iOS persistence with NSUserDefaults and the Documents directory

We are going to look into how persistence is useful in preserving data between closing and re-opening an app. In Storyboard add a button and a slider. Create a an action named saveValue for the button and an outlet named sliderView for the slider, and finally create a global variable named SliderValueKey to hold the slider value.

1) User Defaults is the easiest way in iOS to save app information associated with user preferences. We can store primitive types and object types. It behaves like a dictionary — data is stored as keys and values. iOS keeps a database of these user preference values for each app on the device. Naturally, the values are persisted from one run of an app to another. The NSUserDefaults class implements the Singleton design pattern. That means that there is a class method that will return a reference to the same user defaults object, regardless of where we invoke it in our program.

let SliderValueKey = "Slider Value Key"

@IBOutlet weak var sliderView: UISlider!

override func viewDidLoad() {
    super.viewDidLoad()
    sliderView.value = NSUserDefaults.standardUserDefaults().floatForKey(SliderValueKey)
    println("Slider value at load time: \(sliderView.value)")
}

@IBAction func saveValue(sender: UIButton) {
    println("Slider value at save time: \(sliderView.value)")
    NSUserDefaults.standardUserDefaults().setFloat(sliderView.value, forKey: SliderValueKey)
}

To test the code, run the app and notice that a value of 0.0 is displayed in the console for the slider. Now move the slider to the left or to the right, then press the button to save the current value of the slider. The new value will be noted in the console. Next, close the app and run it again. As you expect, the start value will now be the last value we had before closing the app. This is the most basic form of persistence.

2) Let’s look at how we can write and read files from disk, as another form of persistence. Every time the user writes to disk, the Documents directory will be used.

override func viewDidLoad() {
    super.viewDidLoad()
    if NSFileManager.defaultManager().fileExistsAtPath(fileURL().path!) {
        println("The file \((fileURL().pathComponents?.last)!) already exists!")
    } else {
        println("Creating the file \((fileURL().pathComponents?.last)!) on disk.")
        NSFileManager.defaultManager().createFileAtPath(fileURL().path!, contents: nil, attributes: nil)
    }
}

func fileURL() ->  NSURL {
    let filename = "demo.txt"
    let dirPath = NSSearchPathForDirectoriesInDomains(.DocumentDirectory, .UserDomainMask, true)[0] as! String
    let pathArray = [dirPath, filename]
    let fileURL =  NSURL.fileURLWithPathComponents(pathArray)!
    return fileURL
}

The first time you run the app you will see in the console:

    Creating the file demo.txt on disk.

If you run the app again, this time you will see in the console:

    The file demo.txt already exists!

You can read more about NSUserDefaults and NSFileManager if you’re interested in exploring these topics more.

Until next time!