How to iterate a Swift [AnyObject] array with a downcast?

If the array of [AnyObject] is supposed to be entirely composed of the type you want, you can do a en-mass cast:

func tableView(tableView: NSTableView, sortDescriptorsDidChange oldDescriptors: [AnyObject]) {
    let currentSort = tableView.sortDescriptors as? [NSSortDescriptor]
    for s in currentSort ?? [] {
        println("keyPath: \(s.key()), ascending: \(s.ascending)")
    }
}

This uses [] to substitute an empty array for the nil you get if it turns out one or more of the AnyObject were not of the right type – you’d still need an if…else if you wanted explicit error handling rather than silent failure.

Alternatively if you want your program to trap in case of failure, you could use as! but then it’s a bit of a shame if you find this out in production.

By the way, if the fact that you have to come up with a new name for the unwrapped value was part of what was bothering you, you can just give it the same name:

let x: Int? = 1
if let x = x {
    println(x)
}

which declares a local unwrapped x which masks the outer x within the if.

If some but not all might be of the right type and you only want to operate on those, mapSome is a handy array extension to have around:

extension Array {
    func mapSome<U>(transform: T->U?) -> [U] {
        return lazy(self).map(transform)
            .filter { $0 != nil }
            .map { $0! }.array
    }
}

func tableView(tableView: NSTableView, sortDescriptorsDidChange oldDescriptors: [AnyObject]) {
    let currentSort = tableView.sortDescriptors.mapSome { $0 as? NSSortDescriptor }
    for s in currentSort {
        println("keyPath: \(s.key()), ascending: \(s.ascending)")
    }
}