只需尝试使用此

//MARK:- Extension for String
extension String
{
    func appendLineToURL(fileURL: URL) throws
    {
        try (self + "\n").appendToURL(fileURL: fileURL)
    }

    func appendToURL(fileURL: URL) throws
    {
        let data = self.data(using: String.Encoding.utf8)!
        try data.append(fileURL: fileURL)
    }
}
//MARK:- Extension for File data
extension Data
{
    func append(fileURL: URL) throws {
        if let fileHandle = FileHandle(forWritingAtPath: fileURL.path)
        {
            defer
            {
                fileHandle.closeFile()
            }

            fileHandle.seekToEndOfFile()
            fileHandle.write(self)
        }
        else
        {
            try write(to: fileURL, options: .atomic)
        }
    }
}

用法：

try newLine.appendToURL(fileURL: path!)

这些扩展将附加您正好想要的NewLine，

These Extension will Append Your NewLine Which you exactly looking for to Do,

这只会替换路径上的Line

try newLine.write(to: fileURL, atomically: false, encoding: .utf8)

这将使用现有数据作为新行附加数据

try newLine.appendToURL(fileURL: path!)