[iOS API breaking change] Make splashScreenView of FlutterViewController nullable

[zhongwuzw]

Based on the talk of https://github.com/flutter/engine/pull/34496/files#r923640588, we should make splashScreenView nullable, then Swift users can remove splashScreenView if they set nil.

But it lead to an API-breaking change for Swift users, if Swift users use the getter of splashScreenView, the return type changed from UIView to UIView? .

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[cyanglaz]

LGTM

I'm not familiar with the breaking change process in the engine repro.

@zanderso can you (or someone in the engine team) review for the breaking change process?

[zanderso]

What will this change break?

[cyanglaz]

What will this change break?

see #34496 (comment)

[jmagman]

What will this change break?

@zanderso Specifically it would be a compilation error if the calling code is in Swift and gets the splash screen and stores in a nonnull UIView variable.

error build: Value of optional type 'UIView?' must be unwrapped to a value of type 'UIView'

They would need to update their code to instead store in a UIView?. I imagine this would be very rare, but possible in an add-to-app scenario.

@UIApplicationMain
@objc class AppDelegate: FlutterAppDelegate {
  var splashScreen = UIView()
  override func application(
    _ application: UIApplication,
    didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?
  ) -> Bool {
    var flutterEngine = FlutterEngine(name: "my flutter engine")
    let flutterViewController = FlutterViewController(engine: flutterEngine, nibName: nil, bundle: nil)
    splashScreen = flutterViewController.splashScreenView // compilation error: Value of optional type 'UIView?' must be unwrapped to a value of type 'UIView'

More likely the add-to-app developer is just setting the view. Storing it with let also shouldn't cause issues.

@UIApplicationMain
@objc class AppDelegate: FlutterAppDelegate {
  override func application(
    _ application: UIApplication,
    didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?
  ) -> Bool {
    var flutterEngine = FlutterEngine(name: "my flutter engine")
    let flutterViewController = FlutterViewController(engine: flutterEngine, nibName: nil, bundle: nil)
    let splashScreen = flutterViewController.splashScreenView // compiles successfully

[zanderso]

Thanks @jmagman. It sounds like this would just need a small migration guide.

[zhongwuzw]

@zanderso I'm not familiar with the breaking change process :), I make a migration guide PR, can you help me to review that?

[jmagman]

LGTM, I'll make sure flutter/website#7385 deprecation notice gets merged.