In Swift 3 the syntax is a just a few different than the 1 or 2 version:
let file = "file.txt" //this is the file. we will write to and read from it
let text = "some text" //just a text
if let dir = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask).first {
let path = dir.appendingPathComponent(file)
// writing
do {
try text.write(to: path, atomically: false, encoding: String.Encoding.utf8)
}
catch {/* error handling here */}
//reading
do {
let text2 = try String(contentsOf: path, encoding: String.Encoding.utf8)
}
catch {/* error handling here */}
}

Source: https://stackoverflow.com/questions/24097826/read-and-write-data-from-text-file