Writing to text file in swift 3 - swift3

Question

I am trying to write data that is inputted by a user via UITextField to a text file. I am successfully able to do this by the code I have written below. However, when I tried to save more data it will replace the existing data in the textfile with the new data that is being saved. for example, if I save the string 'hello world' and then save another string saying 'bye'. I will only see the string 'bye' in the textfile. Is there a way I can modify my code so I can see 'hello world' on one line of the textile and 'bye' on another.
#IBAction func btnclicked(_ sender: Any) {
self.savedata(value: answer.text!)
}
func savedata (value: String){
let fileName = "Test"
let DocumentDirURL = try! FileManager.default.url(for: .documentDirectory, in: .userDomainMask, appropriateFor: nil, create: true)
let fileURL = DocumentDirURL.appendingPathComponent(fileName).appendingPathExtension("txt")
print("FilePath: \(fileURL.path)")
let writeString = NSString(string: answer.text!)
do {
// Write to the file
try writeString.write(to: fileURL, atomically: true, encoding: String.Encoding.utf8.rawValue)
} catch let error as NSError {
print("Failed writing to URL: \(fileURL), Error: " + error.localizedDescription)
}
}

Answer 1

Here is an example using FIleHandler, adapted to Swift 3, from here (of course you should add all the error handling code that's missing in my example) :
let dir = FileManager.default.urls(for: FileManager.SearchPathDirectory.cachesDirectory, in: FileManager.SearchPathDomainMask.userDomainMask).first!
let fileurl = dir.appendingPathComponent("log.txt")
let string = "\(NSDate())\n"
let data = string.data(using: .utf8, allowLossyConversion: false)!
if FileManager.default.fileExists(atPath: fileurl.path) {
if let fileHandle = try? FileHandle(forUpdating: fileurl) {
fileHandle.seekToEndOfFile()
fileHandle.write(data)
fileHandle.closeFile()
}
} else {
try! data.write(to: fileurl, options: Data.WritingOptions.atomic)
}

Answer 2

do {
let fileHandle = try FileHandle(forWritingTo:pathWithFileName)
fileHandle.seekToEndOfFile()
let oldData = try String(contentsOf: pathWithFileName,encoding: .utf8).data(using: .utf8)!
var data = periodValue.data(using: .utf8)!
fileHandle.write(data)
fileHandle.closeFile()
} catch {
print("Error writing to file \(error)")
}

Answer 3

Here is a Swift 4 version as an extension to String.
extension String {
func writeToFile(fileName: String) {
guard let dir = FileManager.default.urls(for: .cachesDirectory, in: .userDomainMask).first else {
return
}
let fileUrl = dir.appendingPathComponent(fileName)
guard let data = self.data(using: .utf8) else {
return
}
guard FileManager.default.fileExists(atPath: fileUrl.path) else {
try? data.write(to: fileUrl, options: .atomic)
return
}
if let fileHandle = try? FileHandle(forUpdating: fileUrl) {
fileHandle.seekToEndOfFile()
fileHandle.write(data)
fileHandle.closeFile()
}
}
}