Exposing Information: Observing Events vs State (in Rx)

ObservingState.kt

class UserRepository{
  // note: hot observables that do not replay missed events
  fun logins(): Observable<User> 
  fun logouts(): Observable<Unit>  
}

/**
 * An abstract user with two implementations: 
 * "LoggedIn" and "Logged out"
 */
sealed class User {
  // note: this represents a logged in user, not a log in event.
  data class LoggedIn(val name: String) : User()
  object LoggedOut : User()
}
class UserRepository {
  // emits the state of the user over time (as it changes)
  fun userStates() : Observable<User>
}

class MainActivity : Activity() {
  override fun onStart() {
      userRepository.userStates()
        .filter { user: User -> user is User.LoggedOut }
        .map { user: User -> user as User.LoggedOut }
        .subscribe { navigateToLoginScreen() }
  }
}

User.LoggedOut //current state is emitted upon subscription
User.LoggedIn("Zak")
User.LoggedIn("Zak Taccardi")
User.LoggedOut


val userStates : Observable<User> = userRepository.userStates()
userStates
  .distinctUntilChanged { 
      old: User, new: User -> old.javaClass == new.javaClass 
  }
  // distinctUntilChange() emits first value by default. 
  // Skip it, because we need at least 2 to do the comparison
  .skip(1)
  .map { user: User ->
    if (user is User.LoggedIn) {
        // a user just logged in!
        "${user.name} logged in!"
    } else {
        // a user just logged out!
        "Logged out!"
    }
  }
  .subscribe { log -> println(log) }
// output:
// Zak logged in!
// Logged out!