Generic support

[imsamgarg]

The current implementation restricts ValidationBuilder to be only used in the TextFormFields or wherever the value is in a string. There are some cases where we would need to validate values other than String like DropdownFormField or any custom FormField.

Consider this example

DatePickerFormField(
  onChanged: (v) {
    dueDate = v;
  },
  validator: (DateTime? date) {
    // date validations here...
  },
);

DatePickerFormField is a custom widget I made using FormField. the validator has this signature.

final FormFieldValidator<DateTime>? validator; 

I can not use the current ValidationBuilder in this case. To solve this I've added generics support to the original ValidationBuilder class and rename it to GenericValidationBuilder. To avoid breaking the old code I've added a typedef with the original class name.

// For backwards compatibility
typedef ValidationBuilder = GenericValidationBuilder<String>;
typedef StringValidationBuilder = GenericValidationBuilder<String>;

Please review the PR.

[themisir]

The issue is almost all of the validators in the library are only useful for string values. Adding generic validation support does not provide any value to the users since users will need to provide their own validation functions & messages, strictly limiting capabilities of the library for non string values.