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Fixing Project 10: NSCoding

You've just learned all the core basics of working with UserDefaults, but we're just getting started. You see, above and beyond integers, dates, strings, arrays and so on, you can also save any kind of data inside UserDefaults as long as you follow some rules.

What happens is simple: you use the archivedData() method of NSKeyedArchiver, which turns an object graph into a Data object, then write that to UserDefaults as if it were any other object. If you were wondering, “object graph” means “your object, plus any objects it refers to, plus any objects those objects refer to, and so on.”

The rules are very simple:

  1. All your data types must be one of the following: boolean, integer, float, double, string, array, dictionary, Date, or a class that fits rule 2.
  2. If your data type is a class, it must conform to the NSCoding protocol, which is used for archiving object graphs.
  3. If your data type is an array or dictionary, all the keys and values must match rule 1 or rule 2.

Many of Apple's own classes support NSCoding, including but not limited to: UIColor, UIImage, UIView, UILabel, UIImageView, UITableView, SKSpriteNode and many more. But your own classes do not, at least not by default. If we want to save the people array to UserDefaults we'll need to conform to the NSCoding protocol.

The first step is to modify your Person class to this:

class Person: NSObject, NSCoding {

When we were working on this code in project 10, there were two outstanding questions:

  • Why do we need a class here when a struct will do just as well? (And in fact better, because structs come with a default initializer!)
  • Why do we need to inherit from NSObject?

It's time for the answers to become clear. You see, working with NSCoding requires you to use objects, or, in the case of strings, arrays and dictionaries, structs that are interchangeable with objects. If we made the Person class into a struct, we couldn't use it with NSCoding.

The reason we inherit from NSObject is again because it's required to use NSCoding – although cunningly Swift won't mention that to you, your app will just crash.

Once you conform to the NSCoding protocol, you'll get compiler errors because the protocol requires you to implement two methods: a new initializer and encode().

We need to write some more code to fix the problems, and although the code is very similar to what you've already seen in UserDefaults, it has two new things you need to know about.

First, you'll be using a new class called NSCoder. This is responsible for both encoding (writing) and decoding (reading) your data so that it can be used with UserDefaults.

Second, the new initializer must be declared with the required keyword. This means "if anyone tries to subclass this class, they are required to implement this method." An alternative to using required is to declare that your class can never be subclassed, known as a final class, in which case you don't need required because subclassing isn't possible. We'll be using required here.

Add these two methods to the Person class:

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

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

The initializer is used when loading objects of this class, and encode() is used when saving. The code is very similar to using UserDefaults, but here we’re adding as? typecasting and nil coalescing just in case we get invalid data back.

With those changes, the Person class now conforms to NSCoding, so we can go back to ViewController.swift and add code to load and save the people array.

Let's start with writing, because once you understand that the reading code will make much more sense. As I said earlier, you can write Data objects to UserDefaults, but we don't currently have a Data object – we just have an array.

Fortunately, the archivedData() method of NSKeyedArchiver turns an object graph into a Data object using those NSCoding methods we just added to our class. Because we make changes to the array by adding people or by renaming them, let's create a single save() method we can use anywhere that's needed:

func save() {
    if let savedData = try? NSKeyedArchiver.archivedData(withRootObject: people, requiringSecureCoding: false) {
        let defaults = UserDefaults.standard
        defaults.set(savedData, forKey: "people")
    }
}

So: line 1 is what converts our array into a Data object, then lines 2 and 3 save that data object to UserDefaults. You now just need to call that save() method when we change a person's name or when we import a new picture.

You need to modify our collection view's didSelectItemAt method so that you call self.save() just after calling self.collectionView.reloadData(). Remember, the self is required because we're inside a closure. You then need to modify the image picker's didFinishPickingMediaWithInfo method so that it calls save() just before the end of the method.

And that's it – we only change the data in two places, and both now have a call to save().

Finally, we need to load the array back from disk when the app runs, so add this code to viewDidLoad():

let defaults = UserDefaults.standard

if let savedPeople = defaults.object(forKey: "people") as? Data {
    if let decodedPeople = try? NSKeyedUnarchiver.unarchiveTopLevelObjectWithData(savedPeople) as? [Person] {
        people = decodedPeople
    }
}

This code is effectively the save() method in reverse: we use the object(forKey:) method to pull out an optional Data, using if let and as? to unwrap it. We then give that to the unarchiveTopLevelObjectWithData() method of NSKeyedUnarchiver to convert it back to an object graph – i.e., our array of Person objects.

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