why do we need the service class using singleton in Swift? - ios

Question

I watch realm tutorial from youtube, the instructor using singleton in the service class like the code below :
import Foundation
import RealmSwift
class RealmService {
// singleton
private init() {}
static let shared = RealmService()
var realm = try! Realm()
func save<T: Object>(object: T) {
do {
try realm.write {
realm.add(object)
}
} catch {
post(error)
}
}
func update<T: Object>(object: T, for dictionary: [String: Any?]) {
do {
try realm.write {
for (key,value) in dictionary {
object.setValue(value, forKey: key)
}
}
} catch {
post(error)
}
}
func delete<T: Object>(object: T) {
do {
try realm.write {
realm.delete(object)
}
} catch {
post(error)
}
}
func post(_ error: Error) {
NotificationCenter.default.post(name: Notification.Name.realmError, object: error)
}
func observerRealmErrors(in vc: UIViewController, completion: #escaping(Error?) -> Void ) {
NotificationCenter.default.addObserver(forName: Notification.Name.realmError, object: nil, queue: nil) { (notification) in
completion(notification.object as? Error)
}
}
func stopObservingErrors(in vc: UIViewController) {
NotificationCenter.default.removeObserver(vc, name: NSNotification.Name.realmError, object: nil)
}
}
I understand that A singleton is an object which is instantiated exactly once. but why it needs to use singleton for this purpose? if the code s like below, I understand, because it doesn't make any sense if there is more than one "bobby" as the ID, then it makes sense to create a singleton class
but how about that realm service like the code above? why it needs to use singleton? I have seen some instructor using singleton for the service class
class AccountManager {
static let sharedInstance = AccountManager()
var userInfo = (ID: "Bobby", Password: 01036343984)
// Networking: communicating server
func network() {
// get everything
}
private init() { }
}

Answer 1

You gave an example of a shared property (userInfo) in your second block , but in the first block the instructor uses a singleton to prevent himself from duplicating code everywhere inside the project , and only to have one single unit to do the crud ( create , read , update , delete) operations of the Realm database and it's another use of a singleton class other than sharing a value

Answer 2

It's simple. If you don't try to using a singleton in realm case like you have wrote, you should call realm = try! Realm() method every time when you trying to use realm objects or functions(saving deleting etc..). It will make your code too boring and also can causes many miss point. If you using realm with the singleton code like you have wrote(it wrote with generic functions), you can save your object in realm database with just single line of code like RealmService.save(yourobject).