorderRadius.circular(4)),
    onPressed: () {
      print('hey');

      if (!EmailValidator.validate(_emailController.text)) {
        _emailValid = false;
        return;
      }

      // do login stuffs
    },
  )

文本字段

_textFieldWidget('Email', false, _emailController, TextInputType.emailAddress, _emailValid ? 'not valid' : null)

Widget _textFieldWidget(String label, bool obscureText,
    TextEditingController controller, TextInputType type, String errorText) {
  return new TextField(
    obscureText: obscureText,
    controller: controller,
    keyboardType: type,
    decoration: InputDecoration(
        border: OutlineInputBorder(), labelText: label, errorText: errorText),
  );
}

Answer 1

[如果要验证用户输入，则应签出表单和验证者。在表单中，您可以使用验证器来简化TextFormField －这是从flutter网站获取的示例：

// Create a Form widget.
class MyCustomForm extends StatefulWidget {
  @override
  MyCustomFormState createState() {
    return MyCustomFormState();
  }
}

// Create a corresponding State class.
// This class holds data related to the form.
class MyCustomFormState extends State<MyCustomForm> {
  // Create a global key that uniquely identifies the Form widget
  // and allows validation of the form.
  //
  // Note: This is a GlobalKey<FormState>,
  // not a GlobalKey<MyCustomFormState>.
  final _formKey = GlobalKey<FormState>();

  @override
  Widget build(BuildContext context) {
    // Build a Form widget using the _formKey created above.
    return Form(
      key: _formKey,
      child: Column(
        crossAxisAlignment: CrossAxisAlignment.start,
        children: <Widget>[
          TextFormField(
            validator: (value) {
              if (value.isEmpty) {
                return 'Please enter some text';
              }
              return null;
            },
          ),
          Padding(
            padding: const EdgeInsets.symmetric(vertical: 16.0),
            child: RaisedButton(
              onPressed: () {
                // Validate returns true if the form is valid, or false
                // otherwise.
                if (_formKey.currentState.validate()) {
                  // If the form is valid, display a Snackbar.
                  Scaffold.of(context)
                      .showSnackBar(SnackBar(content: Text('Processing Data')));
                }
              },
              child: Text('Submit'),
            ),
          ),
        ],
      ),
    );
  }
}

有关详细信息，请访问flutter网站https://flutter.dev/docs/cookbook/forms/validation