我的 TextFormField 的验证总是抛出错误

Question

child: TextFormField(
                                controller: emailcontroller,
                                decoration: InputDecoration(
                                    border: InputBorder.none,
                                    hintText: "Email or Phone number",
                                    hintStyle:
                                        TextStyle(color: Colors.grey[400])),
                                keyboardType: TextInputType.emailAddress,
                                validator: (String value) { <= this line throw error
                                  if (value.isEmpty) {
                                    return 'Please Enter Name';
                                  }
                                  return null;
                                },
                                onSaved: (String value) {   <= this line throw error
                                 name = value;
                                },
                              )

"validator: (String value)" throws:

The argument type 'void Function(String)' can't be assigned to the parameter type 'void Function(String?)?'.dartargument_type_not_assignable

"onSaved: (String value)" throws:

The argument type 'void Function(String)' can't be assigned to the parameter type 'void Function(String?)?'.dartargument_type_not_assignable

when I checked a GitHub code I saw the same structure but it's not throwing an error

child: TextFormField(
                 keyboardType: TextInputType.text,
                 decoration: buildInputDecoration(Icons.person, "Full Name"),
                 validator: (String value) {
                   if (value.isEmpty) {
                     return 'Please Enter Name';
                   }
                   return null;
                 },
                 onSaved: (String value) {
                   name = value;
                 },
               ),

"buildInputDecoration" function :

InputDecoration buildInputDecoration(IconData icons,String hinttext) {
  return InputDecoration(
    hintText: hinttext,
    prefixIcon: Icon(icons),
    focusedBorder: OutlineInputBorder(
      borderRadius: BorderRadius.circular(25.0),
      borderSide: BorderSide(
          color: Colors.green,
          width: 1.5
      ),
    ),
    border: OutlineInputBorder(
      borderRadius: BorderRadius.circular(25.0),
      borderSide: BorderSide(
        color: Colors.blue,
        width: 1.5,
      ),
    ),
    enabledBorder:OutlineInputBorder(
      borderRadius: BorderRadius.circular(25.0),
      borderSide: BorderSide(
        color: Colors.blue,
        width: 1.5,
      ),
    ),
  );
}

Answer 1

Your example from GitHub is for older Dart versions without null safety. You are using Dart with null safety where String is not the same type as String? . So just add ? after String to validator function parameter:

    return TextFormField(
      ...
      validator: (String? value) {
        if (value?.isEmpty ?? true) {
          return 'Please Enter Name';
        }
        return null;
      },
      onSaved: (String? value) {
        name = value;
      },
    );

You can also just remove explicit value type:

    return TextFormField(
      ...
      validator: (value) {
        if (value?.isEmpty ?? true) {
          return 'Please Enter Name';
        }
        return null;
      },
      onSaved: (value) {
        name = value;
      },
    );