如何在kotlin中使密封类成为泛型？

Question

是否可以使用下面的AsyncResult类来防止在UserDataAppResult和CreateUserResult中重新定义InFlight、Error和InFlight？

//TODO: use this to make the below classes generic?
sealed class AsyncResult{
  object InFlight : AsyncResult()
  data class Error(val errorMessage: String) : AsyncResult()
  data class Loaded<out T>(val users: T) : AsyncResult()
}

sealed class UserDataAppResult : AppResult() {
  object InFlight : UserDataAppResult()
  data class Error(val errorMessage: String) : UserDataAppResult()
  data class Loaded(val users: List<User>) : UserDataAppResult()
}

sealed class CreateUserResult : AppResult() {
  object InFlight : CreateUserResult()
  data class Error(val errorMessage: String) : CreateUserResult()
  data class Loaded(val users: User) : CreateUserResult()
}

上面的代码可能是这样的吗？

sealed class AsyncResult{
  class InFlight : AsyncResult()
  data class Error(val errorMessage: String) : AsyncResult()
  data class Loaded<out T>(val users: T) : AsyncResult()
}

sealed class UserDataAppResult : AsyncResult()
sealed class CreateUserResult : AppResult()

val activeUsers: Flowable<UserDataAppResult> = appDatabase.userDao().getActiveUsers(appSettings.currentLanguage.ordinal)
    .map<UserDataAppResult> { UserDataAppResult.Loaded(it) }
    .onErrorReturn { UserDataAppResult.Error(it.localizedMessage) }
    .startWith(UserDataAppResult.InFlight)
    .observeOn(AndroidSchedulers.mainThread())
    .share()

fun createUser(): Flowable<CreateUserResult> {

  val userId = UUID.randomUUID().toString()
  val user = User()
  user.id = userId
  return appDatabase.userDao().insertAll(user)
      .map <CreateUserResult> { CreateUserResult.Loaded(user) }
      .onErrorReturn { CreateUserResult.Error(it.localizedMessage) }
      .startWith(CreateUserResult.InFlight)
}

目前没有找到UserDataAppResult.Error，这是有道理的。但是是否可以重用AppResult密封的类层次结构并引入新类型。

Answer 1

这在Kotlin是不可能的。您使用的每个类型都必须在某处有一个显式声明的类。即使在超类中声明了嵌套类，编译器也不会隐式创建类。

对于您的问题，我建议您将代码从组合两个基于继承的层次结构重写为两个组合继承和组合的层次结构之一，或者只是以某种方式重构层次结构，例如(我认为结果的确切实例在不是Loaded的情况下与您无关)：

sealed class AsyncResult {
    object InFlight : AsyncResult()
    data class Error(val errorMessage: String) : AsyncResult()
    sealed class Loaded<out T>(val result: T) : AsyncResult() {
        sealed class UserDataAppResult(users: List<User>) : Loaded<List<User>>(users)
        sealed class CreateUserResult(user: User) : Loaded<User>(user)
    }
}

Answer 2

您的Object在Kotlin中不能有泛型类型，但只需遵循以下示例即可解决此问题：

sealed class ResponseState<out T> {
    object Loading : ResponseState<Nothing>()
    data class Error(val throwable: Throwable) : ResponseState<Nothing>()
    data class Success<T>(val item: T) : ResponseState<T>()
}

写作：

val state = MutableLiveData<ResponseState<MessageModle>>() 


_state.postValue(ResponseState.Loading)

myNetworkCall { response, e
  if (e != null) _state.postValue(ResponseState.Error(e))
  else _state.postValue(ResponseState.Success(response))
}

阅读：

state.observe(..., {state -> 
  when(state) {
    Loading -> showLoading()
    is Error -> showError(state.throwable)
    is Success -> onSuccess(state.item)
  }
}

Answer 3

通过谷歌指南：https://developer.android.com/jetpack/docs/guide

sealed class Resource<T>(
        val data: T? = null,
        val message: String? = null
) {
    class Success<T>(data: T) : Resource<T>(data)
    class Loading<T>(data: T? = null, var refreshing: Boolean = false) : Resource<T>(data)
    class Error<T>(data: T? = null, message: String) : Resource<T>(data, message)
}