在android官方指导的相关应用框架中，用到一个Resource类来表示网络请求的状态与结果
// A generic class that contains data and status about loading this data.
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) : Resource<T>(
