Built-in validators

The validators the package ships, with their exact types, and how to combine them with & and |.

A validator is a function. The Validator<T, E extends Object> typedef is E? Function(T): value in, error out, null means valid. Every built-in takes the error to return as its last argument, so it works with any E.

Validator	`T`	Rejects
`filled(e)`	`String?`	`null`, empty, and whitespace-only strings
`notLongerThan(max, e)`	`String?`	`length > max`; `max` itself passes, `null` passes
`atLeastLength(min, e)`	`String?`	`null`, and `length < min`; `min` itself passes
`exactly(text, e)`	`String?`	anything not equal to `text`
`nothing(e)`	`String?`	any non-empty string — "must be empty"
`positiveInteger(e)` / `nonNegativeInteger(e)`	`String?`	`null`, non-numeric text, and `<= 0` / `< 0` after `int.tryParse`
`positiveDecimal(e)` / `nonNegativeDecimal(e)`	`String?`	the same, with `double.tryParse`
`boundedNonNegativeInteger(max, e)`	`String?`	anything but `0..max`, or the literal string `>max`
`notNull(e)`	`T?`	`null` — the required-dropdown validator
`mustBeTrue(e)`	`bool?`	`null` and `false` — the accept-the-terms validator
`notEmpty(e)`	`List<T>?`	`null` and the empty list

Two helpers wrap another validator:

conditionalValidator(validator, () => enabled) runs validator only while the getter returns true — a rule that applies when a switch is on.
dynamicValidator(() => buildValidator()) rebuilds the validator on every run, for parameters that change at runtime — a maximum that depends on the plan the user picked.

final vatId = AdvancedTextFieldController(
  validator: conditionalValidator(
    filled(MyError.required),
    () => needsInvoice.fieldValue,
  ),
);

notEmpty is for lists

There is no built-in Set validator, so a rule on a multi-select is a one-line closure: (chosen) => chosen.isEmpty ? MyError.pickOne : null.

Combining validators

& requires both sides to pass; | requires one. Both short-circuit left to right, and the left-most error wins:

final email = AdvancedTextFieldController(
  validator: filled(MyError.required) & atLeastLength(5, MyError.tooShort),
);

and([...]) and or([...]) take a list, plus an optional shared error to return instead of the first failing validator's own:

validator: and(
  [filled(MyError.required), atLeastLength(8, MyError.tooShort)],
  MyError.invalidPassword, // one code for the whole chain
),

Both sides must have the same T

The built-in string validators are typed Validator<String?, E> even though AdvancedTextFieldController holds a non-nullable String. Assigning one alone is fine — a function of String? accepts a String. Combining is where it bites: an unannotated closure (value) => … infers String and will not combine with filled. Write custom string rules over String?, like the built-ins — see Custom validators.

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Built-in validators

Combining validators

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