15 Replies Latest reply on Oct 5, 2016 11:57 AM by Rodger Wilson

    Receipt printing connection close duration.

    Alexius Lim

      I'm currently working on an IOS app written in swift to print receipts using the Zebra ZQ520 printer with linkOS v1.4.948. Previously I was facing a problem that the printer sometimes does not print the receipt. The problem seems to be the connection is closed before all the data is sent. So what I did was setting a sleep duration before closing the connection. However this method does not seem to be reliable. Is there any way to check if all the data is sent to the printer before closing the connection? The following is my code:


      func printReceiptLabel(completionHandler: (Bool) -> Void) {

              dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0)) { () -> Void in

                  guard let conn = self.openConnection() else {

                      dispatch_async(dispatch_get_main_queue()) {

                          self.displayAlertMessage("No printers found")





                  let open = conn.open()

                  let receiptGenerator = ReceiptGenerator()


                  if let receiptInfo = self.printReceipt {

                      let receiptString = receiptGenerator.createReceiptString(receiptInfo)

                      let receiptData = receiptString.dataUsingEncoding(NSUTF8StringEncoding) as! NSMutableData


                      if open {

                          let blockSize = 1024

                          let totalSize = receiptData.length

                          var bytesRemaining = totalSize

                          var error: NSError?


                          while bytesRemaining > 0 {

                              print("Bytes sent")

                              let bytesToSend = min(blockSize, bytesRemaining)

                              let range = NSMakeRange(0, bytesToSend)



                              let partialLabel = receiptData.subdataWithRange(range)

                              conn.write(partialLabel, error: &error)


                              bytesRemaining -= bytesToSend


                              receiptData.replaceBytesInRange(range, withBytes: nil, length: 0)





                          dispatch_async(dispatch_get_main_queue(), { () -> Void in



                      } else {

                          dispatch_async(dispatch_get_main_queue()) {


                              self.displayAlertMessage("Error connecting to printer")