Good use case for singleton - ios

Question

In my app, I have a like feature where is are displayed users which liked your profile. Because the user receive push notification when another user like him, I decided to create a singleton ENFeedLikeManager to store my likes array and add object when a notification is coming:
ENFeedLikeManager.swift
let ENFeedLikeInstance = ENFeedLikeManager.sharedInstance
class ENFeedLikeManager: NSObject {
var likeViewController: ENLikeViewController?
var likes = [ENFeedLike]()
static let sharedInstance = ENFeedLikeManager()
override init() {
super.init()
}
func addNotificationLike(like: ENFeedLike) {
guard let likeViewController = likeViewController else {
return
}
likes.insert(like, at: 0)
likeViewController.likeTableView.insertRows(at: [IndexPath.init(row: 0, section: 0)], with: .automatic)
}
}
When the app is launched, I fetch the like data from the server and I store the result in the ENFeedLikeInstance.likes and I works from this array for further operations like displaying the tableView...
AppDelegate.swift (application(_ application: UIApplication, didReceiveRemoteNotification userInfo: [AnyHashable : Any], fetchCompletionHandler completionHandler: #escaping (UIBackgroundFetchResult) -> Void) when it's a new like notification)
let newLikeData = parseLikeNotification(notification: dataDict)
if let user = newLikeData {
ENFeedLikeInstance.addNotificationLike(like: user)
}
I am afraid that by using this kind of singleton, I have some problems of deinitialization.
Have you some advice or another to accomplish that?

Answer 1

You will have issues indeed.
These kind of things should be handle/stored on the server side, and then retrieve via a request every time you need to display the list of like.
If you really want to store them locally, a singleton is not the way to go, cause every time your app is quitted, your singleton will disappear with everything in it. You should store that in:
a sqlite database,
or a coredata,
or a NSUserDefault
If you really want to use a singleton, then you need to make a request every time it is init to grab the list of likes from your server.
Note:
If you choose to store your data locally you can have issues, especially with a list of like that could change quite often, you can end up with a list that is not up to date.
Edit:
Now I understand your concern. So yes it's a bad practice to manage the logic of your controller in your singleton. If you want to do the way you are doing (which is not that bad at the end, but be careful, I would not rely heavily on notifications to display the right list, they are many cases where notifications are not received), your singleton should only keep updated the list of like, and notify your controller about any change done to that list. Then your controller will update the tableview.
So all the logic of the tableview should go in your controller. You could create a delegate and your controller respond to the protocol of that delegate.
You would end up with something like that:
let ENFeedLikeInstance = ENFeedLikeManager.sharedInstance
protocol ENFeedLikeManagerDelegate: class {
func didUpdateLikeList(_ list: [ENFeedLike])
}
class ENFeedLikeManager: NSObject {
weak var delegate: ENFeedLikeManagerDelegate?
var likes = [ENFeedLike]() {
didSet {
delegate?. didUpdateLikeList(likes)
}
}
static let sharedInstance = ENFeedLikeManager()
override init() {
super.init()
}
func addNotificationLike(like: ENFeedLike) {
guard let likeViewController = likeViewController else {
return
}
likes.insert(like, at: 0)
}
}
then you just have to use that delegate in your controller like so:
extension yourViewController: ENFeedLikeManagerDelegate {
func didUpdateLikeList(_ list: [ENFeedLike]) {
// update your tableview here with the new list of like
}
}