Class casting dynamically in swift - swift

Question

I am trying to dyanmically cast to a class in Swift. Is this possible? Here is the code I am trying to use:
let stringClass: AnyClass = NSString.self
let anyObject: AnyObject = "foo"
let string = anyObject as! stringClass
The code fails to compile at the cast. Is this possible and if so, why is the right syntax?
Real use case
Here is the real issue. I am attempting to refactor this code:
switch (value) {
case "valueOne":
viewController = storyboard.instantiateViewController(withIdentifier: "foo") as! FirstViewController
case "valueTwo":
viewController = storyboard.instantiateViewController(withIdentifier: "bar") as! SecondViewController
default:
return nil
}
into:
let controllersDictionary: [String: (String, UIViewController.Type)] = [
"valueOne" : ("bar", FirstViewController.self),
"valueTwo" : ("foo", SecondViewController.self)
]
let tuple = controllersDictionary[value]!
let identifier = tuple.0
let cast = tuple.1
let viewController = storyboard.instantiateViewController(withIdentifier: identifier) as! cast

Answer 1

I'm not sure exactly what you're trying to achieve, but here's a working version of your example:
func cast<T>(value: Any, to type: T) -> T? {
return castedValue as? T
}
let inputValue: Any = "this is a test"
let inputType = String.self()
let casted = cast(value: inputValue, to: inputType)
print(casted)

Answer 2

I'm not seeing what the cast at this point is for. You can write:
let controllersDictionary: [String: String] = [
"valueOne" : "bar",
"valueTwo" : "foo"
]
let identifier = controllersDictionary[value]!
let viewController = storyboard.instantiateViewController(withIdentifier: identifier)
The cast does nothing for you in the code that you have shown. viewController is typed as UIViewController, but it is the correct underlying view controller subclass thanks to polymorphism; whatever the class is in the storyboard, that's the class of this instance.
The only time you need to cast down is when you have to message an instance with a message belonging only to the subclass, and you have not shown any such need at this point in your code.

Answer 3

While there are/will be ways to make this kind of thing work, the Swifty solution (IMO) is to have your desired classes adhere to a protocol that defines the shared behavior you're trying to use, or simply use a super class they have in common
This allows the dynamism requried (in most cases at least) while still allowing the compile-time checks that prevent run time errors.
For your example,
protocol Stringable {
func toString() -> String
}
extension String: Stringable {
func toString() -> String {
return self
}
}
let thing = "foo"
let anything: Any = thing
let test: String? = (anything as? Stringable)?.toString()
Note that this requires "Any" rather than "AnyObject" since you need to cast to a protocol
Since you mentioned ViewControllers, I thought this might help:
static func createViewController<T: UIViewController>(storyboard: String, scene: String) -> T? {
return UIStoryboard(name: storyboard, bundle: nil).instantiateViewControllerWithIdentifier(scene) as? T
}

Answer 4

The x as! Y.Type syntax only works when Y is explicitly stated.
So, the compiler wants x as! NSString. The compiler crash is a bug, I suggest that you file a report.
And just think about it for a second, how would that even help you? stringClass is of type AnyClass, and you're force casting an AnyObject which already conforms to AnyClass. You should cast when you have a static type in mind, because casting doesn't really make any sense otherwise.
If you want to check, however, whether your object's class is a subclass of a particular type, you'd use:
x is Y.Type

Answer 5

Its possible so long as you can provide "a hint" to the compiler about the type of... T. So in the example below one must use : String?.
func cast<T>(_ value: Any) -> T? {
return value as? T
}
let inputValue: Any = "this is a test"
let casted: String? = cast(inputValue)
print(casted) // Optional("this is a test")
print(type(of: casted)) // Optional<String>
Why Swift doesn't just allow us to let casted = cast<String>(inputValue) I'll never know.
One annoying scenerio is when your func has no return value. Then its not always straightford to provide the necessary "hint". Lets look at this example...
func asyncCast<T>(_ value: Any, completion: (T?) -> Void) {
completion(value as? T)
}
The following client code DOES NOT COMPILE. It gives a "Generic parameter 'T' could not be inferred" error.
let inputValue: Any = "this is a test"
asyncCast(inputValue) { casted in
print(casted)
print(type(of: casted))
}
But you can solve this by providing a "hint" to compiler as follows:
asyncCast(inputValue) { (casted: String?) in
print(casted) // Optional("this is a test")
print(type(of: casted)) // Optional<String>
}