Subclassing PFObject

Armor.h

@interface Armor : PFObject<PFSubclassing>
+ (NSString *)parseClassName;
@end

Armor.m

// Import this header to let Armor know that PFObject privately provides most
// of the methods for PFSubclassing.
#import <Parse/PFObject+Subclass.h>

@implementation Armor
+ (void)load {
  [self registerSubclass];
}

+ (NSString *)parseClassName {
  return @"Armor";
}
@end

Armor.swift

class Armor : PFObject, PFSubclassing {
  override class func initialize() {
    struct Static {
      static var onceToken : dispatch_once_t = 0;
    }
    dispatch_once(&Static.onceToken) {
      self.registerSubclass()
    }
  }

  static func parseClassName() -> String {
    return "Armor"
  }
}

Swift-Bridging-Header.h

// Import this header into your Swift bridge header file to
// let Armor know that PFObject privately provides most of
// the methods for PFSubclassing.
#import <Parse/PFObject+Subclass.h>

If I try to subclass with Swift 4 I get "Method 'initialize()' defines Objective-C class method 'initialize', which is not permitted by Swift"