StatefulComputedValue

Computed value for values of type T where the form manager maintains a computed value state of type TState for each managed value.

Stateful computed values are useful when computed values require an expensive computation over data and if it is possible to save the result of such expensive computation and tweak it as data changes instead of running the expensive computation all over again.

As an example, imagine that we have a list of people and that we want to compute the average age of all people. Instead of iterating over the whole list every time a person is added or removed, we can use a stateful computed value to save, as state, the sum of all ages, and simply tweak this sum as people are added or removed. Having access to the sum of all ages as state allows us to implement the computed value function with a complexity of O(1) as opposed to O(N).

The following snippet implements the example above:

object AvgAgeNotOverMax : StatefulComputedValue<Int, Int>() {
    private val ComputedValueContext.people: List<Person> by dependency("../people")

    override suspend fun ComputedValueContext.initState(): Int =
        people.fold(0) { sum, person -> sum + person.age }

    private val ageObserver by observe<Int>("../people/∗/age") { agesSum, event ->
        when (event) {
            is ValueEvent.Init<Int> -> agesSum + event.newValue
            is ValueEvent.Change<Int> -> agesSum + event.newValue - event.oldValue
            is ValueEvent.Destroy<Int> -> agesSum - event.oldValue
            else -> agesSum
        }
    }

    override suspend fun ComputedValueContext.computeFromState(state: Int) = state / people.size
}