How to subclass a UIViewController and add properties in swift?

Question

I want to make a new kind of view controller in Swift that has an additional property that must be explicitly initialized. It doesn't make any sense for this property to be nil or have a default value and the controller will only be initialized programmatically. I tried defining it like this:

class MyController : UIViewController {
  var prop: Int
  init(prop: Int) {
    self.prop = prop
    super.init()
  }

  required init(coder aDecoder: NSCoder?) {
    fatalError("don't serialize this")
  }
}

I tried running this but it crashed because super.init() tries to run the nib constructor which isn't defined, so I tried adding that:

override init(nibName nibNameOrNil: String?, bundle nibBundleOrNil: NSBundle?) {
  super.init(nibName: nibNameOrNil, bundle: nibBundleOrNil)
}

But now the compiler complains that prop isn't being initialized. And this is my question: how can I initialize prop correctly here? I don't want a default value, and anything I set will override the correct value that I set in the other initializer.

I kinda hacked around it by setting some default value in the nib init, but then having my first init do this

self.prop = prop
super.init()
self.prop = prop

But other than being really weird and ugly, that makes me worried that now it is possible to initialize my view controller from a nib and end up with the default value, which would be bad.

What is the correct and idiomatic way to do this in Swift?

How is the view controller to be created? Storyboard? Programmatically? — picciano, May 17 '18 at 13:58
Have you thought about a protocol instead of inheritance? It's much cleaner and seems to currently be the convention for adding properties to existing classes. — kevin, May 17 '18 at 14:18
Alternatively, could the view controller pull in the data it needs rather than having it be set in the init function? — picciano, May 17 '18 at 14:29
Initializers are good for programmatically created viewControllers, but for viewcontrollers created through storyboard [you're out of luck and have to work your way around it](https://stackoverflow.com/a/39400793/5175709) — mfaani, Aug 27 '19 at 20:30

score 5 · Accepted Answer · answered May 17 '18 at 14:14

5

At some point the view controller must be initialized by calling init(nibName:bundle:) or init(coder:)

Try this:

class MyViewController: UIViewController {

    var prop: Int

    init(prop: Int, nibName nibNameOrNil: String? = nil, bundle nibBundleOrNil: Bundle? = nil) {
        self.prop = prop
        super.init(nibName:nibNameOrNil, bundle: nibBundleOrNil)
    }

    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }

}

answered May 17 '18 at 14:14

picciano

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Crap, the problem is that my view controller doesn't subclass UIViewController directly, there's an intermediate that needs to run. I left it out of the question initially because I didn't think it mattered... – Max May 17 '18 at 14:16
Might need to see that as well then. You should be able to just substitute your subclass for the `UIViewController`, but generally I like to discourage subclassing in favor of Protocols. Most functionality you want to accomplish with a "base" view controller can really be done with Protocols. But that may be another question to post... – picciano May 17 '18 at 14:20
Actually, this did help me figure it out! The problem is that the superclass was calling `[super init]` rather than `[super initWithNibName:nil bundle:nil]`. The first is just a convenience method for the second, but because of polymorphism it ended up calling the nib init in `MyViewController`, which I didn't want! Changing it the explicit second form ensures that this doesn't happen. I think I actually demonstrate this in my question by calling `super.init()`, same problem. – Max May 17 '18 at 14:30

JaredH · Answer 2 · 2018-05-17T14:12:42.600

1

Try the following

class MyController: UIViewController {

    var prop: Int

    required init?(coder aDecoder: NSCoder) {
        fatalError()
    }

    init(prop: Int) {
        self.prop = prop
        super.init(nibName: nil, bundle: nil)
    }
}

edited May 17 '18 at 14:12

answered May 17 '18 at 13:57

JaredH

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Doesn't work for me using Swift 4, UIViewController does not have `init(frame:)` and must call designated init method. I think you are thinking of UIView – Scriptable May 17 '18 at 14:04
But then I need to define init(frame:)? – Max May 17 '18 at 14:04

How to subclass a UIViewController and add properties in swift?

2 Answers2