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Return instancetype in Swift

I'm trying to make this extension:

extension UIViewController

class func initialize(storyboardName: String, storyboardId: String) -> Self

let storyboad = UIStoryboard(name: storyboardName, bundle: nil)
let controller = storyboad.instantiateViewControllerWithIdentifier(storyboardId) as! Self

return controller




But I get compile error:

error: cannot convert return expression of type 'UIViewController' to
return type 'Self'

Is it possible? Also I want to make it as init(storyboardName: String, storyboardId: String)

init(storyboardName: String, storyboardId: String)

3 Answers
3

Similar as in Using 'self' in class extension functions in Swift, you can define a generic helper method which infers the type of self from the calling context:

extension UIViewController

class func instantiateFromStoryboard(storyboardName: String, storyboardId: String) -> Self

return instantiateFromStoryboardHelper(storyboardName, storyboardId: storyboardId)


private class func instantiateFromStoryboardHelper<T>(storyboardName: String, storyboardId: String) -> T

let storyboard = UIStoryboard(name: storyboardName, bundle: nil)
let controller = storyboard.instantiateViewControllerWithIdentifier(storyboardId) as! T
return controller




Then

let vc = MyViewController.instantiateFromStoryboard("name", storyboardId: "id")


compiles, and the type is inferred as MyViewController.

MyViewController

Update for Swift 3:

extension UIViewController

class func instantiateFromStoryboard(storyboardName: String, storyboardId: String) -> Self

return instantiateFromStoryboardHelper(storyboardName: storyboardName, storyboardId: storyboardId)


private class func instantiateFromStoryboardHelper<T>(storyboardName: String, storyboardId: String) -> T

let storyboard = UIStoryboard(name: storyboardName, bundle: nil)
let controller = storyboard.instantiateViewController(withIdentifier: storyboardId) as! T
return controller




Another possible solution, using unsafeDowncast:

unsafeDowncast

extension UIViewController

class func instantiateFromStoryboard(storyboardName: String, storyboardId: String) -> Self

let storyboard = UIStoryboard(name: storyboardName, bundle: nil)
let controller = storyboard.instantiateViewController(withIdentifier: storyboardId)
return unsafeDowncast(controller, to: self)




Self is determined at compile-time, not runtime. In your code, Self is exactly equivalent to UIViewController, not "the subclass that happens to be calling this." This is going to return UIViewController and the caller will have to as it into the right subclass. I assume that's what you were trying to avoid (though it is the "normal Cocoa" way to do it, so just returning UIViewController is probably the best solution).

Self

Self

UIViewController

UIViewController

as

UIViewController

Note: You should not name the function initialize in any case. That's an existing class function of NSObject and would cause confusion at best, bugs at worst.

initialize

NSObject

But if you want to avoid the caller's as, subclassing is not usually the tool to add functionality in Swift. Instead, you usually want generics and protocols. In this case, generics are all you need.

as

func instantiateViewController<VC: UIViewController>(storyboardName: String, storyboardId: String) -> VC
let storyboad = UIStoryboard(name name: storyboardName, bundle: nil)
let controller = storyboad.instantiateViewControllerWithIdentifier(storyboardId) as! VC

return controller



This isn't a class method. It's just a function. There's no need for a class here.

let tvc: UITableViewController = instantiateViewController(name: name, storyboardId: storyboardId)


Another way is to use a protocol, which also allows you to return Self.

Self

protocol StoryboardGeneratable



extension UIViewController: StoryboardGeneratable



extension StoryboardGeneratable where Self: UIViewController

static func initialize(storyboardName: String, storyboardId: String) -> Self

let storyboad = UIStoryboard(name: storyboardName, bundle: nil)
let controller = storyboad.instantiateViewController(withIdentifier: storyboardId) as! Self
return controller




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