Day 49
Day 49 관련
Project 12, part two
Alan Perlis once said “fools ignore complexity; pragmatists suffer it; some can avoid it; geniuses remove it.” Today you’re going to see that in Swift code: rather than try to simplify the code for NSCoding, the Swift team found a way to remove it entirely using the Codable protocol.
Sometimes you won’t have a choice and need to use NSCoding, but where possible Codable is both easier and safer to use. You do need to know them both, though, and by seeing them both used to solve the same problem hopefully you’ll able to decide for yourself which you prefer.
Before you get down to work, here’s another fun Swift in-joke for you: sometimes you might see T-shirts with a picture of a fish followed by “able”, but before you scratch your head wondering what “fishable” might mean please remember that “COD” is a type of fish!
Today you should work through the Codable chapter and wrap up for project 12, complete its review, then work through all three of its challenges.
Fixing Project 10: Codable
Fixing Project 10: Codable
NSCoding is a great way to read and write data when using UserDefaults, and is the most common option when you must write Swift code that lives alongside Objective-C code.
However, if you’re only writing Swift, the Codable protocol is much easier. We already used it to load petition JSON back in project 7, but now we’ll be loading and saving JSON.
There are three primary differences between the two solutions:
- The
Codablesystem works on both classes and structs. We madePersona class becauseNSCodingonly works with classes, but if you didn’t care about Objective-C compatibility you could make it a struct and useCodableinstead. - When we implemented
NSCodingin the previous chapter we had to writeencode()andinit()calls ourself. WithCodablethis isn’t needed unless you need more precise control - it does the work for you. - When you encode data using
Codableyou can save to the same format thatNSCodinguses if you want, but a much more pleasant option is JSON –Codablereads and writes JSON natively.
All three of those combined means that you can define a struct to hold data, and have Swift create instances of that struct directly from JSON with no extra work from you.
Anyway, to demonstrate more of Codable in action I’d like you to close project12a and open project12b – this should be identical to project 10, because it doesn’t contain any of the NSCoding changes.
First, let’s modify the Person class so that it conforms to Codable:
class Person: NSObject, Codable {
…and that’s it. Yes, just adding Codable to the class definition is enough to tell Swift we want to read and write this thing. So, now we can go back to ViewController.swift and add code to load and save the people array.
As with NSCoding we’re going to create a single save() method we can use anywhere that's needed. This time it’s going to use the JSONEncoder class to convert our people array into a Data object, which can then be saved to UserDefaults. This conversion might fail, so we’re going to use if let and try? so that we save data only when the JSON conversion was successful.
Add this method to ViewController.swift now:
func save() {
let jsonEncoder = JSONEncoder()
if let savedData = try? jsonEncoder.encode(people) {
let defaults = UserDefaults.standard
defaults.set(savedData, forKey: "people")
} else {
print("Failed to save people.")
}
}
Just like with the NSCoding example you need to modify our collection view's didSelectItemAt method so that you call self?.save() just after calling self.collectionView.reloadData(). Again, remember that adding 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.
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 {
let jsonDecoder = JSONDecoder()
do {
people = try jsonDecoder.decode([Person].self, from: savedPeople)
} catch {
print("Failed to load people")
}
}
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 an instance of JSONDecoder to convert it back to an object graph – i.e., our array of Person objects.
Once again, note the interesting syntax for decode() method: its first parameter is [Person].self, which is Swift’s way of saying “attempt to create an array of Person objects.” This is why we don’t need a typecast when assigning to people – that method will automatically return [People], or if the conversion fails then the catch block will be executed instead.
Wrap up
Details
You will use UserDefaults in your projects. That isn't some sort of command, just a statement of inevitability. If you want to save any user settings, or if you want to save program settings, it's just the best place for it. And I hope you'll agree it is (continuing a trend!) easy to use and flexible, particularly when your own classes conform to Codable.
As you saw, the NSCoding protocol is also available. Yes, it takes extra work to use, and can be quite annoying when your data types have lots of properties you need to save, but it does have the added benefit of Objective-C compatibility if you have a mixed codebase.
One proviso you ought to be aware of: please don't consider UserDefaults to be safe, because it isn't. If you have user information that is private, you should consider writing to the keychain instead – something we'll look at in project 28.
Review what you learned
Anyone can sit through a tutorial, but it takes actual work to remember what was taught. It’s my job to make sure you take as much from these tutorials as possible, so I’ve prepared a short review to help you check your learning.
Click here to review what you learned in project 12.
Challenge
One of the best ways to learn is to write your own code as often as possible, so here are three ways you should try your new knowledge to make sure you fully understand what’s going on:
- Modify project 1 so that it remembers how many times each storm image was shown – you don’t need to show it anywhere, but you’re welcome to try modifying your original copy of project 1 to show the view count as a subtitle below each image name in the table view.
- Modify project 2 so that it saves the player’s highest score, and shows a special message if their new score beat the previous high score.
- Modify project 5 so that it saves the current word and all the player’s entries to
UserDefaults, then loads them back when the app launches.
Wrap up - Additional
