On iOS, plugins should have access to a UiViewController

[mehmetf]

In any scenario (add-to-app or greenfield apps), we need to have a way of publishing an appropriate UiViewController to plugins so that they can be used in native APIs that push other native view controllers.

The current solutions are ugly. We either have to ask the app to provide it or we have to write ugly hacks within plugin code: https://github.com/flutter/plugins/blob/master/packages/google_sign_in/google_sign_in/ios/Classes/GoogleSignInPlugin.m#L210-L224.

Let's fix it.

[gaaclarke]

[[[[UIApplication sharedInstance] delegate] window] rootViewController]?

The link in the description is dead.

[vashworth]

@stuartmorgan-g This is fixed with #174168, correct?

[stuartmorgan-g]

Yes, my #104117 was a dup of this because I didn't know it had been filed.