[UIScene] Using implicit engine misses willConnectScene event #186547

Steps to reproduce

This came up when reading:

For didFinishLaunching

The old behavior before UIScene migration is that, if FlutterViewController is created programmatically, even if it's created as child of the rootVC (like what this integration test is doing), plugins still won't receive didFinishLaunching event, because rootVC's viewDidLoad is called after didFinishLaunching returns.

However, if FlutterViewController is created from storyboard (again, as child of the rootVC), plugins will receive didFinishLaunching event.

This behavior is odd, but since it's been this way, we should keep it untouched.

For sceneWillConnect

First of all, there's no original behavior before UIScene migration (sceneWillConnect is only applicable after UIScene migration)

From my observation, the current behavior for FlutterViewController created as the child of the rootVC:

• If FlutterViewController is created programmatically:
  • If engine is created in app delegate's didFinishLaunching, plugins will receive sceneWillConnect
  • If engine is created afterwards, plugins won't receive sceneWillConnect
• If FlutterViewController is created from Storyboard, then plugins will always receive sceneWillConnect.

(for both cases above, FlutterViewController is created as a child of the rootVC)

This behavior is a bit confusing and can be brittle:

• If a developer makes FlutterViewController a child of the rootVC, the intention is to receive sceneWillConnect, regardless whether they use storyboard or not.
• The fact that Apple changed storyboard loading order once (and caused so much headache) hinted that the current setup is quite brittle, and could very well cause more headache down the road.

Proposal

The only factor that matters here is: "when is FlutterViewController added to the scene?" - if it's added in the same run-loop as scene connection, we should pipe sceneWillConnect; if it's added afterwards, we should not pipe sceneWillConnect. Using storyboard or not should be irrelevant.

In this particular case, where a developer makes FlutterViewController a child of the rootVC, we should always pipe sceneWillConnect, regardless of using storyboard or not.

Why not just check if VC is added before sceneWillConnect is called?

Because for some scenarios (e.g. SwiftUI), VC is added after sceneWillConnect is called. But it still happens in the same run-loop.

Can we just use scene connection's notification?

Note that scene connection notification is sent after sceneWillConnect is called, but before viewDidLoad of rootVC is called. So if we rely on the notification will also miss some edge cases.

The simplest approach is to rely on willEnterForeground and didEnterBackground, which is called after view is loaded, which is the best timing to capture this edge case. The drawback would be that it's called with a delay. However, it's still on the same run-loop (just like the connection notification path).