ios 15 prewarming causing appwillLaunch method when prewarm is done

Question

We had an app in production which was reporting very high time to interact(tti) for ios 15 prewarm.

TTI = timewhenViewController is loaded - mainStartTime

mainStart time is measured inside AppDelegate.swift's willFinishLaunchingWithOptions method like this

mainStartTime = Date()

and when first view controller is loaded we measure tti as

tti = -(mainStartTime.timeIntervalSinceNow)

We observed that for prewarm scenarios mainStartTime was coming very early (approx 2 hours before user even launches the app).

I checked online but found no documentation. Just wanted to know can it possibly happen that prewarm is calling willFinishLaunchingWithOptions method while prewarming the app.

Answer 1

Apple's documentation is incorrect, here is the behaviour observed on iOS 15:

UIApplicationMain() always runs, including during prewarming.

What happens after this depends on whether your app uses the UIScene life-cycle.

• For apps that do support scenes:

  • application:didFinishLaunchingWithOptions: may be called (doesn't always happen)
  • scene:willConnectToSession:options: is not called - in fact the SceneDelegate is not created until the app is opened.

• For apps that do not support scenes:

  • application:didFinishLaunchingWithOptions: is not called.