swift indexOf for [AnyObject] array

Question

Trying to get the index of an array ([AnyObject]). What's the part that I'm missing?

extension PageViewController : UIPageViewControllerDelegate {
      func pageViewController(pageViewController: UIPageViewController, willTransitionToViewControllers pendingViewControllers: [AnyObject]) {
        let controller: AnyObject? = pendingViewControllers.first as AnyObject?
        self.nextIndex = self.viewControllers.indexOf(controller) as Int?
      }
    }

I have tried with Swift 1.2 this approach:

func indexOf<U: Equatable>(object: U) -> Int? {
    for (idx, objectToCompare) in enumerate(self) {
      if let to = objectToCompare as? U {
        if object == to {
          return idx
        }
      }
    }
    return nil
  }

Maybe try it the other way, by casting `self.viewControllers` as `[UIViewController]`? — Aderstedt, Aug 19 '15 at 12:22
@Aderstedt: I'm getting: `Cannot invoke 'indexOf' with an argument list of type '(AnyObject?)'`. Any idea? — el.severo, Aug 19 '15 at 12:34
As so often, the *question mark* in the error message is significant. — Martin R, Aug 19 '15 at 12:57

nhgrif · Accepted Answer · 2015-08-19T13:19:28.963

We need to cast the object we're testing to a UIViewController, since we know that are array of controllers is holding UIViewControllers (and we know that UIViewControllers conform to Equatable.

extension PageViewController : UIPageViewControllerDelegate {
    func pageViewController(pageViewController: UIPageViewController, willTransitionToViewControllers pendingViewControllers: [AnyObject]) {
        if let controller = pendingViewControllers.first as? UIViewController {
            self.nextIndex = self.viewControllers.indexOf(controller)
        }
    }
}

The logic behind the error is that in order for the indexOf method to compare the object you pass in, it must compare them using the == operator. The Equatable protocol specifies that the class has implemented this function, so this is what indexOf requires its arguments conform to.

Objective-C doesn't have this same requirement, but the actual Objective-C implementation ends up meaning that the argument is compared with objects in the array using the isEqual: method (which NSObject and therefore all Objective-C classes implement).

score 0 · Answer 2 · answered Aug 19 '15 at 12:36

0

You have to cast the viewController property to an Array object:

if let controllers = self.viewControllers as? [UIViewController] {
    self.nextIndex = controllers.indexOf(controller)
}

answered Aug 19 '15 at 12:36

Tim Specht

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@nhgrif why would that be expensive? – Tim Specht Aug 19 '15 at 12:47
Sorry... this particular part might be necessary if `self.viewControllers` isn't already declared as a `[UIViewController]` (it's not clear from the question). But you missed the part where we need to cast `controller` to a `UIViewController`, as my answer describes. – nhgrif Aug 19 '15 at 12:48
Actually, upon a second read of the question, casting the array itself is entirely unnecessary. Look at the implementation of `indexOf` that's being used. – nhgrif Aug 19 '15 at 12:53

swift indexOf for [AnyObject] array

2 Answers2