NSInvalidArgumentException unrecognized selector crash due to OverlayScrollViewDelegateProxy deallocation issue

[bshuttkg]

Describe the bug

Memory deallocation issue in the OverlayScrollViewDelegateProxy.

Environnement

• Device: iPhone 16 Pro Simulator
• OS: iOS 18.2
• OverlayContainer Version: aa7bd20

Description

As you know, the proxy sets itself as the UIScrollViewDelegate:

scrollView.delegate = self

For our use case, the scrollView is a UITableView; so the delegate is a UITableViewDelegate.
As a result, it will be sent messages like tableView:viewForHeaderInSection:.

The framework does consider this, as we see in the following code to forward on other messages:

override func responds(to aSelector: Selector!) -> Bool {
    let originalDelegateRespondsToSelector = originalDelegate?.responds(to: aSelector) ?? false
    return super.responds(to: aSelector) || originalDelegateRespondsToSelector
}

override func forwardingTarget(for aSelector: Selector!) -> Any? {
    if originalDelegate?.responds(to: aSelector) == true {
        return originalDelegate
    } else {
        return super.forwardingTarget(for: aSelector)
    }
}

where:

private weak var originalDelegate: UIScrollViewDelegate?

But there is a bug here, a few:

1. In ARC in Swift: Basics and Beyond, we see how Swift is non-lexical. Above, in forwardingTarget(for:), originalDelegate may respond to a selector and be deallocated when it is returned (because it is weak)
2. originalDelegate may cause responds(to:) to return true, but have been deallocated in forwardingTarget(for:)

The second is why we are seeing crashes in our app.
Just before the crash, forwardingTarget(for:) is called with selector tableView:viewForHeaderInSection: and the originalDelegate is nil.

Returning nil for this scenario in forwardingTarget(for:) does not stop the invocation.

So we are experiencing the following crash in our app:

'NSInvalidArgumentException', reason: '-[OverlayContainer.OverlayScrollViewDelegateProxy tableView:viewForHeaderInSection:]: unrecognized selector sent to instance'

It looks to me like you might have done the same as this SO answer.
Currently, I don't think it's a correct implementation of a proxy in Swift for weak references.

[gaetanzanella]

Hi @bshuttkg, thank you for the feedback!

Could you try to make a PR for it? 🙏
There is a test and it sounds like you understand the problem :)

[bshuttkg]

Hey @gaetanzanella, thank you for getting back to me 🙏 .

Currently, I'm not sure that there is a Swift solution for the selector proxy pattern we are attempting here.
This might be an Objective-C thing, but I would like to be wrong about that.

I've tried returning nil in forwardingTarget(for:) when the originalDelegate is deallocated, but the invocation seems to be invoked anyway.

If Objective-C was an option (probably not) then:

• This blog discusses how this might be achieved using NSObject
• WeakForwarder is an implementation using NSProxy

[gaetanzanella]

Did you succeed to create a test to reproduce the issue? in OverlayScrollViewDelegateTest