/// <summary>
    /// Connection对象  这个类是TcpConnection和 UDPConnnection连接类的父类
    /// Connection由以下五个文件组成 大家注意到每个类前面都有个 partial关键字
    /// ConnectionCreate.cs <1>
    /// ConnectionDelegatesHandlers.cs <2>
    /// ConnectionIncomingData.cs <3>
    /// ConnectionSendClose.cs <4>
    /// ConnectionStatic.cs  <5>
    /// </summary>
    /// 这部分负责接收数据，并交个networkcomms的优先级线程池处理
    public abstract partial class Connection
    {
        /// <summary>
        /// The <see cref="PacketBuilder"/> for this connection
        /// </summary>
        protected PacketBuilder packetBuilder;

        /// <summary>
        /// The current incoming data buffer
        /// </summary>
        protected byte[] dataBuffer;

        /// <summary>
        /// The total bytes read so far within dataBuffer
        /// </summary>
        protected int totalBytesRead;

        /// <summary>
        /// The thread listening for incoming data should we be using synchronous methods.
        /// </summary>
        protected Thread incomingDataListenThread = null;

        /// <summary>
        /// A connection specific method which triggers any requisites for accepting incoming data
        /// </summary>
        protected abstract void StartIncomingDataListen();

        /// <summary>
        /// Attempts to use the data provided in packetBuilder to recreate something usefull. If we don‘t have enough data yet that value is set in packetBuilder.
        /// </summary>
        /// <param name="packetBuilder">The <see cref="PacketBuilder"/> containing incoming cached data</param>
        protected void IncomingPacketHandleHandOff(PacketBuilder packetBuilder)
        {
            try
            {
                if (NetworkComms.LoggingEnabled) NetworkComms.Logger.Trace(" ... checking for completed packet with " + packetBuilder.TotalBytesCached.ToString() + " bytes read.");

                if (packetBuilder.TotalPartialPacketCount == 0)
                    throw new Exception("Executing IncomingPacketHandleHandOff when no packets exist in packetbuilder.");

                //Loop until we are finished with this packetBuilder
                int loopCounter = 0;
                while (true)
                {
                    //If we have ended up with a null packet at the front, probably due to some form of concatentation we can pull it off here
                    //It is possible we have concatenation of several null packets along with real data so we loop until the firstByte is greater than 0
                    if (packetBuilder.FirstByte() == 0)
                    {
                        if (NetworkComms.LoggingEnabled) NetworkComms.Logger.Trace(" ... null packet removed in IncomingPacketHandleHandOff() from " + ConnectionInfo + ", loop index - " + loopCounter.ToString());

                        packetBuilder.ClearNTopBytes(1);

                        //Reset the expected bytes to 0 so that the next check starts from scratch
                        packetBuilder.TotalBytesExpected = 0;

                        //If we have run out of data completely then we can return immediately
                        if (packetBuilder.TotalBytesCached == 0) return;
                    }
                    else
                    {
                        //First determine the expected size of a header packet
                        int packetHeaderSize = packetBuilder.FirstByte() + 1;

                        //Do we have enough data to build a header?
                        if (packetBuilder.TotalBytesCached < packetHeaderSize)
                        {
                            if (NetworkComms.LoggingEnabled) NetworkComms.Logger.Trace(" ... ... more data required for complete packet header.");

                            //Set the expected number of bytes and then return
                            packetBuilder.TotalBytesExpected = packetHeaderSize;
                            return;
                        }

                        //We have enough for a header
                        PacketHeader topPacketHeader;
                        using(MemoryStream headerStream = packetBuilder.ReadDataSection(1, packetHeaderSize - 1))
                            topPacketHeader = new PacketHeader(headerStream, NetworkComms.InternalFixedSendReceiveOptions);

                        //Idiot test
                        if (topPacketHeader.PacketType == null)
                            throw new SerialisationException("packetType value in packetHeader should never be null");

                        //We can now use the header to establish if we have enough payload data
                        //First case is when we have not yet received enough data
                        if (packetBuilder.TotalBytesCached < packetHeaderSize + topPacketHeader.PayloadPacketSize)
                        {
                            if (NetworkComms.LoggingEnabled) NetworkComms.Logger.Trace(" ... ... more data required for complete packet payload. Expecting " + (packetHeaderSize + topPacketHeader.PayloadPacketSize).ToString() + " total packet bytes.");

                            //Set the expected number of bytes and then return
                            packetBuilder.TotalBytesExpected = packetHeaderSize + topPacketHeader.PayloadPacketSize;
                            return;
                        }
                        //Second case is we have enough data
                        else if (packetBuilder.TotalBytesCached >= packetHeaderSize + topPacketHeader.PayloadPacketSize)
                        {
                            //We can either have exactly the right amount or even more than we were expecting
                            //We may have too much data if we are sending high quantities and the packets have been concatenated
                            //no problem!!
                            SendReceiveOptions incomingPacketSendReceiveOptions = IncomingPacketSendReceiveOptions(topPacketHeader);
                            if (NetworkComms.LoggingEnabled) NetworkComms.Logger.Debug("Received packet of type ‘" + topPacketHeader.PacketType + "‘ from " + ConnectionInfo + ", containing " + packetHeaderSize.ToString() + " header bytes and " + topPacketHeader.PayloadPacketSize.ToString() + " payload bytes.");

                            //If this is a reserved packetType we call the method inline so that it gets dealt with immediately
                            bool isReservedType = false;
                            foreach (var tName in NetworkComms.reservedPacketTypeNames)
                            {
                                //isReservedType |= topPacketHeader.PacketType == tName;
                                if (topPacketHeader.PacketType == tName)
                                {
                                    isReservedType = true;
                                    break;
                                }
                            }

                            //Only reserved packet types get completed inline
                            if (isReservedType)
                            {
#if WINDOWS_PHONE
                                var priority = QueueItemPriority.Normal;
#else
                                var priority = (QueueItemPriority)Thread.CurrentThread.Priority;
#endif

                                PriorityQueueItem item = new PriorityQueueItem(priority, this, topPacketHeader, packetBuilder.ReadDataSection(packetHeaderSize, topPacketHeader.PayloadPacketSize), incomingPacketSendReceiveOptions);
                                if (NetworkComms.LoggingEnabled) NetworkComms.Logger.Trace(" ... handling packet type ‘" + topPacketHeader.PacketType + "‘ inline. Loop index - " + loopCounter.ToString());
                                NetworkComms.CompleteIncomingItemTask(item);
                            }
                            else
                            {
                                QueueItemPriority itemPriority = (incomingPacketSendReceiveOptions.Options.ContainsKey("ReceiveHandlePriority") ? (QueueItemPriority)Enum.Parse(typeof(QueueItemPriority), incomingPacketSendReceiveOptions.Options["ReceiveHandlePriority"]) : QueueItemPriority.Normal);
                                PriorityQueueItem item = new PriorityQueueItem(itemPriority, this, topPacketHeader, packetBuilder.ReadDataSection(packetHeaderSize, topPacketHeader.PayloadPacketSize), incomingPacketSendReceiveOptions);

                                //QueueItemPriority.Highest is the only priority that is executed inline
#if !WINDOWS_PHONE
                                if (itemPriority == QueueItemPriority.Highest)
                                {
                                    if (NetworkComms.LoggingEnabled) NetworkComms.Logger.Trace(" ... handling packet type ‘" + topPacketHeader.PacketType + "‘ with priority HIGHEST inline. Loop index - " + loopCounter.ToString());
                                    NetworkComms.CompleteIncomingItemTask(item);
                                }
                                else
                                {
                                    int threadId = NetworkComms.CommsThreadPool.EnqueueItem(item.Priority, NetworkComms.CompleteIncomingItemTask, item);
                                    if (NetworkComms.LoggingEnabled) NetworkComms.Logger.Trace(" ... added completed " + item.PacketHeader.PacketType + " packet to thread pool (Q:" + NetworkComms.CommsThreadPool.QueueCount.ToString() + ", T:" + NetworkComms.CommsThreadPool.CurrentNumTotalThreads.ToString() + ", I:" + NetworkComms.CommsThreadPool.CurrentNumIdleThreads.ToString() + ") with priority " + itemPriority.ToString() + (threadId > 0 ? ". Selected threadId=" + threadId.ToString() : "") + ". Loop index=" + loopCounter.ToString() + ".");
                                }
#else
                                int threadId = NetworkComms.CommsThreadPool.EnqueueItem(item.Priority, NetworkComms.CompleteIncomingItemTask, item);
                                if (NetworkComms.LoggingEnabled) NetworkComms.Logger.Trace(" ... added completed " + item.PacketHeader.PacketType + " packet to thread pool (Q:" + NetworkComms.CommsThreadPool.QueueCount.ToString() + ", T:" + NetworkComms.CommsThreadPool.CurrentNumTotalThreads.ToString() + ", I:" + NetworkComms.CommsThreadPool.CurrentNumIdleThreads.ToString() + ") with priority " + itemPriority.ToString() + (threadId > 0 ? ". Selected threadId=" + threadId.ToString() : "") + ". Loop index=" + loopCounter.ToString() + ".");
#endif
                            }

                            //We clear the bytes we have just handed off
                            if (NetworkComms.LoggingEnabled) NetworkComms.Logger.Trace("Removing " + (packetHeaderSize + topPacketHeader.PayloadPacketSize).ToString() + " bytes from incoming packet buffer from connection with " + ConnectionInfo +".");
                            packetBuilder.ClearNTopBytes(packetHeaderSize + topPacketHeader.PayloadPacketSize);

                            //Reset the expected bytes to 0 so that the next check starts from scratch
                            packetBuilder.TotalBytesExpected = 0;

                            //If we have run out of data completely then we can return immediately
                            if (packetBuilder.TotalBytesCached == 0) return;
                        }
                        else
                            throw new CommunicationException("This should be impossible!");
                    }

                    loopCounter++;
                }
            }
            catch (Exception ex)
            {
                //Any error, throw an exception.
                if (NetworkComms.LoggingEnabled) NetworkComms.Logger.Fatal("A fatal exception occured in IncomingPacketHandleHandOff(), connection with " + ConnectionInfo + " be closed. See log file for more information.");

                NetworkComms.LogError(ex, "CommsError");
                CloseConnection(true, 45);
            }
        }

        /// <summary>
        /// Handle an incoming CheckSumFailResend packet type
        /// </summary>
        /// <param name="packetDataSection"></param>
        internal void CheckSumFailResendHandler(MemoryStream packetDataSection)
        {
            //If we have been asked to resend a packet then we just go through the list and resend it.
            SentPacket packetToReSend;
            lock (sentPacketsLocker)
            {
                string checkSumRequested = NetworkComms.InternalFixedSendReceiveOptions.DataSerializer.DeserialiseDataObject<string>(packetDataSection, 
                    NetworkComms.InternalFixedSendReceiveOptions.DataProcessors, NetworkComms.InternalFixedSendReceiveOptions.Options);

                if (sentPackets.ContainsKey(checkSumRequested))
                    packetToReSend = sentPackets[checkSumRequested];
                else
                    throw new CheckSumException("There was no packet sent with a matching check sum");
            }

            //If we have already tried resending the packet 10 times something has gone horribly wrong
            if (packetToReSend.SendCount > 10) throw new CheckSumException("Packet sent resulted in a catastropic checksum check exception.");

            if (NetworkComms.LoggingEnabled) NetworkComms.Logger.Warn(" ... resending packet due to MD5 mismatch.");

            //Increment send count and then resend
            packetToReSend.IncrementSendCount();
            SendPacket(packetToReSend.Packet);
        }
    }