I'd like to add a share extension to my app (an Action app extension, I think). The extension would appear when users share a photo in the Photos app (and, ideally, Safari). If you tapped my app icon on the share sheet, iOS would pass the photo to my app and switch the user from Photos or Safari to my full app, with the shared photo(s) available for my app to work with.
I know this is possible, because Instagram (a third-party app) works exactly like this. If you look at an image in the Photos app, tap Share and then tap Instagram, iOS will background the Photos app, activate the Instagram app and let you edit and post your photo in the main Instagram app.
It seems like NSExtensionContext#open(_:completionHandler:) might do this if I add a custom URL to my main app, but the documentation for that says:
Each extension point determines whether to support this method, or under which conditions to support this method. In iOS, the Today and iMessage app extension points support this method.
That would rule out an Action, Photo Editing or Share extension. But then how does Instagram do this, and how can I achieve the same in my app?
I know that it's possible for an Action, Photo Editing or Share extension to open as a mini-app on top of the app providing the content. But coordinating the IPC for that is much, much more work (for my particular app) than just switching the user over to the app, with full access to all the functionality and data that my main app usually has access to.