第 20 课：组合状态文案与重试提示

sealed interface State {
    data object Loading : State
    data class Ready(val count: Int) : State
    data class Error(val message: String) : State
}
fun label(state: State) = when (state) {
    State.Loading -> "加载中"
    is State.Ready -> "共 ${state.count} 节"
    is State.Error -> state.message.ifBlank { "加载失败" }
}
fun canRetry(state: State) = state is State.Error && state.message.isNotBlank()
fun display(state: State): String =
    label(state) + if (canRetry(state)) " · 可重试" else ""
fun main() {
    println(display(State.Error("网络失败"))) // 网络失败 · 可重试
    println(display(State.Error("  "))) // 加载失败
}