test.swift

import UIKit

let attr = NSAttributedString(string: "Hello, playground", attributes: [NSBackgroundColorAttributeName:UIColor.redColor()])


var shouldBeOne = 1
attr.enumerateAttribute(NSBackgroundColorAttributeName, inRange: NSMakeRange(0, attr.length), options:[]) { (value, range, stop) -> Void in
  shouldBeOne += 1
}
print(shouldBeOne) //<- prints "1"


var shouldBeZero = 0
attr.enumerateAttribute("not a real attribute", inRange: NSMakeRange(0, attr.length), options:[]) { (value, range, stop) -> Void in
  shouldBeZero += 1
}
print(shouldBeZero) //<- prints "1"