Angel3 Request Validator

Pub Version (including pre-releases) Null Safety Gitter License

Validation library based on the matcher library, with Angel3 support. Why re-invent the wheel, when you can use the same validators you already use for tests?

This library runs both on the server, and on the client. Thus, you can use the same validation rules for forms on the server, and on the frontend.

For convenience's sake, this library also exports matcher.


Creating a Validator

import 'package:angel3_validate/angel3_validate.dart';

main() {
    var validator = Validator({
        'username': isAlphaNum,
        'multiple,keys,with,same,rules': [isString, isNotEmpty],
        'balance': [
        'nested': [
            [bar, baz]

Validating data

The Validator will filter out fields that have no validation rules. You can rest easy knowing that attackers cannot slip extra data into your applications.

main() {
    var result = validator.check(formData);

    if (!result.errors.isNotEmpty) {
        // Invalid data
    } else {
        // Safely handle filtered data
        return someSecureOperation(;

You can enforce validation rules, and throw an error if validation fails.

main() {
    try {
        // `enforce` will return the filtered data.
        var safeData = validator.enforce(formData);
    } on ValidationException catch(e) {

Required Fields

Fields are optional by default.

Suffix a field name with a '*' to mark it as required, and to throw an error if it is not present.

main() {
    var validator = Validator({
        'googleId*': isString,
        // You can also use `requireField`
        requireField('googleId'): isString,

Forbidden Fields

To prevent a field from showing up in valid data, suffix it with a '!'.

Default values

If not present, default values will be filled in before validation. This means that they can still be used with required fields.

final Validator todo = Validator({
    'text*': isString,
    'completed*': isBool
}, defaultValues: {
    'completed': false

Default values can also be parameterless, synchronous functions that return a single value.

Custom Validator Functions

Creating a whole Matcher class is sometimes cumbersome, but if you pass a function to the constructor, it will be wrapped in a Matcher instance.

(It simply returns the value of calling predicate.)

The function must synchronously return a bool.

main() {
    var validator = Validator({
        'key*': (key) {
            var file = File('whitelist.txt');
            return file.readFileSync().contains(key);

Custom Error Messages

If these are not present, angel3_validate will attempt to generate a coherent error message on its own.

    'age': [greaterThanOrEqualTo(18)]
}, customErrorMessages: {
    'age': 'You must be an adult to see this page.'

The string {{value}} will be replaced inside your error message automatically.


Oftentimes, fields that we want to validate as numbers are passed as strings. Calling autoParse will correct this before validation.

main() {
    var parsed = autoParse({
        'age': '34',
        'weight': '135.6'
    }, ['age', 'weight']);


You can also call checkParsed or enforceParsed as a shorthand.


This is a helper function to extract only the desired keys from a Map.

var inputData = {'foo': 'bar', 'a': 'b', '1': 2};
var only = filter(inputData, ['foo']);

print(only); // { foo: bar }

Extending Validators

You can add situation-specific rules within a child validator. You can also use extend to mark fields as required or forbidden that originally were not. Default value and custom error message extension is also supported.

final Validator userValidator = Validator({
    'username': isString,
    'age': [

To mark a field as now optional, and no longer required, suffix its name with a '?'.

var ageIsOptional = userValidator.extend({
    'age?': [

Note that by default, validation rules are simply appended to the existing list. To completely overwrite existing rules, set the overwrite flag to true.

register(Map userData) {
    var teenUser = userValidator.extend({
        'age': lessThan(18)
    }, overwrite: true);    

Bundled Matchers

This library includes some Matchers for common validations, including:

  • isAlphaDash: Asserts that a String is alphanumeric, but also lets it contain dashes or underscores.
  • isAlphaNum: Asserts that a String is alphanumeric.
  • isBool: Asserts that a value either equals true or false.
  • isEmail: Asserts that a String complies to the RFC 5322 e-mail standard.
  • isInt: Asserts that a value is an int.
  • isNum: Asserts that a value is a num.
  • isString: Asserts that a value is a String.
  • isNonEmptyString: Asserts that a value is a non-empty String.
  • isUrl: Asserts that a String is an HTTPS or HTTP URL.

The remaining functionality is effectively implemented by the matcher package.

Nested Validators

Very often, the data we validate contains other data within. You can pass a Validator instance to the constructor, because it extends the Matcher class.

main() {
    var bio = Validator({
        'age*': [isInt, greaterThanOrEqualTo(0)],
        'birthYear*': isInt,
        'countryOfOrigin': isString

    var book = Validator({
        'title*': isString,
        'year*': [
            (year) {
                return year <=;

    var author = Validator({
        'bio*': bio,
        'books*': [
    }, defaultValues: {
        'books': []

Use with Angel

server.dart exposes seven helper middleware:

  • validate(validator): Validates and filters req.bodyAsMap, and throws an AngelHttpException.BadRequest if data is invalid.
  • validateEvent(validator): Sets to the result of validation on a service event.
  • validateQuery(validator): Same as validate, but operates on req.query.
  • autoParseBody(fields): Auto-parses numbers in req.bodyAsMap.
  • autoParseQuery(fields): Same as autoParseBody, but operates on req.query.
  • filterBody(only): Filters unwanted data out of req.bodyAsMap.
  • filterQuery(only): Same as filterBody, but operates on req.query.
import 'package:angel3_framework/angel3_framework.dart';
import 'package:angel3_validate/server.dart';

final Validator echo = Validator({
    'message*': (String message) => message.length >= 5

final Validator todo = Validator({
    'text*': isString,
    'completed*': isBool
}, defaultValues: {
    'completed': false

void main() async {
    var app = Angel();

    app.chain([validate(echo)]).post('/echo', (req, res) async {
        res.write('You said: "${req.bodyAsMap["message"]}"');


    await app.startServer();


Cross-platform validation library based on matcher.
Support for using angel_validate with the Angel Framework.