Incoming

sealed class Incoming : OnlineMessageSource

接收到的一条消息的 MessageSource

FromFriend	`abstract class FromFriend : Incoming`
FromGroup	`abstract class FromGroup : Incoming`
FromTemp	`abstract class FromTemp : Incoming`
Key	`companion object Key : Key<Incoming>`

fromId	发送人. `val fromId: Long`
sender	消息发送人. 可能为机器人或好友或群员. 即类型必定为 Bot, QQ 或 Member `abstract val sender: User`
target2	`val target2: Any`
targetId	消息发送目标. `val targetId: Long`

此 Key 指代的 Message 类型名. 一般为 class.simpleName, 如 "QuoteReply", "PlainText"

val typeName: String