let fileName = "file.txt"
let text = "Line 1\nLine 2"

let dir = FileManager.default.urls(
    for: .documentDirectory,
    in: .userDomainMask).first!
let filePath = dir.appendingPathComponent(fileName)

do {
    try text.write(to: filePath,
        atomically: false,
        encoding: String.Encoding.utf8)
} catch(let e) {
    print(e.localizedDescription)
}