Swift: How to initialise a let constant in a UIViewController subclass

Question

I want the model to be passed in by the implementing developer and I want that to be mandatory.

let model: PagingTutorialModel

init(withModel model: PagingTutorialModel) {
    self.model = model
    super.init(nibName: nil, bundle: nil)
}

required init?(coder aDecoder: NSCoder) {
    super.init(coder: aDecoder)
}

Error: Property 'self.model' not initialised at super.init call

No. Please don't switch to optionals for the sake of appeasing the compiler unless an optional is actually the correct representation of your data. — nhgrif, May 06 '16 at 12:02
[Recommended reading regarding init with coder](http://stackoverflow.com/a/32108404/2792531). — nhgrif, May 06 '16 at 12:02
I'm using storyboards yes. Funny how saying that out loud now makes everything seem so clear. If I am using storyboards then the controller is initialised when the scene is segue'd to ... so assigning to a let constant during initialisation is simply not possible unless we avoid storyboards all together. Would you mind posting this as an answer, and I will accept it. — , May 06 '16 at 12:38
You could implement the `NSCoding` protocol for your `PagingTutorialModel` class and call `init(coder:)` on it...see http://nshipster.com/nscoding/ — Aaron Rasmussen, May 06 '16 at 13:24

score 3 · Answer 1 · answered May 06 '16 at 12:48

View controllers cannot realistically have let values unless you're assign them a default value.

let foo = 3

The problem is, when we initialize view controllers from storyboards, iOS initializes the view controller with init(coder:). We don't have an opportunity to pass values in during initialization, so we cannot have let properties.

score 1 · Accepted Answer · answered May 06 '16 at 12:04

The reason is, that in init?(coder:) you also need to set the model. Depending of you setup, one solution could be to trap in init?(coder:):

required init?(coder aDecoder: NSCoder) {
    fatalError("Not implemented")
}

But doing so, you can't initialize this view controller from a storyboard. But in my opinion Storyboards are only for prototyping.

Swift: How to initialise a let constant in a UIViewController subclass

2 Answers2