DataBindingObservablesExtensions.kt

package ru.campuz.base

import android.databinding.Observable
import android.databinding.ObservableField

fun <T, R> ObservableField<T>.map(mapper: (T?) -> R): ObservableField<R> {
  return MappedObservableField(this, mapper)
}

class MappedObservableField<T, R>(
    val observableField: ObservableField<T>,
    val mapper: (T?) -> R
) : ObservableField<R>() {
  init {
    observableField.get()?.let(mapper)
    observableField.addOnPropertyChangedCallback(object : Observable.OnPropertyChangedCallback() {
      override fun onPropertyChanged(sender: Observable?, propertyId: Int) {
        this@MappedObservableField.set(observableField.get().let(mapper))
      }
    })
  }
}

fun <T, V, R> combineLatest(o1: ObservableField<T>, o2: ObservableField<V>, mapper: (T?, V?) -> R): ObservableField<R> {
  return CombineLatestObservableField(o1, o2, mapper)
}

fun <T, V, R> ObservableField<T>.combineWith(o2: ObservableField<V>, mapper: (T?, V?) -> R): ObservableField<R> {
  return combineLatest(this, o2, mapper)
}

class CombineLatestObservableField<T, V, R>(
    val o1: ObservableField<T>,
    val o2: ObservableField<V>,
    val mapper: (T?, V?) -> R
) : ObservableField<R>() {

  var o1LastValue: T? = null
  var o2LastValue: V? = null

  init {
    update(o1.get(), o2.get())
    o1.addOnPropertyChangedCallback(object : Observable.OnPropertyChangedCallback() {
      override fun onPropertyChanged(sender: Observable?, propertyId: Int) {
        update(o1.get(), o2LastValue)
      }
    })
    o2.addOnPropertyChangedCallback(object : Observable.OnPropertyChangedCallback() {
      override fun onPropertyChanged(sender: Observable?, propertyId: Int) {
        update(o1LastValue, o2.get())
      }
    })
  }

  private fun update(v1: T?, v2: V?) {
    o1LastValue = v1
    o2LastValue = v2
    mapper(v1, v2)
  }
}

Example.kt

val oRegistrationStatus = ObservableFieldProperty(RegistrationStatus.AVAILABLE)

val oParticipationStatus = ObservableFieldProperty(ParticipationStatus.NOT_PARTICIPATING)
val oParticipateButtonVisible = oParticipationStatus.map { it == ParticipationStatus.NOT_PARTICIPATING }
val oParticipateButtonActive = oRegistrationStatus.map { it == RegistrationStatus.AVAILABLE }
val oCancelParticipationButtonVisible = oParticipationStatus.map { it == ParticipationStatus.PARTICIPATING }