Deliver callbacks to App Delegates that use fast forwarding

[tetek]

Hey,

I've been tracking down an unwanted behaviour in a SwiftUI app using Firebase.
I set my AppDelegate with UIApplicationDelegateAdaptor.

After adding Firebase I stopped receiving:

    func application(
        _ application: UIApplication,
        didRegisterForRemoteNotificationsWithDeviceToken deviceToken: Data
    )

after calling UIApplication.shared.registerForRemoteNotifications().

It turns out UIApplicationDelegateAdaptor installs SwiftUI's own SwiftUI.AppDelegate as the
application delegate, which implements only a handful of selectors itself and forwards the rest to
the app's delegate via forwardingTargetForSelector:.

With the App Delegate Proxy active (the default), createSubclassWithObject: records the original
implementation using class_getInstanceMethod(realClass, sel), which is NULL for a forwarded
selector. Adding the donor method then makes message lookup succeed, so the runtime no longer
consults the forwarding target, and each donor's trailing if (realIMP) is false — the callback
reaches the interceptors but is never delivered to the app's delegate, with no diagnostic.

This isn't limited to APNS: application:continueUserActivity:restorationHandler: and
application:openURL:options: are lost the same way for any app using the adaptor together with a
GoogleUtilities-based SDK.

This PR makes each donor fall back to the forwarding target when the original implementation is
NULL, leaving behaviour unchanged for delegates that implement the selector themselves.
Seven tests cover this: forwarded APNS callbacks, continueUserActivity, interceptors still being
notified, return values, completion handlers, merging the fetch result with interceptors, and
a forwarding target that doesn't implement the selector.
Related: firebase/firebase-ios-sdk#13566, firebase/firebase-ios-sdk#14751,
firebase/firebase-ios-sdk#10417

[daymxn]

Thanks for the PR! There's a few things that need addressing before we move forward with it, but the core idea looks solid to me.

This PR should also include a changelog entry (see this file). Just add an # Unreleased section and add an entry there (eg; like this).

[tetek]

Hey @daymxn, thanks for the review. All suggestions applied.

[paulb777]

@tetek Based on the CI run, it looks the change doesn't build on macOS. Please address.

[paulb777]

Thanks @tetek - I asked an agent to take a look and it came back with the following:

Thanks @tetek, and thanks for working through daymxn's feedback. I verified this against the real SwiftUI.AppDelegate with a minimal @UIApplicationDelegateAdaptor app on iOS 18.4, 26.5 and 27.0 simulators.

• On main: after proxyOriginalDelegateIncludingAPNSMethods, the adaptor receives only handleEventsForBackgroundURLSession, which SwiftUI implements itself. The APNS callbacks, openURL:options: and continueUserActivity are dropped, both return NO, and the fetch completion reports .failed.
• With this PR: all six reach the adaptor, both return YES, and the fetch result is .newData.

A few small things before merge:

1. Dispatch group. donor_didReceiveRemoteNotification:fetchCompletionHandler: resolves the forwarding target twice: once to decide on dispatch_group_enter, and again inside forwardSelector:. If the second lookup returns nil, the group is never left, and the system completion handler never runs. Could you enter the group, call forwardSelector: once, and dispatch_group_leave if it returns nil?
2. Test crash on regression. In testForwardingAppDelegateForwardsCompletionHandlers, if forwarding regresses, calling the stored completion handlers calls a nil block and crashes the test run. Please guard those calls, and consider asserting that the fetch result is UIBackgroundFetchResultNewData.
3. Optional test. Add one with an interceptor registered plus a forwarding delegate for didReceiveRemoteNotification:fetchCompletionHandler:, checking that the original completion runs once with the merged result.
4. PR description. Please update it (there are six tests now) and reference Swizzle breaks Application Delegates in Swift firebase/firebase-ios-sdk#13566, didRegister/didFailToRegisterForRemoteNotifications callbacks not called when FirebaseAuth is linked via SPM firebase/firebase-ios-sdk#14751 and SwiftUI and Objective-C inter-op with Swizzling firebase/firebase-ios-sdk#10417.
5. Nit. Wrap the CHANGELOG entry at 80 columns like the other entries.