.required()

Adds one validation: the value is not null. Inside a schema, a missing key counts as null, so .required() is also what makes a field mandatory. It says nothing about emptiness: '' and [] pass. The return type drops the ?, so l.string().required() is a StringValidator<String> and success data is a String. It chains in any position. Optional by default explains the design.

Signature

StringValidator<String> required({String? message, MessageBuilder? messageBuilder})
// and the same on every other validator type, each returning its non-nullable form

Parameters

Parameter	Type	Meaning
message	String?	Replaces the default error message.
messageBuilder	MessageBuilder?	Builds the message from the issue. message wins when both are given. See Message builders.

Issue

Code required. Default message: value is required. value is replaced by the field name when there is one.

Code generation

The generator adds .required() to every parameter that is non-nullable, not dynamic, and has no default (a constructor default, @Default, or @JsonKey(defaultValue:)). Dart’s required keyword plays no part.

Example

Hand-written

final name = l.string().required();

Generated

required String name,          // l.string().required()
required String? note,         // l.string()
@Default('en') String locale,  // l.string()

name.validate(input)+2-1

1. +valid: 'Ada'
2. +valid: ''
3. -invalid: nullvalue is required