// Packet is the unit of data.
type Packet interface {
	fmt.Stringer
}

// Protocol use to pack/unpack Packet.
type Protocol interface {
	// return the size need for pack the Packet.
	PackSize(p Packet) int
	// PackTo pack the Packet to w.
	// The return value n is the number of bytes written;
	// Any error encountered during the write is also returned.
	PackTo(p Packet, w io.Writer) (int, error)
	// Pack pack the Packet to new created buf.
	Pack(p Packet) ([]byte, error)
	// try to unpack the buf to Packet. If return len > 0, then buf[:len] will be discard.
	// The following return conditions must be implement:
	// (nil, 0, nil) : buf size not enough for unpack one Packet.
	// (nil, len, err) : buf size enough but error encountered.
	// (p, len, nil) : unpack succeed.
	Unpack(buf []byte) (Packet, int, error)
}

func SetLogger(l Logger)

const (
	// EventAccept mean server accept a new connect.
	EventAccept EventType = iota
	// EventConnected mean client connected to a server.
	EventConnected
	// EventRecv mean conn recv a packet.
	EventRecv
	// EventClosed mean conn is closed.
	EventClosed
)

// Handler is the event callback.
// p will be nil when event is EventAccept/EventConnected/EventClosed
type Handler interface {
	OnEvent(et EventType, c *Conn, p Packet)
}

// 1. create protocol and handler.
// ...

// 2. create opts.
opts := xtcp.NewOpts(handler, protocol)

// 3. create server.
server := xtcp.NewServer(opts)

// 4. start.
go server.ListenAndServe("addr")

// 1. create protocol and handler.
// ...

// 2. create opts.
opts := xtcp.NewOpts(handler, protocol)

// 3. create client.
client := NewConn(opts)

// 4. start
go client.DialAndServe("addr")

func (c *Conn) Send(buf []byte) error

func (h *myhandler) OnEvent(et EventType, c *Conn, p Packet) {
	switch et {
		case EventRecv:
			...
	}
}

// StopMode define the stop mode of server and conn.
type StopMode uint8

const (
	// StopImmediately mean stop directly, the cached data maybe will not send.
	StopImmediately StopMode = iota
	// StopGracefullyButNotWait stop and flush cached data.
	StopGracefullyButNotWait
	// StopGracefullyAndWait stop and block until cached data sended.
	StopGracefullyAndWait
)