How to pass a (changing) variable between two view controllers? - swift

Question

I have two view controllers. VC1 & VC2. VC1 is passing a variable which keeps changing & updating every second on VC1, in my case, it is the nearest beacon which is handled by a method in VC1.
VC1 code:
var id: Int = 0 // initializing
// send data to VC2
override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
guard let vc2 = segue.destination as? navigationScreenVC else { return }
vc2.id2 = id
}
VC2 code:
var id2: Int = 0
override func viewDidLoad() {
super.viewDidLoad()
print(VC2)
}
It is working where it sends the first value it encounters, but not when the value keeps changing, so I want that to be sent as soon as it triggers a change.
I tried to do didSet{} but it doesn't work that way.

Answer 1

Use a delegate pattern.
In VC2:
protocol VC2Delegate: AnyObject {
var id2: Int { get }
}
class VC2 {
weak var delegate: VC2Delegate?
override func viewDidLoad() {
super.viewDidLoad()
print(delegate?.id2)
}
}
In VC1:
class VC1: UIViewController, VC2Delegate {
...
var id2: Int = 0
override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
guard let vc2 = segue.destination as? navigationScreenVC else { return }
vc2.delegate = self
}
...
}

Answer 2

A ViewController should not be managing stuff after it stopped being visible. You should managing this in a separate class and waiting for updates from a delegate, lets say:
protocol BeaconUpdateListener : AnyObject {
func currentBeaconIdWasUpdated(to newValue: Int)
}
class BeaconManager {
struct DelegateWrapper {
weak var delegate : BeaconUpdateListener?
}
static let delegates = [DelegateWrapper]()
static var currentId : Int = -1 {
didSet {
delegates.forEach { (delegate) in
delegate.delegate?.currentBeaconIdWasUpdated(to: currentId)
}
}
}
}
Sample code, missing details. You could make your own or update this one. Now, having that data outside your UI code makes it easier to use from anywhere else, and update that code in the future. This way, you "subscribe" to id updates like this:
BeaconManager.delegates.append(OBJECT_THAT_NEEDS_TO_BE_NOTIFIED)
... update your id like this:
BeaconManager.currentId = 65421687543152
... and wait for updates like this:
class VC2 : ViewController, BeaconUpdateListener {
func currentBeaconIdWasUpdated(to newValue: Int) {
// Do stuff once i receive the update
}
// ...
}