Using Swift Enums to deal with Storyboard Segues

I like when the code is explicit. Unlike Brent Simmons, I don’t feel comfortable using string literals in code especially not when I’m still fleshing things out and change values in Interface Builder a lot.

In Objective-C, I would’ve used plain old constants. For storyboard segues, for example:

NSString const * kSegueToThere = @"goThere";
NSString const * kUnwindSegueFromThere = @"unwindFromThere";

I usually ended up writing methods like isUnwindSegueForXYZ: to get rid of the repeating string equality check noise. That’s a lot more boilerplate code. Both don’t read well.

- (void)prepareForSegue:(UIStoryboardSegue *)segue {

    if ([segue.identifier isEqualToString:kSegueToThere]) {
        // ...
    } else if ([self isUnwindSegueFromThere:segue]) {
        // ...
    }
}

Brent has a point, though: it doesn’t improve readability a lot, especially since I adopt naming conventions which are telling the intent: uppercase identifiers designate a view controller scene; lowercase identifiers designate a segue; lowercase identifiers starting with “unwind”, well, guess what.

With Swift, I now have a lot more readable way to group constants: enums.

enum Destination: String {
    case There = "showThis"
    case Elsewhere = "showThat"
    
    init?(_ segue: UIStoryboardSegue) {
        self.init(rawValue: segue.identifier!)
    }
}

In prepareForSegue(_:,sender:), where these values are used the most, I end up with this:

override func prepareForSegue(segue: UIStoryboardSegue, sender: AnyObject?) {
    
    switch Destination(segue)! {
    case .There: // ...
    case .Elsewhere: // ...
    }
}

I’m very happy with that.

I feel comfortable force-unwrapping the enum in some cases. It’s okay when there should be no unknown case. If that’s not your favorite way to create runtime errors, using the new guard statement in Swift 2.0 will produce a much more lenient alternative.

Update 2016-04-25: Protocol extensions as mixins to the rescue, you can make things even better with enums + associated values.

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