Form validators that can be used directly via code or constructed from JSON to provide more dynamic validation.

Add the repo to your Flutter pubspec.yaml file.

dependencies:
  form_validation: <<version>> 

Then run...

flutter packages get

The library provides a set of built-in validators while also providing a mechanism for applications to provide their own validators. All the built-in validators are able to be deserialized via JSON. They all expect an attribute of type to match a specific value when being desearialized.

Note: With the sole exception of RequiredValidator, all built in validators will pass on null or empty values.

The library provides a default set of English error messages for each validator's error message. This library uses the static_translations library for the string and language management, see it for details on how to override the defaults or provide values for other languages.

The Validator class can be used to decode a list of child ValueValidator entries. Each of the built-in validators can be deserialized via JSON. In addition to being able to deserialize from JSON, each of the built-in validators supports serializing to a JSON compatible map via toJson or an actual JSON encoded string via toJsonString.

The overall struction needs to be:

{
  "validators": [
    // One or more of the JSON objects shown below
  ]
}

{
  "allowNegative": <bool>, // Default: true; states whether negative values are allowed or not
  "type": "currency"
}

{
  "type": "email"
}

{
  "length": <int>, // The maximum length the value may be
  "type": "max_length"
}

{
  "length": <int>, // The maximum number the value may be
  "type": "max_number"
}

{
  "length": <int>, // The minimum length the value may be
  "type": "min_length"
}

{
  "length": <int>, // The minimum number the value may be
  "type": "min_number"
}

{
  "type": "number"
}

{
  "type": "phone_number"
}

{
  "type": "required"
}

The Validator supports custom validators being added either directly through classes extending the ValueValidator abstract class and passing them in via the constructor. Alternatively, an application may register a validator type with Validator using the registerCustomValidatorBuilder function.

class MyCustomValidator extends ValueValidator {
  static const type = 'my_custom_validator';

  static MyCustomValidator fromDynamic(dynamic map) {
    MyCustomValidator({
      // initialization args go here
    });

    MyCustomValidator result;

    if (map != null) {
      assert(map['type'] == type);

      result = MyCustomValidator(
        // Do additional JSON conversion here
      )
    })

    return result;
  }

  Map<String, dynamic> toJson() => {
    // add additional attributes here
    "type": type,
  }

  String validate({
    @required BuildContext context,
    @required String label,
    @required String value,
  }) {
    String error;

    // In general, validators should pass if the value is empty.  Combine 
    // validators with the RequiredValidator to ensure a value is non-empty.
    if (value?.isNotEmpty == true) {
      // Do processing to determine if the value is valid or not
    }

    return error;
  }
}

...

void main() {
  Validator.registerCustomValidatorBuilder(
    MyCustomValidator.type,
    MyCustomValidator.fromDynamic,
  );

  // start app
} 

...

var jsonStr = '''
{
  "validators": [{
    "type": "required"
  }, {
    "type": "my_custom_validator"
  }]
}
'''

// This will create a validation chain with the RequiredValidator as well as the
// MyCustomValidator defined above
var validator = Validator.fromDynamic(json.decode(jsonStr));