Handle API States

State.kt

sealed class State<T> {
    class Loading<T> : State<T>()

    data class Success<T>(val data: T) : State<T>()

    data class Error<T>(val message: String) : State<T>()

    companion object {

        /**
         * Returns [State.Loading] instance.
         */
        fun <T> loading() = Loading<T>()

        /**
         * Returns [State.Success] instance.
         * @param data Data to emit with status.
         */
        fun <T> success(data: T) = Success(data)

        /**
         * Returns [State.Error] instance.
         * @param message Description of failure.
         */
        fun <T> error(message: String) = Error<T>(message)
    }

}

inline fun <reified T> State<T>.doIfSuccess(callback: (value: T) -> Unit) {
    if (this is State.Success) {
        callback(data)
    }
}

inline fun <reified T> State<T>.doIfFailure(callback: (error: String?, throwable: Throwable?) -> Unit) {
    if (this is State.Error) {
        callback(message, throwable)
    }
}

// Mapping Result
inline fun <reified T, reified R> State<T>.map(transform: (T) -> R) : State<R> {
    return when(this) {
        is State.Success -> State.Success(transform(data))
        is State.Error -> this
}