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")
                }
                return
            }
           
            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)
                    }
                   
                    NSThread.sleepForTimeInterval(NSTimeInterval(0.5))
                    conn.close()
                    dispatch_async(dispatch_get_main_queue(), { () -> Void in
                        completionHandler(true)
                    })
                } else {
                    dispatch_async(dispatch_get_main_queue()) {
                        completionHandler(false)
                        self.displayAlertMessage("Error connecting to printer")
                    }
                }
 
            }
        }
    }