Swift MVVM - use protocol to handle viewModel events - swift

Question

I'm trying to use MVVM with delegate protocols. When something changes in the view model I want to trigger it in the view controller.
When I want to use protocols to handle the view model's event on a view controller, I can not set the protocol to the view controller for my view model class.
It gives me the error:
Argument type (SecondViewController) -> () -> SecondViewController does not conform to expected type SecondViewModelEvents
How can I do this the right way?
Here is the code for my view model:
protocol SecondViewModelEvents {
func changeBackground()
}
class SecondViewModel:NSObject {
var events:SecondViewModelEvents?
init(del:SecondViewModelEvents) {
self.events = del
}
func loadDataFromServer() {
self.events?.changeBackground()
}
}
And for my view controller class:
class SecondViewController: UIViewController,SecondViewModelEvents {
let viewModel = SecondViewModel(del: self) //Argument type '(SecondViewController) -> () -> SecondViewController' does not conform to expected type 'SecondViewModelEvents'
#IBAction func buttonPressed(_ sender: Any) {
self.viewModel.loadDataFromServer()
}
func changeBackground() {
self.view.backgroundColor = UIColor.red
}
}

Answer 1

You're trying to initialize the view model variable and pass the view controller as a delegate which at this point is not fully initialized.
Try checking out the very informative and very detailed Initialization page in the official Swift language guide.
Since this is a protocol used for this specific purpose, we can safely constrain it to classes (notice the : class addition to your code.
protocol SecondViewModelEvents: class {
func changeBackground()
}
It's good practice to use more descriptive naming, and also using weak references for delegate objects in order to avoid strong reference cycles.
class SecondViewModel {
weak var delegate: SecondViewModelEvents?
init(delegate: SecondViewModelEvents) {
self.delegate = delegate
}
func loadDataFromServer() {
delegate?.changeBackground()
}
}
You can try to use an optional view model, which will get initialized in an appropriate place, like awakeFromNib():
class SecondViewController: UIViewController, SecondViewModelEvents {
var viewModel: SecondViewModel?
override func awakeFromNib() {
super.awakeFromNib()
viewModel = SecondViewModel(delegate: self)
}
#IBAction func buttonPressed(_ sender: Any) {
viewModel?.loadDataFromServer()
}
func changeBackground() {
view.backgroundColor = UIColor.red
}
}
Or an alternative approach would be to initialize a non-optional view model in the UIViewController required initializer:
// ...
var viewModel: SecondViewModel
required init?(coder aDecoder: NSCoder) {
super.init(coder: aDecoder)
self.viewModel = SecondViewModel(delegate: self)
}
// ...

Answer 2

You need to use lazy initialization as,
lazy var viewModel = SecondViewModel(del: self)
OR
lazy var viewModel = { [unowned self] in SecondViewModel(del: self) }()