Swift 3：如何将 UTF8 数据流（每个字符 1、2、3 或 4 个字节）转换为字符串？

Question

在我的应用程序中，一个 tcp 客户端正在处理来自远程 tcp 服务器的数据流。当接收到的字符是 1 字节字符时，一切正常。当 tcp 服务器发送特殊字符如“ü”（十六进制“c3b5”-> 一个 2 字节字符）时，我开始遇到问题。

这是 Swift 3 代码行，只要接收到的数据包含一些超过 1 个字节的 UTF8 字符，就会得到一个 nil 字符串：

let convertedString = String(bytes: data, encoding: String.Encoding.utf8)

知道如何解决这个问题吗？基本上，传入的流可能包含编码为 UTF8 的 1 字节或 2 字节字符，我需要将数据流转换为字符串而不会出现问题。

这是我遇到问题的整个代码部分：

func startRead(for task: URLSessionStreamTask) {
    task.readData(ofMinLength: 1, maxLength: 65535, timeout: 300) { (data, eof, error) in
        if let data = data {
            NSLog("stream task read %@", data as NSData)

            let convertedString1 = String(data: data, encoding: String.Encoding(rawValue: String.Encoding.utf8.rawValue))

            if let convertedString = String(bytes: data, encoding: String.Encoding.utf8) {

                self.partialMessage = self.partialMessage + convertedString

                NSLog(convertedString)

                // Assign lengths (delimiter, MD5 digest, minimum expected length, message length)
                let delimiterLength = Constants.END_OF_MESSAGE_DELIMITER.lengthOfBytes(using: String.Encoding.utf8)
                let MD5Length = 32 // 32 characters -> hex representation of 16 bytes
                // 3 = CR+LF+1 char at least
                let minimumExpectedMessageLength = MD5Length + delimiterLength + 3
                let messageLength = self.partialMessage.lengthOfBytes(using: String.Encoding.utf8)

                // Check for delimiter and minimum expected message length (2 char msg + MD5 digest + delimiter)
                if (self.partialMessage.contains(Constants.END_OF_MESSAGE_DELIMITER)) &&
                    (messageLength >= minimumExpectedMessageLength) {

                    var message = self.partialMessage

                    // Get rid of optional CR+LF
                    var lowBound = message.index(message.endIndex, offsetBy: -1)
                    var hiBound = message.index(message.endIndex, offsetBy: 0)
                    var midRange = lowBound ..< hiBound

                    let optionalCRLF = message.substring(with: midRange)

                    if (optionalCRLF == "\r\n") || (optionalCRLF == "\0") {  // Remove CR+LF if present
                        lowBound = message.index(message.endIndex, offsetBy: -1)
                        hiBound = message.index(message.endIndex, offsetBy: 0)
                        midRange = lowBound ..< hiBound
                        message.removeSubrange(midRange)
                    }

                    // Check for delimiter proper position (has to be at the end)
                    lowBound = message.index(message.endIndex, offsetBy: -delimiterLength)
                    hiBound = message.index(message.endIndex, offsetBy: 0)
                    midRange = lowBound ..< hiBound

                    let delimiter = message.substring(with: midRange)

                    if (delimiter == Constants.END_OF_MESSAGE_DELIMITER)  // Delimiter in proper position?
                    {
                        // Acquire the MD digest
                        lowBound = message.index(message.endIndex, offsetBy: -(MD5Length+delimiterLength))
                        hiBound = message.index(message.endIndex, offsetBy: -(delimiterLength))
                        midRange = lowBound ..< hiBound
                        let receivedMD5 = message.substring(with: midRange)

                        // Acquire the deframed message (normalized message)
                        lowBound = message.index(message.startIndex, offsetBy: 0)
                        hiBound = message.index(message.endIndex, offsetBy: -(MD5Length+delimiterLength))
                        midRange = lowBound ..< hiBound
                        let normalizedMessage = message.substring(with: midRange)

                        // Calculate the MD5 digest on the normalized message
                        let calculatedMD5Digest = normalizedMessage.md5()

                        // Debug
                        print(delimiter)
                        print(normalizedMessage)
                        print(receivedMD5)
                        print(calculatedMD5Digest!)

                        // Check for the integrity of the data
                        if (receivedMD5.lowercased() == calculatedMD5Digest?.lowercased()) || self.noMD5Check  // TEMPORARY
                        {
                            if (normalizedMessage == "Unauthorized Access")
                            {
                                // Update the authorization status
                                self.authorized = false

                                // Stop the refresh control
                                if let refreshControl = self.refreshControl {
                                    if refreshControl.isRefreshing {
                                        refreshControl.endRefreshing()
                                    }
                                }

                                // Stop the stream
                                NSLog("stream task stop")
                                self.stop(task: task)

                                // Shows an alert
                                self.showAlert(title: NSLocalizedString("Unauthorized Access", comment: "Unauthorized Access Title"), message: NSLocalizedString("Please login with the proper Username and Password before to send any command!", comment: "Unauthorized Access Message"))                                    
                            }
                            else if (normalizedMessage == "System Busy")
                            {
                                // Stop the refresh control
                                if let refreshControl = self.refreshControl {
                                    if refreshControl.isRefreshing {
                                        refreshControl.endRefreshing()
                                    }
                                }

                                // Stop the stream
                                NSLog("stream task stop")
                                self.stop(task: task)

                                // Shows an alert
                                self.showAlert(title: NSLocalizedString("System Busy", comment: "System Busy Title"), message: NSLocalizedString("The system is busy at the moment. Only one connection at a time is allowed!", comment: "System Busy Message"))
                            }
                            else if (normalizedMessage == "Error")
                            {
                                // Stop the refresh control
                                if let refreshControl = self.refreshControl {
                                    if refreshControl.isRefreshing {
                                        refreshControl.endRefreshing()
                                    }
                                }

                                // Stop the stream
                                NSLog("stream task stop")
                                self.stop(task: task)

                                // Shows an alert
                                self.showAlert(title: NSLocalizedString("Error", comment: "Error Title"), message: NSLocalizedString("An error occurred during the execution of the command!", comment: "Command Error Message"))
                            }
                            else if (normalizedMessage == "ErrorMachineRunning")
                            {
                                // Stop the refresh control
                                if let refreshControl = self.refreshControl {
                                    if refreshControl.isRefreshing {
                                        refreshControl.endRefreshing()
                                    }
                                }

                                // Stop the stream
                                NSLog("stream task stop")
                                self.stop(task: task)

                                // Shows an alert
                                self.showAlert(title: NSLocalizedString("Error", comment: "Error Title"), message: NSLocalizedString("The command cannot be executed while the machine is running", comment: "Machine Running Message 1")+"!\r\n\n "+NSLocalizedString("Trying to execute any command in this state could be dangerous for both people and machinery", comment: "Machine Running Message 2")+".\r\n\n "+NSLocalizedString("Please stop the machine and leave the automatic or semi-automatic modes before to provide any command", comment: "Machine Running Message 3")+".")
                            }
                            else if (normalizedMessage == "Command Not Recognized")
                            {
                                // Stop the refresh control
                                if let refreshControl = self.refreshControl {
                                    if refreshControl.isRefreshing {
                                        refreshControl.endRefreshing()
                                    }
                                }

                                // Stop the stream
                                NSLog("stream task stop")
                                self.stop(task: task)

                                // Shows an alert
                                self.showAlert(title: NSLocalizedString("Error", comment: "Error Title"), message: NSLocalizedString("Command not recognized!", comment: "Command Unrecognized Message"))
                            }
                            else
                            {
                                // Stop the refresh control
                                if let refreshControl = self.refreshControl {
                                    if refreshControl.isRefreshing {
                                        refreshControl.endRefreshing()
                                    }
                                }

                                // Stop the stream
                                NSLog("stream task stop")
                                self.stop(task: task)

                                //let testMessage = "test\r\nf3ea0b9bff4a2c79e60acf6873f4a1ce</EOM>\r\n"
                                //normalizedMessage = testMessage

                                // Process the received csv file
                                self.processCsvData(file: normalizedMessage)
                            }
                        }
                        else
                        {
                            // Stop the refresh control
                            if let refreshControl = self.refreshControl {
                                if refreshControl.isRefreshing {
                                    refreshControl.endRefreshing()
                                }
                            }

                            // Stop the stream
                            NSLog("stream task stop")
                            self.stop(task: task)

                            // Shows an alert
                            self.showAlert(title: NSLocalizedString("Data Error", comment: "Data Error Title"), message: NSLocalizedString("The received data cannot be read since it's corrupted or incomplete!", comment: "Data Error Message"))
                        }

                    }
                    else
                    {
                        // Stop the refresh control
                        if let refreshControl = self.refreshControl {
                            if refreshControl.isRefreshing {
                                refreshControl.endRefreshing()
                            }
                        }

                        // Stop the stream
                        NSLog("stream task stop")
                        self.stop(task: task)

                        // Shows an alert
                        self.showAlert(title: NSLocalizedString("Data Error", comment: "Data Error Title"), message: NSLocalizedString("The received data cannot be read since it's corrupted or incomplete!", comment: "Data Error Message"))
                    }
                }
            }
        }
        if eof {
            // Stop the refresh control
            if let refreshControl = self.refreshControl {
                if refreshControl.isRefreshing {
                    refreshControl.endRefreshing()
                }
            }

            // Refresh the tableview content
            self.tableView.reloadData()

            // Stop the stream
            NSLog("stream task end")
            self.stop(task: task)

        } else if error == nil {
            self.startRead(for: task)
        } else {
            // We ignore the error because we'll see it again in `didCompleteWithError`.
            NSLog("stream task read error")
        }
    }
}

Answer 1

data 代表整个字符串的数据，而不仅仅是一个子字符串，这一点至关重要。如果您尝试从整个字符串的部分数据转换子字符串，很多情况下会失败。

它适用于 1 字节字符，因为无论你在哪里截断数据流，部分数据仍然代表一个有效的字符串。但是一旦开始处理多字节字符，部分数据流很容易导致数据的第一个或最后一个字节只是多字节字符的一部分。这会阻止数据被正确解释。

因此，在尝试将数据转换为字符串之前，您必须确保使用给定字符串的所有字节构建一个 data 对象。

通常，您应该以字节数开始数据。假设前 4 个字节代表一些商定的“字节顺序”中的 32 位整数。您读取这 4 个字节以获取长度。然后你读取数据，直到你得到更多的字节。然后你就知道你在消息的末尾了。

尝试在数据末尾使用“消息结束”标记的问题在于，“消息结束”标记可能会在读取中拆分。无论哪种方式，您都需要重构代码以在数据级别进行处理，并且在读取所有字符串数据之前不要尝试将数据转换为字符串。

Answer 2

如您所知，单个 UTF-8 字符是 1、2、3 或 4 个字节。 对于您的情况，您需要处理 1 个或 2 个字节的字符。并且您的接收字节序列可能未与“字符边界”对齐。 但是，正如 rmaddy 所指出的，String.Encoding.utf8 的字节序列必须以右边界开始和结束。

现在，有两种方法可以处理这种情况。 正如 rmaddy 建议的那样，一种是首先发送长度并计算传入的数据字节数。 这样做的缺点是您还必须修改传输（服务器）端，这可能是不可能的。

另一种选择是逐字节扫描传入的序列并跟踪字符边界，然后构建合法的 UTF-8 字节序列。 幸运的是，UTF-8 的设计使您可以轻松识别字符边界的位置 通过查看字节流中的任何字节。具体来说，1、2、3 和 4 字节 UTF-8 字符的第一个字节分别以 0xxxxxxx、110xxxxxx、1110xxxx 和 11110xxx 开头，第二个..第四个字节 在位表示中都是 10xxxxxx。这让您的生活更轻松。

如果您从 1 字节 UTF-8 字符之一中选择“消息结束”标记， 您可以在不考虑字节序列的情况下轻松成功地检测 EOM，因为它是单个字节并且不会出现在 2..4 字节字符中的任何位置。