ViewModel, LiveData and Transformations.map

Question

New to Kotlin and Android development and related and I’m not certain what to do to apply some business logic and convert a value. I have a List of a class and I’d like to modify one of the values in the class while leaving everything else in the class unharmed. Once I get to the view model, I’m not certain of how to access the time value in my class to modify it. I’d appreciate if someone would point me in the right direction.

Entity and Dao

import org.threeten.bp.Instant

data class ActionDetails(val time: Instant,
                     val firstName: String,
                     ... )

@Query("SELECT time, first_name as firstName...")
fun liveStatus(): LiveData<List<ActionDetails>>

ViewModel

class MainViewModel(private val repository: DataRepository) : ViewModel() {

    private val _actions: LiveData<List<ActionDetails>>
    val actions: LiveData<List<ActionDetails>>
        get() = _actions

    init {
        _actions = Transformations.map(repository.liveStatus()) {
            //Convert Instant value per business rules and convert to formatted string
            time -> ...

        }
    }
}

Answer 1

You can't change just 1 value. Create a new data class that represents the desired full object after the business logic transformation

e.g. from

data class ActionDetails(val time: Date, val firstName: String, val lastName: String)

to

data class DisplayItem(val time: String, val firstName: String, val lastName: String, val fullName: String)

Then transform your livedata from List<ActionDetails> to List<DisplayItem>. To do that you can use the Iterable#map function that applies a transformation to each element of the list and returns the resulting list.

Transformations.map(repository.liveStatus()) { list ->
    list.map { item ->
        val formattedTime = item.time.toString() // whatever you need
        val fullName = "${item.firstName} ${item.lastName}"
        DisplayItem(formattedTime, item.firstName, item.lastName, fullName)
    }
}

Sidenote: if it's just 1 small thing that you want to change, maybe don't use Transformations.map but simply format the string at the place where it's displayed, e.g. in a adapter view holder