How to save a struct with NSCoding

Question

How can I save my struct with NSCoding so that it doesn´t change even if the user closes the app? I would appreciate it if you could also show me how to implement the missing code correctly.

UPDATE with two new functions below: Here is my code:

    struct RandomItems: Codable
{
    var items : [String]
    var seen  = 0

    init(items:[String], seen: Int)
    {
        self.items = items
        self.seen = seen
    }

    init(_ items:[String])
    { self.init(items: items, seen: 0) }


    mutating func next() -> String
    {
        let index = Int(arc4random_uniform(UInt32(items.count - seen)))
        let item  = items.remove(at:index)
        items.append(item)
        seen = (seen + 1) % items.count
        return item
    }
    func toPropertyList() -> [String: Any] {
        return [
            "items": items,
            "seen": seen
        ]
    }


    }





override func viewWillDisappear(_ animated: Bool) {
    UserDefaults.standard.set(try? PropertyListEncoder().encode(quotes), forKey:"quote2")
}

override func viewDidAppear(_ animated: Bool) {
     if let data = UserDefaults.standard.value(forKey:"quote2") as? Data {
        let quote3 = try? PropertyListDecoder().decode(Array<RandomItems>.self, from: data)
    }

}




  }


extension QuotesViewController.RandomItems {
init?(propertyList: [String: Any]) {
    return nil
}

}

How can I make sure the whole Array is covered here?

You need to make it a class. You will need also to subclass NSObject and make it conforms to NSCoding — Leo Dabus, Oct 18 '17 at 17:43
Possible duplicate of [Save struct in class to NSUserDefaults using Swift](https://stackoverflow.com/questions/25546716/save-struct-in-class-to-nsuserdefaults-using-swift) — TNguyen, Oct 18 '17 at 17:53

DerrickHo328 · Answer 1 · 2017-10-20T15:30:59.507

7

For structs you should be using the new Codable protocol. It is available since swift 4 and is highly recommended.

struct RandomItems: Codable
{
    var items: [String]
    var seen = 0
}

extension RandomItems {
     init?(propertyList: [String: Any]) {
           ...
     }
}

// Example usage
let a = RandomItems(items: ["hello"], seen: 2)
let data: Data = try! JSONEncoder().encode(a)
UserDefaults.standard.set(data, forKey: "MyKey") // Save data to disk
// some time passes
let data2: Data = UserDefaults.standard.data(forKey: "MyKey")! // fetch data from disk
let b = try! JSONDecoder().decode(RandomItems.self, from: data2)

Update

It looks like the Original Poster is nesting the struct inside of another class. Here is another example where there struct is nested.

class QuotesViewController: UIViewController {

    struct RandomItems: Codable
    {
        var items: [String]
        var seen = 0
    }
}

extension QuotesViewController.RandomItems {
    init(_ items:[String])
        { self.items = items }

    init?(propertyList: [String: Any]) {
        guard let items = propertyList["items"] as? [String] else { return nil }
        guard let seen = propertyList["seen"] as? Int else { return nil }
        self.items = items
        self.seen = seen
    }
}

// example usage
let a = QuotesViewController.RandomItems(items: ["hello"], seen: 2)
let data: Data = try! JSONEncoder().encode(a)
UserDefaults.standard.set(data, forKey: "MyKey") // Save data to disk
// some time passes
let data2: Data = UserDefaults.standard.data(forKey: "MyKey")! // fetch data from disk
let b = try! JSONDecoder().decode(QuotesViewController.RandomItems.self, from: data2)

edited Oct 20 '17 at 15:30

answered Oct 18 '17 at 20:32

DerrickHo328

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Where exactly do I have to insert the three lines with the declaration of variables? @Calimari328 – Joy Lucas Oct 19 '17 at 17:54
@JoyLucas I'm not sure I understand your question. Can you rephrase it? – DerrickHo328 Oct 19 '17 at 17:56
let a = .... //Error: Cannot invoke initializer for type 'QuotesViewController.RandomItems' with an argument list of type '(items: [String], seen: Int)' @Calimari328 How can I fix this? – Joy Lucas Oct 19 '17 at 18:02
Well, Maybe your struct needs the right access modifiers or you need to implement a public initializer. Or Maybe you are adding types to your struct that aren't Codable. It's hard to tell with such little info and not knowing how your are trying to use it. – DerrickHo328 Oct 19 '17 at 18:10
What does your struct look like? – DerrickHo328 Oct 19 '17 at 18:17
I edited the struct in my question above @Calimarie328 – Joy Lucas Oct 19 '17 at 18:34
I get this Error for the extension line: Declaration is only valid at file scope @Calimari328 – Joy Lucas Oct 19 '17 at 19:03
Well, the extension needs to be in the same file where RandomItems is Defined. If it actually is, then perhaps you are trying to make an extension inside of a class definition which is not allowed. You need to move it to the global scope. I'll Update my answer to answer my assumption about your code... – DerrickHo328 Oct 19 '17 at 19:27
That works but I still got errors below, I updated my question @Calimari328 – Joy Lucas Oct 19 '17 at 19:44
The variable RandomItems comes from an array within this class @Calimari328 – Joy Lucas Oct 19 '17 at 19:47
@JoyLucas You may want to take a closer look at my answer. – DerrickHo328 Oct 19 '17 at 19:50
Yes, that works now, but what does the String "hello" and the int 2 exactly mean? @Calimari328 – Joy Lucas Oct 19 '17 at 19:58
They are random data I put in to initialize the struct – DerrickHo328 Oct 19 '17 at 20:00
Becuase I get an error (updated my question) @Calimari328 – Joy Lucas Oct 19 '17 at 20:04
move your initializer to the extension – DerrickHo328 Oct 19 '17 at 20:56
Which initializer? @Calimari328 – Joy Lucas Oct 20 '17 at 11:56
@JoyLucas you have two of them in your example. Move all of them. – DerrickHo328 Oct 20 '17 at 12:34
How can I finally load the saved struct if the user opens the app again? Calimari328 – Joy Lucas Oct 20 '17 at 15:22
You can use NSUserDefaults to save/read data to disk. Then you can use JSONDecoder to turn it from data to a struct – DerrickHo328 Oct 20 '17 at 15:25
Where do I have to put this code if I want the struct to be saved every time the user exits the class and to be loaded again if the user enters the class again? @Calimari328 – Joy Lucas Oct 20 '17 at 18:18
@JoyLucas I can tell that you are very very new to this. Spend some time learning about the UIViewController lifecycle and you may find your answer. https://i.stack.imgur.com/g19fw.png – DerrickHo328 Oct 20 '17 at 18:24
I thought about using viewWillDisappear and viewDidAppear @Calimari328 – Joy Lucas Oct 20 '17 at 19:10
That could very well suit your needs. – DerrickHo328 Oct 20 '17 at 19:23
Looks like your saving it in viewWillDissappear then fetching it in viewDidAppear but you aren't really doing anything with it. – DerrickHo328 Oct 20 '17 at 19:34
I tried something new but that doesn´t work either @Calimari328 – Joy Lucas Oct 20 '17 at 19:54

How to save a struct with NSCoding

1 Answers1