Improve handling of nullable values in Stores

[jwstegemann]

Handling nullable values in Stores

If you have a Store with a nullable content, you can use orDefault to derive a non-nullable Store from it, that transparently translates a null-value from its parent Store to the given default-value and vice versa.

In the following case, when you enter some text in the input and remove it again, you will have a value of null in your nameStore:

val nameStore = storeOf<String?>(null)

render {
    input {
        nameStore.orDefault("").also { formStore ->
            value(formStore.data)
            changes.values() handledBy formStore.update
        }
    }
}

In real world, you will often come across nullable attributes of complex entities. Then you can often call orDefault directly on the SubStore you create to use with your form elements:

@Lenses
data class Person(val name: String?)

//...

val applicationStore = storeOf(Person(null))

//...

val nameStore = applicationStore.sub(Person.name()).orDefault("")

Calling sub on a Store with nullable content

To call sub on a nullable Store only makes sense, when you have checked, that its value is not null:

@Lenses
data class Person(val name: String)

//...

val applicationStore = storeOf<Person>(null)

//...

applicationStore.data.render { person ->
    if (person != null) { // if person is null you would get NullPointerExceptions reading or updating its SubStores
        val nameStore = customerStore.sub(Person.name())
        input {
            value(nameStore.data)
            changes.values() handledBy nameStore.update
        }
    }
    else {
        p { + "no customer selected" }
    }
}