How to use optional binding in switch statement in prepare(segue:) - swift

Question

In swift you can use a cool feature of the switch statement in prepare(segue:) to create cases based on the type of the destination view controller:
Example:
override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
switch segue.destination {
case let detailViewController as DetailViewController:
detailViewController.title = "DetailViewController"
}
case let otherViewController as OtherViewController:
otherViewController.title = "OtherViewController"
}
}
However, what if the segue is triggered by a split view controller, so the destination is a navigation controller, and what you really want to do is switch on the class of the navigation controller's top view controller?
I want to do something like this:
case let nav as UINavigationController,
let detailViewController = nav.topViewController as? DetailViewController:
//case code goes here
Where I have the same construct that I use in a multiple part if let optional binding.
That doesn't work. Instead, I have to do a rather painful construct like this:
case let nav as UINavigationController
where nav.topViewController is DetailViewController:
guard let detailViewController = nav.topViewController as? DetailViewController
else {
break
}
detailViewController.title = "DetailViewController"
That works, but it seems needlessly verbose, and obscures the intent. Is there a way to use a multi-part optional binding in a case of a switch statment like this in Swift 3?

Answer 1

I worked out a decent solution to this problem.
It involves doing some setup before the switch statement, and then using a tuple in the switch statement. Here's what that looks like:
override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
let dest = segue.destination
let navTopVC = (dest as? UINavigationController)?.topViewController
switch (dest, navTopVC) {
case (_, let top as VC1):
top.vc1Text = "Segue message for VC1"
case (_, let top as VC2):
top.vc2Text = "Segue message for VC2"
case (let dest as VC3, nil):
dest.vc3Text = "Segue message for VC3"
default:
break
}
}

Answer 2

You might find this extension useful…
extension UIStoryboardSegue {
var destinationNavTopViewController: UIViewController? {
return (destination as? UINavigationController)?.topViewController ?? destination
}
}
Then you can simply…
override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
switch segue.destinationNavTopViewController {
case let detailViewController as? DetailViewController:
// case code goes here
}
Note that the ?? destination makes sure the return value is non-optional, and also allows it to work in places where the destination could also be a non-navigation controller.

Answer 3

I don't think there is a way to do this with switch and case, but you can do something closer to what you are looking for with if and case (Update: as Hamish pointed out, the case isn't even needed for this scenario) or just normal if let:
override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
if let nav = segue.destination as? UINavigationController,
let detailViewController = nav.topViewController as? DetailViewController {
detailViewController.title = "DetailViewController"
}
if let otherViewController? = segue.destination as? OtherViewController {
otherViewController.title = "OtherViewController"
}
}
Since your switch statement in this example isn't really going to ever be verified by the compiler as handling all cases (because you need to create a default case), there is no added benefit to using switch instead of just if let

Answer 4

Optional binding doesn't really lend itself to switches like you're trying to do.
I understanding the desire to use switches rather than simple if and if else, but it's a bit different conceptually from what switch is meant to do.
Anyway, here are the two options I use for most situations
func prepare(for segue: UIStoryboardSegue, sender: Any?) {
switch segue.destination {
case is DetailViewController:
segue.destination.title = "DetailViewController"
case is OtherViewController:
segue.destination.title = "OtherViewController"
default:
break
}
}
or
func prepare(for segue: UIStoryboardSegue, sender: Any?) {
if let controller = seuge.destination as? DetailViewController {
controller.title = "DetailViewController"
}
else if let controller = seuge.destination as? OtherViewController {
controllercontroller.title = "OtherViewController"
}
}

Answer 5

In my opinion, you should use segue identifiers for controlling flow in this switch. But to answer your question, this should work for you.
switch (segue.destination as? UINavigationController)?.topViewController ?? segue.destination {
...
}
The thing is, according to grammar, you have only one pattern (in your case it is binding) per item in a case item list. Since you have only one item on the input but want to use two patterns, you either want to normalize input (which is in this case appropriate as you can see above) or extend item list (which is inappropriate in this case but I show an example below).
switch ((segue.destination as? UINavigationController)?.topViewController, segue.destination) {
case (let tvc, let vc) where (tvc ?? vc) is DetailViewController:
// TODO: If you await your DetailViewController in navigation or without.
break
case (let tvc as DetailViewController, _):
// TODO: If you await your DetailViewController in navigation but other vc is not in a navigation vc.
default:
fatalError()
}