How to pass data properly when two VC is on same screen - swift

Question

In my app, it has two ViewControllers on below same screen.
ViewControllerA is an UIViewController, which includes a tableView.
ViewControllerB is a containerView, which is on top of UITabBarController(root VC) as a child.
For now, I load data(songs array, index, etc.) in viewWillAppear of ViewControllerA, that would be in background queue. Meanwhile, ViewControllerB requires the same data when ViewControllerA's data loading is complete.
Currently, I'm using Dependency Injection to keep a unique data source. Declare modelController reference in ViewController A and B, and use getter and setter to binding the data changes.
The problem is the songs array fetched from modelController in ViewControllerB is nil. Because ViewControllerA will set data on modelController when loading is completed. And ViewControllerB will use it from modelController in its viewWillAppear. But these two VC are on the same screen, how could I guaranty which one will go first? Also data loading is in background queue, so it is suppose to be delayed. Then if ViewControllerB needs that data in first place, it can't be.
My current thought is to load data on ViewControllerB too, so it will not rely on ViewControllerA's result. Or add some observer/listener to message ViewControllerB when data is ready. I think it is kind of architecture stuff, I need advices from experienced guys, any hints are appreciated!
Create and inject the ModelController
func scene(_ scene: UIScene, willConnectTo session: UISceneSession, options connectionOptions: UIScene.ConnectionOptions) {
guard let _ = (scene as? UIWindowScene) else { return }
guard let rootViewController = window?.rootViewController as? TabBarController else {
fatalError("Unexpected Root View Controller")
}
// create dependency injection here
rootViewController.modelController = ModelController()
AudioManager.shared.modelController = rootViewController.modelController
}
ModelController
enum PlayMode: String {
case cycle = "repeat"
case cycleOne = "repeat.1"
case shuffle = "shuffle"
}
class ModelController {
var songs: [Song]? = nil
var position: Int = 0
var playMode = PlayMode.cycle
}
Getter and Setter
var modelController: ModelController!
var songs: [Song]?
{
get { modelController.songs }
set { modelController.songs = newValue }
}
var position: Int
{
get { modelController.position }
set { modelController.position = newValue }
}
Load data in ViewControllerA.
override func viewWillAppear(_ animated: Bool) {
super.viewWillAppear(true)
loadData()
}
func loadData() {
self.showSpinner()
songs?.removeAll()
DispatchQueue.global(qos: .background).async { [weak self] in
self?.songs = DataFetcher.shared.fetchMetaData().0
DispatchQueue.main.async {
print("data load complete")
self?.table.reloadData()
self?.removeSpinner()
}
}
}

Answer 1

so what you said is also possible
observer/listener to message ViewControllerB when data is ready
You can have a shared data class which both the vcA and vcB use. For example
class SharedData {
static let shared = SharedData()
// Use didLoad as a flag for both ViewControllers
var didLoad: Bool {
return self.data != nil
}
var data: SomeData!
private init() {
fetchData()
}
}
In this answer there is a function called fetchData which is in the same class. You can invoke the fetchData function in the background, and use the didLoad or data as flags for displaying/calling any other function you require. This class is also a singleton meaning there will be one instance of this class throughout your applications - So you can use this anywhere in your application.
This is merely an example. You can extend this to your needs such as adding a delegate to listen for callbacks when data is loaded so you can do something in both VCs.
Or you can simply use a protocol from vcA (assuming vcA triggers the network call) and once the data is loaded you can pass it to vcB