Updating a UILabel via protocol results in crash (found nil) - ios

Question

I want to implement the MVP pattern for a new app. So the View shouldn't have any logic besides one that exclusively concerns UI elements. Therefore I want to request initial data from an "Interpreter" (interpreting user input in later code), which in turn requests data from my model and gives it to the "Presenter". The presenter holds a protocol with functions of the view.
The problem is: Calling updateUIData() from the presenter results in a
fatal error: unexpectedly found nil while unwrapping an Optional value
while calling the function from within the View at the same position is working just fine.
I suspect the error comes from the initialization of the specific MainViewController in the init of the presenter, but I don't know how to resolve this, if my guess is right.
Here's my (relevant) code:
MainViewController:
class MainViewController: UIViewController {
lazy var interpreter = Interpreter() // lazy needed b/c Interpreter holds Presenter which holds MainViewController
#IBOutlet var dateLabel: UILabel!
#IBOutlet var totalTimeLabel: UILabel!
override func viewDidLoad() {
super.viewDidLoad()
// updateUIData()
requestData()
}
func requestData() {
interpreter.requestData()
}
}
extension MainViewController: MainViewSetters {
func updateUIData() {
dateLabel.text = "Data"
totalTimeLabel.text = "loaded"
}
}
MainViewSetters (Protocol):
protocol MainViewSetters {
func updateUIData()
}
Interpreter:
class Interpreter {
let presenter = Presenter()
func requestData() {
// normally: get data from model and pass it to presenter
presenter.presentData()
}
}
Presenter:
class Presenter {
let mainView: MainViewSetters
init(withMainViewController mainVC: MainViewSetters = MainViewController()) {
mainView = mainVC
}
func presentData() {
mainView.updateUIData()
}
}

Answer 1

Your problem here is that you are not passing the reference to MainViewController to your instance of Presenter.
This code :
lazy var interpreter = Interpreter()
Should be more like this : (Type is needed here because with lazy the compiler can't infer properly)
lazy var interpreter: Interpreter = Interpreter(for: self)
You then have to create a special initializer in Interpreter which will pass the viewController instance to its presenter property :
class Interpreter {
let presenter: Presenter
init(for viewController: MainViewSetters) {
presenter = Presenter(withMainViewController: viewController)
}
func requestData() {
// normally: get data from model and pass it to presenter
presenter.presentData()
}
}
I also highly suggest you to remove the default value to Presenter's init method, it's very unlikely you'll want to assign a random instance of MainViewController as mainView of any Presenter object.
Finally, please note that this code is creating a retain cycle and neither your MainViewController instance nor your Presenter instance will be deallocated. This is due to the fact the Presenter class holds a strong reference to the MainViewController instance with its property mainView. To fix this you have to mark the mainView as weak as well as making it optional.
Please see the fixed implementation below :
class Presenter {
weak var mainView: MainViewSetters?
init(withMainViewController mainVC: MainViewSetters) {
mainView = mainVC
}
func presentData() {
mainView?.updateUIData()
}
}
For weak to be acceptable on a property of type MainViewSetters (which is not a real type but only a protocol) you have to specify that its a protocol that will only be applied to classes :
protocol MainViewSetters: class {
func updateUIData()
}

Answer 2

You are initializing interpreter passing a default MainViewController().
Change that code from:
lazy var interpreter = Interpreter()
to
lazy var interpreter = Interpreter(withMainViewController: self)