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()
	
}

----------------------------------------

sealed class 与枚举的区别:

	枚举的类型只能有单个实例

	sealed class
		各类型可以单个可以多个,单个用object,多个用class,类型有参数则可以有多种状态所以用class
		是枚举的一种扩展