Why TextFormField's onSaved method requires a nullable String?-CodePudding

Minimum reproducible code:

final GlobalKey<FormState> _globalKey = GlobalKey();
  
@override
Widget build(BuildContext context) {
  return Scaffold(
    body: Column(
      children: [
        Form(
          key: _globalKey,
          child: TextFormField(onSaved: (String? string) {},
        ),
        ElevatedButton(
          onPressed: () => _globalKey.currentState!.save(),
          child: Text('Save'),
        ),
      ],
    ),
  );
}

Even if I don't enter anything (or bring this field in focus), clicking the Save button still gives me a non-null String. So, why does the onSaved requires a nullable String? in its parameter list i.e. why can't I do the following as onSaved always seems to have a non nullable value.

onSaved: (String string) {}, // Error

CodePudding user response：

I think it's because you can leave the field empty. If the input type is for example numbers you can't convert an empty field to int.

CodePudding user response：

onSaved is defined as FormFieldSetter<String>? onSaved,

And FormFieldSetter is

typedef FormFieldSetter<T> = void Function(T? newValue);

So it expects nullable datatype on callback

solution is

 onSaved: (String? string) {},