Declare a static variable in a function

[Swift] GetCountryNames

class func countryNames() -> [String] {
        struct Static {
            static var instance: [String]?
        }
        if let names = Static.instance {
            return names
        }
        
        var names = [String]()
        for code in NSLocale.ISOCountryCodes() as [String] {
            let identifier = NSLocale.localeIdentifierFromComponents([NSLocaleCountryCode: code])
            let name = NSLocale.currentLocale().displayNameForKey(NSLocaleIdentifier, value: identifier)
            if name != nil {
                names.append(name)
            }
        }
        Static.instance = names.sorted{ (name1: String, name2: String) -> Bool in
            switch name1.localizedCaseInsensitiveCompare(name2) {
            case .OrderedAscending: return true
            default: return false
            }
        }
        return Static.instance!
    }