Method load() defines Objective-C class method 'load', which is not permitted by Swift 1.2

Question

I'm using Parse and I'm creating a PFObject subclass conforming to the protocol PFSubclassing! It was working all fine, but now I'm using Swift 1.2 and it gives me this error:

1. override class func load() {
2.      self.registerSubclass()
3. }

On line 1: Method 'load()' defines Objective-C class method 'load', which is not permitted by Swift 1.2

Anyone have this problem yet? How can I fix?

Answer 1

There is an NSHispster article about method swizzling that touches on this in different context:

Unfortunately, a load class method implemented in Swift is never called by the runtime, rendering that recommendation an impossibility. Instead, we're left to pick among second-choice options:

• Implement method swizzling in initialize. This can be done safely, so long as you check the type at execution time and wrap the swizzling in dispatch_once (which you should be doing anyway).

• Implement method swizzling in the app delegate. Instead of adding method swizzling via a class extension, simply add a method to the app delegate to be executed when application(_:didFinishLaunchingWithOptions:) is called. Depending on the classes you're modifying, this may be sufficient and should guarantee your code is executed every time.

Link: http://nshipster.com/swift-objc-runtime/

-

More info form dev forums:

Swift 1.1 allowed you to define "+load" methods with "class func load()", but they were not actually run at startup time of your app like Objective-C +load methods are. Swift 1.2 bans them to avoid the impression that this might work.

Link: https://devforums.apple.com/message/1102025#1102025

-

tl;dr initialize() and didFinishLaunchingWithOptions seem to be decent places for such things in Swift.

Answer 2

Try this :

override class func initialize() {
   var onceToken : dispatch_once_t = 0;
   dispatch_once(&onceToken) {
      self.registerSubclass()
   }
}

The Parse documentation has been updated : https://www.parse.com/docs/ios/guide#objects-subclassing-pfobject

Answer 3

I'm calling registerSubclass() method in AppDelegate before Parse.setApplicationId for every subclass of PFObject and it works.

Answer 4

Overriding load() never worked with Swift. Earier it was simply not called. I filed a bug for Apple back then (Bug ID 18423731), and recently I got a response that the issue has been addressed by explicitly informing the developer that this is not allowed in Swift.

extension UIButton {
    // !! never called
    override public class func load() { // Method 'load()' defines Objective-C class method 'load', which is not permitted by Swift 1.2
        super.load()
        println("not called earlier anyway");
    }
}

So.... don't. Even if the documentation says otherwise.

Answer 5

I got it working by replacing it with:

override class func initialize() {
}