day-048

Day 48: Project 12: User Defaults, Part One

Follow along at https://www.hackingwithswift.com/100/48.

📒 Field Notes

This day covers the first part of Project 12: User Defaults in Hacking with Swift.

Project 12 is a technique project — geared towards refactoring Project 10 to use UserDefaults. You can find my original version of Project 10 in Day 42. However, I also copied everything over to this day's folder to extend from where I left off.

With that in mind, Day 48 focuses on several specific topics:

• Reading and writing basics: UserDefaults
• Fixing Project 10 with NSCoding

Reading and writing basics: UserDefaults

UserDefaults offers a handy way of persisting data for an application, locally on the device.

Worth noting, though, is that it's not meant to be a heavy, production-level database. In most cases, it seems like a place where we'd store user preferences or loosely-structured miscellany that would be nice to persist while the user has our app downloaded.

For our current app, we want to persist Person instances. UserDefaults can probably pass as a legitimate approach since these are fairly lightweight (each instance stores an image file name, not the file directly), and it's the only kind of data our app is dealing with. Adding more models to the mix, though, would probably warrant a solution more along the lines of Core Data. I found this article to be a really good breakdown of the various persistance approaches Apple gives us.

With all that being said, we'll mainly be interested in reading and writing objects. UserDefaults instances have a fairly straightforward, if not-very-type-safe API that revolves around A) calling set with a key and a value, and B) calling object(forKey:), using the same key across each.

The extra type information needs to be applied by us if we want to ensure that we've retrieved the correct object. For example:

let array = defaults.object(forKey:"SavedArray") as? [String] ?? [String]()

let dict = defaults.object(forKey: "SavedDict") as? [String: String] ?? [String: String]()

Furthermore, objects stored in UserDefaults need to be property lists. Here's how Apple elaborates on that:

A default object must be a property list—that is, an instance of (or for collections, a combination of instances of) NSData, NSString, NSNumber, NSDate, NSArray, or NSDictionary. If you want to store any other type of object, you should typically archive it to create an instance of NSData.

This means that for for custom types like our Persons, we'll need to write them in and read them out as Data objects — and define them in the first place so that they support such encoding and decoding. Which leads to Strategy 1...

Fixing Project 10 with NSCoding

The main ideas here are that we want our Person type to conform to the NSCoding protocol by 1) being a class 🙂 and 2) implementing NSCoding's init(coder:) and encode(with:) functions.

init?(coder aDecoder: NSCoder) {
    name = aDecoder.decodeObject(forKey: "name") as? String ?? ""
    imageName = aDecoder.decodeObject(forKey: "imageName") as? String ?? ""
}

func encode(with aCoder: NSCoder) {
    aCoder.encode(name, forKey: "name")
    aCoder.encode(imageName, forKey: "imageName")
}

This unlocks the ability to read and write Persons to and from UserDefaults anywhere else in our app. There our many different ways to structure that in and of itself. For now, though, I made an extension off of UIViewController:

extension UIViewController {

    func save(
        people: [Person],
        toDefaults userDefaults: UserDefaults = UserDefaults.standard
    ) {
        if let savedData = try? NSKeyedArchiver.archivedData(withRootObject: people, requiringSecureCoding: false) {
            userDefaults.setValue(savedData, forKey: "people")
        }
    }

    func getPeople(fromDefaults userDefaults: UserDefaults = UserDefaults.standard) -> [Person]? {
        if let savedPeople = userDefaults.object(forKey: "people") as? Data {
            if let decodedPeople = try? NSKeyedUnarchiver.unarchiveTopLevelObjectWithData(savedPeople) as? [Person] {
                return decodedPeople
            }
        }

        return nil
    }
}

🔗 Additional/Related Links

Apple Docs: UserDefaults