private void HandleConnection(IAsyncResult result)
{
AcceptConnection();
TcpClient client = server.EndAcceptTcpClient(result);
try
{
NetworkStream ns = client.GetStream();
while (true)
{
while (!ns.DataAvailable)
{
;
}
while (client.Available < 3)
{
;
}
byte[] incomingData = new byte[client.Available];
int bytesCount = ns.Read(incomingData, 0, client.Available);
string decodedData = Encoding.UTF8.GetString(incomingData);
if (Regex.IsMatch(decodedData, "^GET", RegexOptions.IgnoreCase))
{
byte[] response = MakeHandshake(decodedData);
ns.Write(response, 0, response.Length);
}
else
{
bool fin = (incomingData[0] & 0b10000000) != 0,
mask = (incomingData[1] & 0b10000000) != 0;
int opcode = incomingData[0] & 0b00001111,
msglen = incomingData[1] - 128,
offset = 2;
if (msglen == 126)
{
msglen = BitConverter.ToUInt16(new byte[] { incomingData[3], incomingData[2] }, 0);
offset = 4;
}
else if (mask)
{
byte[] decoded = new byte[msglen];
byte[] masks = new byte[4] {
incomingData[offset], incomingData[offset + 1], incomingData[offset + 2], incomingData[offset + 3]
};
offset += 4;
for (int i = 0; i < msglen; ++i)
{
decoded[i] = (byte)(incomingData[offset + i] ^ masks[i % 4]);
}
string text = Encoding.UTF8.GetString(decoded);
string writeJson;
ClientInput input = JsonConvert.DeserializeObject <ClientInput>(text);
if (!input.Authenticate())
{
writeJson = Status.Create(403, "Forbidden: Authentication failed.");
ns.Write(Encoding.Default.GetBytes(writeJson), 0, Encoding.Default.GetBytes(writeJson).Length);
client.Close();
break;
}
if (input.command != "print")
{
writeJson = Status.Create(400, "Bad Request: Incorrect command. Please try again.");
ns.Write(Encoding.Default.GetBytes(writeJson), 0, Encoding.Default.GetBytes(writeJson).Length);
client.Close();
break;
}
ZplString zpl = new ZplString(input.zpl);
if (!zpl.Validate())
{
writeJson = Status.Create(400, "Bad Request: The ZPL string is formatted incorrectly. Please send a properly formatted ZPL string.");
ns.Write(Encoding.Default.GetBytes(writeJson), 0, Encoding.Default.GetBytes(writeJson).Length);
client.Close();
break;
}
Printer printer = new Printer();
if (printer.GetName() == "")
{
writeJson = Status.Create(500, "Internal Server Error: A valid printer could not be found.");
ns.Write(Encoding.Default.GetBytes(writeJson), 0, Encoding.Default.GetBytes(writeJson).Length);
client.Close();
break;
}
printer.Print(zpl.text);
Dictionary <String, String> logEntry = new Dictionary <String, String>
{
{ "IPAddress", SystemHelper.GetLocalIP() },
{ "ComputerName", SystemHelper.GetComputerName() },
{ "ClientUsername", "username" },
{ "PrintContent", zpl.text }
};
PrintLogRepository.Log(logEntry);
writeJson = Status.Create(200, "OK: Action completed");
ns.Write(Encoding.Default.GetBytes(writeJson), 0, Encoding.Default.GetBytes(writeJson).Length);
client.Close();
}
else
{
Console.WriteLine("mask bit not set");
}
Console.WriteLine();
}
}
}
catch (Exception ex)
{
ExceptionLogRepository.Log(ex.Message);
}
}
// The delegate that processes incoming print requests and, if possible, prints them.
private void HandleConnection(IAsyncResult result)
{
// This call doesn't make sense. Isn't this the method we called to pass this method as a delegate?
// So if we're in this method, we know we had to have successfully called AcceptConnection() already,
// and that method doesn't do anything besides specify this method. Why are we calling this?
AcceptConnection();
TcpClient client = server.EndAcceptTcpClient(result);
try
{
NetworkStream ns = client.GetStream();
// QUESTION/COMMENT- I'm not sure why we have this while loop. It would make sense if we were reading incoming data in a synchronous way, but
// the rest of the code acts like result has all the data we're ever going to get.
while (true)
{
// QUESTION/COMMENT- Silly questions, but ones any coder will ask - if we never start getting data
// in stream for some reason, or the client only sends two bytes of data for some weird
// reason, these loops are infinite. Is that a problem?
// The space between the parentheses and the semicolon looks deliberate, given the rest of
// the formatting, so I'm taking that for a convention to indicate an empty while loop.
while (!ns.DataAvailable)
{
;
}
// QUESTION/COMMENT- Below, is 3 a magic value for us in some way? If so, I'd like to replace with a named const.
// From reading the next lines of code, it seems like once we get the data, we get it all - when we
// assign bytesCount, we never check again to see if it has changed - so what's up for us that we're not
// saying "while (client.Available == 0)"?
// COMMENT - Available is the number of bytes of data received from the network and available to be read.
// Wait until we have some until starting work.
while (client.Available < 3)
{
;
}
// incomingData holds all the data we got in the print request, which includes metadata as well as the
// data that we want to print.
byte[] incomingData = new byte[client.Available];
int bytesCount = ns.Read(incomingData, 0, client.Available);
// We only use decodedData to check for a handshake request from the client. For an actual print request,
// we just use the incomingData byte array.
string decodedData = Encoding.UTF8.GetString(incomingData);
// If the data we received starts with GET (case-insensitive), then just
// send a handshake response to the client. Don't do any other processing.
if (Regex.IsMatch(decodedData, "^GET", RegexOptions.IgnoreCase))
{
byte[] response = MakeHandshake(decodedData);
ns.Write(response, 0, response.Length);
}
// It wasn't a handshake request - it's a real print request.
else
{
// We do a lot of low-level bit operations in this block to get and interpret the control bytes
// from the client request.
// First, we do a bitwise comparison on the first two bytes of data to figure out if the fin and mask flags
// have been set.
// 0b at the beginning of a number tells C# that the rest of the number are literal bits.
// fin is true if the first bit of the first incoming byte is 1,
// mask is true if the first bit of the second incoming byte is 1.
// If the mask bit isn't set, we're not going to print anything.
// QUESTION/COMMENT- We never use fin. Why do we bother getting it?
bool fin = (incomingData[0] & 0b10000000) != 0,
mask = (incomingData[1] & 0b10000000) != 0;
// The last 4 bits of the first incoming byte give us the numeric value for opcode.
// We strip the first 4 bits by AND-ing them with 0s.
int opcode = incomingData[0] & 0b00001111,
// The last seven bits of the second incoming byte give us our message length.
// We can find out what the last seven bits are by subtracting the 8th bit, which is worth 128
// if it's 1. That leaves us with a message length of 0-127 bytes.
msglen = incomingData[1] - 128,
// by default we assume that the first two bytes are metadata and the data starts at byte 3
// (0-indexed)
offset = 2;
// If the rightmost bit of the message length byte is 0 and the six bits to the right of that are 1's,
// (126), that's a flag that the msglen is actually
// a number that is two bytes long, that it is stored in bytes 3 and 4, and that our print data starts at byte, not byte 3.
// QUESTION/COMMENT- 126 might make a good named constant.
if (msglen == 126)
{
// The 0 argument just means start reading at the first (0-index) byte of the byte array we pass in the first argument.
msglen = BitConverter.ToUInt16(new byte[] { incomingData[3], incomingData[2] }, 0);
offset = 4;
}
// QUESTION/COMMENT- Is this an error? We went to some trouble in the lines above to get a two-byte message length,
// and an offset value that works with it, but with the "else" below, we're never going to use it.
else if (mask)
{
// Get an array of msglen bytes to hold our print data.
byte[] decoded = new byte[msglen];
// The four bytes after the bytes with the opcode, msglen, fin and mask values are data masks that we'll use to
// transform the data we read. The print data actually starts after the mask bytes.
byte[] masks = new byte[4] {
incomingData[offset], incomingData[offset + 1], incomingData[offset + 2], incomingData[offset + 3]
};
offset += 4;
// QUESTION/COMMENT- is there a reason we use ++i in the for loop below? As far as I can tell it has the same effect as i++, but people are
// really used to seeing i++ and when they see ++i, it makes them stop and wonder why and look for something they're missing.
// If we just like using ++i, that's fine, I'm just offering my two cents from a training and onboarding perspectie.
// COMMENT - read all of the actual incoming data, starting at the offset.
// We use the mask bytes to transform the byte data we received and which we will ultimately deserialize below into
// an input object.
// To transform the data, as we read each data byte, we apply a bitwise exclusive OR operation (true if the bits compared are different,
// false otherwise) to the data byte and one of the mask bytes. We apply the mask bytes to the data bytes in groups of four.
// The first data byte we read gets the first mask byte, the second data byte the second mask byte, etc., then we start again
// with the fifth data byte getting the first mask byte.
for (int i = 0; i < msglen; ++i)
{
decoded[i] = (byte)(incomingData[offset + i] ^ masks[i % 4]);
}
// Turn our transformed data byte array into a JSON string.
string text = Encoding.UTF8.GetString(decoded);
// The output HTTP response we'll send back to the client.
string writeJson;
// Finally, we have can turn our input JSON string into an input object.
ClientInput input = JsonConvert.DeserializeObject <ClientInput>(text);
if (!input.Authenticate())
{
writeJson = Status.Create(403, "Forbidden: Authentication failed.");
ns.Write(Encoding.Default.GetBytes(writeJson), 0, Encoding.Default.GetBytes(writeJson).Length);
client.Close();
break;
}
// QUESTION/COMMENT- "print" might be a good named constant.
if (input.command != "print")
{
writeJson = Status.Create(400, "Bad Request: Incorrect command. Please try again.");
ns.Write(Encoding.Default.GetBytes(writeJson), 0, Encoding.Default.GetBytes(writeJson).Length);
client.Close();
break;
}
// Get a Zebra Print Language (ZPL) string from the input and validate it.
ZplString zpl = new ZplString(input.zpl);
if (!zpl.Validate())
{
writeJson = Status.Create(400, "Bad Request: The ZPL string is formatted incorrectly. Please send a properly formatted ZPL string.");
ns.Write(Encoding.Default.GetBytes(writeJson), 0, Encoding.Default.GetBytes(writeJson).Length);
client.Close();
break;
}
Printer printer = new Printer();
if (printer.GetName() == "")
{
writeJson = Status.Create(500, "Internal Server Error: A valid printer could not be found.");
ns.Write(Encoding.Default.GetBytes(writeJson), 0, Encoding.Default.GetBytes(writeJson).Length);
client.Close();
break;
}
// Finally, we print to the printer we found in the new Printer() call above and log the whole interaction.
printer.Print(zpl.text);
// QUESTION/COMMENT- do we want to log errors? The way the code is written, we break on any error that we know how to
// handle, like invalid ZPL data or printer not found, so we'll never see errors in our log. Exceptions go to the
// ExceptionLogRepository, but errors we know how to handle are just lost.
Dictionary <String, String> logEntry = new Dictionary <String, String>
{
{ "IPAddress", SystemHelper.GetLocalIP() },
{ "ComputerName", SystemHelper.GetComputerName() },
{ "ClientUsername", "username" },
{ "PrintContent", zpl.text }
};
PrintLogRepository.Log(logEntry);
writeJson = Status.Create(200, "OK: Action completed");
ns.Write(Encoding.Default.GetBytes(writeJson), 0, Encoding.Default.GetBytes(writeJson).Length);
client.Close();
// QUESTION/COMMENT- We don't have a break here, so we stay in while(true). Is that what we want to do?
// We just closed the client, which we instantiated just to handle the result passed to this method,
// so it doesn't seem like there's anything more for the while loop to do.
}
else
{
// QUESTION/COMMENT- the code reads a little easier if you put a quick exit condition like this before all
// the lengthy data processing logic.
Console.WriteLine("mask bit not set");
}
Console.WriteLine();
}
}
}
catch (Exception ex)
{
ExceptionLogRepository.Log(ex.Message);
}
}
