Swift Array of dictionaries extension sortInPlace

Question

Using Swift 2.1 I'm trying to make a function that sorts an array of dictionaries by the date value for the key dateKey.

I want to add this as an extension to the Array type, so I can call it using someArray.sortDictionariesByDate(dateKey: String, dateFormatter: NSDateFormatter)

extension Array where Element: CollectionType {
  mutating func sortDictionariesByDate(dateKey: String, dateFormatter: NSDateFormatter) {
    sortInPlace {
      dateFormatter.dateFormat = "yyyy-MM-dd'T'HH:mm:ss.SSSZ"
      if let dateStringA: String = ($0 as? [String: AnyObject])?[dateKey] as? String, let dateStringB = ($1 as? [String: AnyObject])?[dateKey] as? String {
        if let dateA: NSDate = dateFormatter.dateFromString(dateStringA), dateB: NSDate = dateFormatter.dateFromString(dateStringB) {
          return dateA.compare(dateB) == .OrderedAscending
        }
      }
      return false
    }
  }
}

This works fine as long as the dictionaries are typed as [String: AnyObject], but if I use it on a dictionary of type [String: String] it doesn't work since it's unable to cast String to AnyObject. I assume this is because String is a Struct not a Class. I've also tried to typecast the elements to [String: Any] instead, but then it won't work regardless of using dictionaries of type [String: AnyObject] or [String: String].

Is there any cast I can use to support dictionaries with key type String and any value type (String, AnyObject etc.), or perhaps a where clause or protocol conformance that can be added to the extension to avoid casting completely?

EDIT: Here are two example arrays as per request

var sampleArray1: [[String: AnyObject]] = [["date": "2015-10-24T13:00:00.000Z", "foo": "bar"], ["date": "2015-10-24T14:00:00.000Z", "foo": "bar"]]
sampleArray1.sortDictionariesByDate("date", dateFormatter: NSDateFormatter())
var sampleArray2: [[String: String]] = [["date": "2015-10-24T13:00:00.000Z", "foo": "bar"], ["date": "2015-10-24T14:00:00.000Z", "foo": "bar"]]
sampleArray2.sortDictionariesByDate("date", dateFormatter: NSDateFormatter())

Answer 1

First issue: you are comparing strings, not dates. Fortunately, your string's format make it directly comparable as both a string and the date value it represents. Hence, you don't need to convert it to NSDate at all.

The second issue is that typecasting Dictionary<Key,Value1> to Dictionary<Key,Value2> wholesale doesn't work, even when Value1 is covariant on Value2. It may work in trivial example like this...

let a : [String: String] = ["name": "David", "location": "Chicago"]
let b = a as [String: AnyObject]

...because the compiler can see the value type (String) in a and optimize it by compile time. For dynamic situations like yours, there's no information available ahead of time.

---

When you need dynamism, you can always go back to the old friend, Objective-C:

extension Array where Element: CollectionType {
    mutating func sortDictionariesByDate(dateKey: String) {
        self.sortInPlace {
            if let a = $0 as? NSDictionary, b = $1 as? NSDictionary {
                return (a[dateKey] as? String) < (b[dateKey] as? String)
            } else {
                return false
            }
        }
    }
}

// Example:
var sampleArray1: [[String: AnyObject]] = [
    ["date": "2015-10-24T13:00:00.000Z", "foo": "bar"],
    ["date": "2015-10-24T14:00:00.000Z", "foo": "bar"],
    ["date": "2015-10-24T10:00:00.000Z", "foo": "bar"]
]
var sampleArray2: [[String: String]] = [
    ["date": "2015-10-24T13:00:00.000Z", "foo": "bar"], 
    ["date": "2015-10-24T14:00:00.000Z", "foo": "bar"],
    ["date": "2015-10-24T10:00:00.000Z", "foo": "bar"]
]

sampleArray1.sortDictionariesByDate("date")
sampleArray2.sortDictionariesByDate("date")

Note that since you are now comparing strings rather than date, no NSDateFormatter is needed.