Day 48
Day 48 관련
Project 12, part one
Douglas Adams once said, “most of the time spent wrestling with technologies that don't quite work yet is just not worth the effort for end users, however much fun it is for nerds like us.” And of course he was right: when software doesn’t quite work we sometimes see it as a challenge to find a workaround, whereas everyone else in the world just gets annoyed or gives up.
Think about how often you see a Save button in iOS. Hardly ever, right? This isn’t an accident: iOS makes it look like all apps are running all the time when really they get backgrounded or even terminated all the time, but users don’t want to have to think about saving files before a program is quit.
This behavior is a great example of how Apple takes away the annoyance for end users – they don’t have to expend the effort of managing data or worrying about programs, which means they can instead focus on just using their device for the things they actually care about.
Now it falls to us. Project 10 worked great, except that it doesn’t save the pictures users add. Today you’re going to learn one of the ways we can fix that, and we’ll look at a different option tomorrow.
Today you have three topics to work through, and you’ll learn about UserDefaults, NSCoding and more.
Setting up
Setting up
This is our fourth technique project, and we're going to go back to project 10 and fix its glaring bug: all the names and faces you add to the app don't get saved, which makes the app rather pointless!
We're going to fix this using a new class called UserDefaults and a new protocol NSCoding – it’s similar in intent to the Codable protocol we used when working with JSON, but the former is available only to Swift developers whereas the latter is also available to Objective-C developers.
Along the way you’ll also use the classes NSKeyedArchiver and NSKeyedUnarchiver (for use with NSCoding), and JSONEncoder and JSONDecoder (for use with Codable), all of which are there to convert custom objects into data that can be written to disk.
Putting all that together, we're going to update project 10 so that it saves its people array whenever anything is changed, then loads when the app runs.
We're going to be modifying project 10 twice over, once for each method of solving the problem, so I suggest you take a copy of the project 10 folder twice – call the copies project12a and project12b.
Reading and writing basics: UserDefaults
Reading and writing basics: UserDefaults
You can use UserDefaults to store any basic data type for as long as the app is installed. You can write basic types such as Bool, Float, Double, Int, String, or URL, but you can also write more complex types such as arrays, dictionaries and Date – and even Data values.
When you write data to UserDefaults, it automatically gets loaded when your app runs so that you can read it back again. This makes using it really easy, but you need to know that it's a bad idea to store lots of data in there because it will slow loading of your app. If you think your saved data would take up more than say 100KB, UserDefaults is almost certainly the wrong choice.
Before we get into modifying project 10, we're going to do a little bit of test coding first to try out what UserDefaults lets us do. You might find it useful to create a fresh Single View App project just so you can test out the code.
To get started with UserDefaults, you create a new instance of the class like this:
let defaults = UserDefaults.standard
Once that's done, it's easy to set a variety of values – you just need to give each one a unique key so you can reference it later. These values nearly always have no meaning outside of what you use them for, so just make sure the key names are memorable.
Here are some examples:
let defaults = UserDefaults.standard
defaults.set(25, forKey: "Age")
defaults.set(true, forKey: "UseTouchID")
defaults.set(CGFloat.pi, forKey: "Pi")
You can also use the set() to store strings, arrays, dictionaries and dates. Now, here's a curiosity that's worth explaining briefly: in Swift, strings, arrays and dictionaries are all structs, not objects. But UserDefaults was written for NSString and friends – all of which are 100% interchangeable with Swift their equivalents – which is why this code works.
Using set() for these advanced types is just the same as using the other data types:
defaults.set("Paul Hudson", forKey: "Name")
defaults.set(Date(), forKey: "LastRun")
Even if you're trying to save complex types such as arrays and dictionaries, UserDefaults laps it up:
let array = ["Hello", "World"]
defaults.set(array, forKey: "SavedArray")
let dict = ["Name": "Paul", "Country": "UK"]
defaults.set(dict, forKey: "SavedDict")
That's enough about writing for now; let's take a look at reading.
When you're reading values from UserDefaults you need to check the return type carefully to ensure you know what you're getting. Here's what you need to know:
integer(forKey:)returns an integer if the key existed, or 0 if not.bool(forKey:)returns a boolean if the key existed, or false if not.float(forKey:)returns a float if the key existed, or 0.0 if not.double(forKey:)returns a double if the key existed, or 0.0 if not.object(forKey:)returnsAny?so you need to conditionally typecast it to your data type.
Knowing the return values are important, because if you use bool(forKey:) and get back "false", does that mean the key didn't exist, or did it perhaps exist and you just set it to be false?
It's object(forKey:) that will cause you the most bother, because you get an optional object back. You're faced with two options, one of which isn't smart so you realistically have only one option!
Your options:
- Use
as!to force typecast your object to the data type it should be. - Use
as?to optionally typecast your object to the type it should be.
If you use as! and object(forKey:) returned nil, you'll get a crash, so I really don't recommend it unless you're absolutely sure. But equally, using as? is annoying because you then have to unwrap the optional or create a default value.
There is a solution here, and it has the catchy name of the nil coalescing operator, and it looks like this: ??. This does two things at once: if the object on the left is optional and exists, it gets unwrapped into a non-optional value; if it does not exist, it uses the value on the right instead.
This means we can use object(forKey:) and as? to get an optional object, then use ?? to either unwrap the object or set a default value, all in one line.
For example, let's say we want to read the array we saved earlier with the key name SavedArray. Here's how to do that with the nil coalescing operator:
let array = defaults.object(forKey:"SavedArray") as? [String] ?? [String]()
So, if SavedArray exists and is a string array, it will be placed into the array constant. If it doesn't exist (or if it does exist and isn't a string array), then array gets set to be a new string array.
This technique also works for dictionaries, but obviously you need to typecast it correctly. To read the dictionary we saved earlier, we'd use this:
let dict = defaults.object(forKey: "SavedDict") as? [String: St
Fixing Project 10: NSCoding
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:
- 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. - If your data type is a class, it must conform to the
NSCodingprotocol, which is used for archiving object graphs. - 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.
