Swift 3

import UIKit

// Save String to file
let fileName = "TextFile"
let documentDirectory = try FileManager.default.urlForDirectory(.documentDirectory, in: .userDomainMask, appropriateFor: nil, create: true)

var fileURL = try documentDirectory.appendingPathComponent(fileName).appendingPathExtension("txt")

print("FilePath: \(fileURL.path)")

var toFileString = "Text to write"
do {
  // Write to file
  try toFileString.writeToURL(fileURL, atomically: true, encoding: NSUTF8StringEncoding)
} catch let error as NSError {
  print("Failed writing to URL: \(fileURL), Error:\(error.localizedDescription)")
}

// Reading
var fromFileString = ""
do {
  fromFileString = try String(contentsOfURL: fileURL)
} catch let error as NSError {
  print("Failed reading from URL: \(fileURL), Error: " + error.localizedDescription)
}
print("Text input from file: \(fromFileString)")

Swift 2

import UIKit

// Save String to file
let fileName = "TextFile"
let DocumentDirectoryURL = try! NSFileManager.defaultManager().URLForDirectory(.DocumentDirectory, inDomain: .UserDomainMask, appropriateForURL: nil, create: true)

let fileURL = DocumentDirectoryURL.URLByAppendingPathComponent(fileName).URLByAppendingPathExtension("txt")
print("FilePath: \(fileURL.path)")

var toFileString = "Text to write"
do {
  // Write to file
  try toFileString.writeToURL(fileURL, atomically: true, encoding: NSUTF8StringEncoding)
} catch let error as NSError {
  print("Failed writing to URL: \(fileURL), Error:\(error.localizedDescription)")
}

// Reading
var fromFileString = ""
do {
  fromFileString = try String(contentsOfURL: fileURL)
} catch let error as NSError {
  print("Failed reading from URL: \(fileURL), Error: " + error.localizedDescription)
}
print("Text input from file: \(fromFileString)")