New Validation approach

[jamowei]

First, you need to specify a Validation for your data-model. Therefore, you can use one of the two new global convenience functions:

// creates a Validation for data-model D with metadata T and validation-messages of M
fun <D, T, M> validation(validate: MutableList<M>.(Inspector<D>, T?) -> Unit): Validation<D, T, M>

// creates a Validation for data-model D and validation-messages of M
fun <D, M> validation(validate: MutableList<M>.(Inspector<D>) -> Unit): Validation<D, Unit, M>

These functions are available in the commonMain source set, so you can create your Validation object right next to your data classes to keep them together. Example:

@Lenses
data class Person(
    val name: String = "",
    val height: Double = 0.0,
) {
    companion object {
        val validation = validation<Person, String> { inspector ->
            if(inspector.data.name.isBlank()) add("Please give the person a name.")
            if(inspector.data.height < 1) add("Please give the person a correct height.")
        }
    }
}

Then you can call your Validation everywhere (e.g. JVM- or JS-site) to get a list of messages which shows if your model is valid or not. We recommend extending your validation messages from the ValidationMessage interface. Then your validation message type must implement the path which is important for matching your message to the corresponding attribute of your data-model and the isError value which is needed to know when your model is valid or not:

data class MyMessage(override val path: String, val text: String) : ValidationMessage {
    override val isError: Boolean = text.startsWith("Error")
}

// change your Validation to use your own validation message
val validation = validation<Person, MyMessage> { inspector ->
    val name = inspector.sub(Person.name())
    if (name.data.isBlank())
        add(MyMessage(name.path, "Error: Please give the person a name."))

    val height = inspector.sub(Person.height())
    if (height.data < 1)
        add(MyMessage(height.path, "Error: Please give the person a correct height."))
}

// then you can use the valid attribute to check if your validation result is valid or not
val messages: List<MyMessage> = Person.validation(Person())
messages.valid  // == false

New ValidatingStore

We introduce a new type of Store which we call ValidatingStore. This Store has the same properties as a RootStore and additionally a Validation which it uses to validate the stored model. With these additional information you get next to your known data flow also a messages flow which you can use to render out your list of validation messages. Furthermore, you can decide on store creation if you want to automatically validate your model after an update to your store takes place. Then you have to set the validateAfterUpdate flag and the flow of messages gets accordingly updated. Otherwise, you can call the validate(data: D, metadata: T? = null) function inside your own handlers to update the list of messages by your own. For cases, you want to reset your validation state to an empty list or to a specific list of messages, you can use the resetMessages(messages: List<M> = emptyList()) function. Example:

// create your ValidatingStore
val myValidationStore = object : ValidatingStore<Person, Unit, MyMessage>(
    Person(), Person.validation, validateAfterUpdate = true, id = "myPerson"
) {
    // check if your model is valid and then save
    val save = handle { person ->
        if(validate(person).valid) {
            localStorage.setItem(person.name, person.height.toString())
            Person()
        } else person
    }
    //...
}

render {
    //...
    input {
        val name = myValidationStore.sub(Person.name())
        value(name.data)
        changes.values() handledBy name.update
    }
    //...
    button {
        +"Save"
        clicks handledBy myValidationStore.save
    }

    // render out the messages
    myValidationStore.messages.renderEach(MyMessage::path) {
        p {
            +it.text
            if(it.isError) inlineStyle("color: red")
        }
    }
}

[jamowei]

Opened up a ticket at Jetbrains Youtrack: https://youtrack.jetbrains.com/issue/KT-50897

[ghost]

Some proposals for the API docs.

Besides this looks really good! I hardly can wait to use this :-)

I have some proposals for the release notes too. We should talk about those afterwards.