When accepting user-input, it is a nice idea to validate the data before processing it any further.

To do validation in fritz2, you first have to create a Validation object. To do so you can use the global validation function which has the following type parameters:

  • the type of data to validate
  • a type for metadata you want to forward from your Handlers to your validation (or Unit by default if you do not need this)
  • a type describing the validation-results (like a message, etc.). Should implement the minimal ValidationMessage interface.

It is recommended to put your validation code inside the companion object of your data-class in the commonMain source-set of your multiplatform-project. Code in commonMain can be used in jsMain (frontend) and jvmMain (backend).

Inside the validation function you have the Inspector for your data model which gives you the right paths next to the data by using the same lens-functions as in your sub-stores by calling the sub() method. The resulting SubInspectors then have two attributes data and path.

To add a validation-message to the list of messages, just use the add function.

data class Message(override val path: String, val severity: Severity, val text: String): ValidationMessage {
override val isError: Boolean = severity > Severity.Warning

data class Person(
val name: String,
val age: Int
) {
companion object {
val validation: Validation<Person, Unit, Message> = validation<Person, Message> { inspector ->
val name = inspector.sub(
if( {
add(Message(name.path, Severity.Error, "Please provide a name"))

val age = inspector.sub(Person.age())
if( < 1) {
add(Message(age.path, Severity.Error, "Please correct the age"))
} else if( > 100) {
add(Message(age.path, Severity.Warning, "Is the person really older then 100 years‽"))

enum class Severity {

You can structure and implement your concrete validation-rules with everything Kotlin offers.

Now you can use the Validation object in your jsMain code:

val store: ValidatingStore<Person, Unit, Message> = storeOf(Person("Chris", 42), Person.validation)
val msgs: Flow<List<Message>> = store.messages

By default, a ValidatingStore automatically validates its data after it gets changed to update the list of messages. You can access this validation-messages with store.messages. This gives you a Flow<List<M>> where M is your ValidationMessage-type. You can handle the Flow of your messages like any other Flow of a List, for example by rendering your messages to html:

// create some messages
store.update(Person("", 101))

render {
ul {
store.messages.renderEach {
li(baseClass = {

If you want to start the validation process in a specific hander you can do so by implementing the ValidatingStore by yourself:

object PersonStore: ValidatingStore<Person, Unit, Message>(Person("", 0), Person.validation) {
val save = handle {
if(validate(it).valid) {
// send request to server...
Person("", 0)
} else it
val reset = handle {
resetMessages() // empties the list of messages
Person("", 0)

By calling the resetMessages() function you can manually reset the list of messages if needed.

Have a look at a more complete example here.