Using Arrays in Singleton Class in iOS - ios

Question

PetInfo.class
class PetInfo {
static let shared: PetInfo = PetInfo()
lazy var petArray: [PetInfo] = []
var PetID:Int
var PetName:String
...
init(){ .. }
}
ViewController.swift
class ViewController: UIViewController {
var PetArray = PetInfo.shared.petArray
override func viewDidLoad() {
super.viewDidLoad()
let pet = PetInfo()
pet.PetName = "Jack"
PetArray.append(pet). **Create Object and gave a name**
print(PetArray[0].PetName) //works!
}
}
secondViewController.swift
class secondViewController: UIViewController {
var PetArray = PetInfo.shared.petArray
override func viewDidLoad() {
super.viewDidLoad()
let label: UILabel = {
let label = UILabel()
...
label.text = PetArray[0].PetName **tried to print**
return label
}()
view.addSubview(label)
}
}
I want to share PetArray array in all of the view controllers.(It's more than two.)
It put data in the first VC, but doesn't work in the Second VC.
How can I share this array using a Singleton pattern?
Except for the array, It works perfect.(like String.. PetID, PetName.. )

Answer 1

Array in swift is implemented as Struct, which means Array is a value type and not a reference type. Value types in Swift uses copy on write (COW) mechanism to handle the changes to their values.
So in your ViewController when you assigned
var PetArray = PetInfo.shared.petArray
your PetArray was still pointing to the same array in your PetInfo.shared instance (I mean same copy of array in memory) . But as soon as you modified the value of PetArray by using
PetArray.append(pet)
COW kicks in and it creates a new copy of petArray in memory and now your PetArray variable in your ViewController and PetInfo.shared.petArray are no longer pointing to same instances instead they are pointing to two different copies of array in memory.
So all the changes you did by using PetArray.append(pet) is obviously not reflected when you access PetInfo.shared.petArray in secondViewController
What can I do?
remove PetArray.append(pet) and instead use PetInfo.shared.petArray.append(pet)
What are the other issues in my code?
Issue 1:
Never use Pascal casing for variable name var PetArray = PetInfo.shared.petArray instead use camel casing var petArray = PetInfo.shared.petArray
Issue 2:
class PetInfo {
static let shared: PetInfo = PetInfo()
lazy var petArray: [PetInfo] = []
var PetID:Int
var PetName:String
...
init(){ .. }
}
This implementation will not ensure that there exists only one instance of PetInfo exists in memory (I mean it cant ensure pure singleton pattern), though you provide access to instance of PetInfo using a static variable named shared there is nothing which stops user from creating multiple instances of PetInfo simply by calling PetInfo() as you did in let pet = PetInfo()
rather use private init(){ .. } to prevent others from further creating instances of PetInfo
Issue 3:
You are holding an array of PetInfo inside an instance of PetInfo which is kind of messed up pattern, am not really sure as to what are you trying to accomplish here, if this is really what you wanna do, then probably you can ignore point two (creating private init) for now :)

Answer 2

I think the best solution to use Combine and Resolver frameworks. Works perfectly in my case with shared arrays.
In your case it could be
import Combine
import Resolver // need to add pod 'Resolver' to Podfile and install it first
// Data Model
struct PetInfo: Codable {
var PetID:Int
var PetName:String
...
}
// Repository to read manage data (read/write/search)
class PetRepository: ObservableObject {
#Published var petArray = Array<PetInfo>()
override init() {
super.init()
load()
}
private func load() {
// load pets info from server
}
}
Need to add AppDelegate+Injection.swift that will contain repository registration:
import Foundation
import Resolver
extension Resolver: ResolverRegistering {
public static func registerAllServices() {
// Services
register { PetRepository() }.scope(application)
}
}
Then use it in any controllers
import UIKit
import Combine
import Resolver
class ViewController: UIViewController {
#LazyInjected var petRepository: PetRepository
private var cancellables = Set<AnyCancellable>()
override func viewDidLoad() {
super.viewDidLoad()
petRepository.$petArray
.receive(on: DispatchQueue.main)
.debounce(for: 0.8, scheduler: RunLoop.main)
.sink { [weak self] petInfos in
// set UI here
}
.store(in: &cancellables)
}
}

Answer 3

If you want PetInfo to be a singleton, make its initializer private:
class PetInfo {
static let shared: PetInfo = PetInfo()
lazy var petArray: [PetInfo] = []
var PetID:Int
var PetName:String
...
private init(){ .. } // !!
}
This way, any attempt to create new instances (like you do in your first ViewController) will fail, and will remind you to always use PetInfo.shared to access the singleton.