BUILD_LIBRARY_FOR_DISTRIBUTION and IBOutlet property cannot have non-'objc' class type

After more trial and error, I find it is not directly related to more than one subclass. It is related to subclassing something from an external framework and then using it.

This is a case of defining a class in one framework and subclassing it in another. This example has 2 frameworks: "Framework 1" and "Framework 2".

First I define a subclass in Framework 1.

// Defined in Framework 1
open class ImageSubclass: UIImageView {
}

Then I use it in Framework 1.

// Used in Framework 1.  This compiles.
class MyViewController: UIViewController {
    @IBOutlet weak var imageView: ImageSubclass!
}

I switch to Framework 2 and use that class in an outlet. This also works.

// Used in Framework 2.  This compiles
class MyOtherViewController: UIViewController {
    @IBOutlet weak var imageView: ImageSubclass!
}

Still in Framework 2, I subclass what I defined in Framework 1.

// Subclass in Framework 2.
open class BetterImageSubclass: ImageSubclass {
}

Then I use that subclass in an outlet. This gives me the error you can see on line 3's comment.

// Use new subclass in Framework 2.
class MyBetterOtherViewController: UIViewController {
    // Error: @IBOutlet property cannot have non-'@objc' class type 'BetterImageSubclass'
    @IBOutlet weak var imageView: BetterImageSubclass!
}

So I can't see why using it directly makes it think it is an Objective-C class (which it is), and then subclassing it makes it NOT be an Objective-C class (which is still is).

Interestingly, if I do this I get "Explicit '@objc' on subclass of 'ImageSubclass' requires iOS 13.0.0". So this is not an option either.

@objc open class BetterImageSubclass: ImageSubclass {
}

It looks like this is iOS 13 only. When switching all projects to that as a target, the errors went away. So we'll have to stay with the lipo and friends solution for the time being.

I meet the error '@objc' static property in extension of subclass of 'SideMenuManager' requires iOS 13.0.0 after I changed BUILD_LIBRARY_FOR_DISTRIBUTION=YES.

What I've done is to extend the framework that I just made swiftmodule with a @objc static property.

I get it that it's an iOS 13 feature. My question is like what is mentioned in this original post: why I manged to compile my app before I make the framework swift module, while it fails after I do that?