seal class

在kotlin1.1前，sealclass只能在类内部定义，在kotlin1.1后，可以定义在同一个文件中

sealed class PlayerCmd {
    class Play(val url: String, val position: Long = 0): PlayerCmd()
    class Seek(val position: Long): PlayerCmd()
    object Pause: PlayerCmd()
    object Resume: PlayerCmd()
    object Stop: PlayerCmd()
}
enum class PlayerState{
    IDLE, PAUSE, PLAYING
}