Handoff

I am developing an application for iOS and macOS and I would like to implement the Handoff feature for it. The application works such that the iOS versions stores its data locally and the macOS version should only work when the iOS version is running and on reachable by Handoff. The macOS version should pull its data from the iOS application and submit any changes to the iOS version to persist.

This is not exactly the intended use-case for Handoff and I suspect in this spike I might find it is not feasible to implement this. Maybe Handoff won't be fast enough or have high enough throughput to exchange data through it as if it was a generalized data transfer channel.

I am unable to determine the feasibility of this without implementing the feature and finding out for myself, hence this repository.

Apple documentation includes a Handoff tutorial I will be working off of, pulling in any other sources as/if needed:

https://developer.apple.com/documentation/foundation/task_management/implementing_handoff_in_your_app

I started off with iOS where I replaced the initial Text with a TextField and I bound its text to a variable on the view. On each change to the binding, I think I should update the activity. At the same time, the tutorial says to override the updateUserActivityState method of the view controller. I don't know how to get at the root view controller in Swift and according to this Stack Overflow question, it is not exactly straightforward:

https://stackoverflow.com/a/59994872/2715716

There is also this HackingSwift article about view controllers in SwiftUI:

https://www.hackingwithswift.com/books/ios-swiftui/wrapping-a-uiviewcontroller-in-a-swiftui-view

Between these two resources I think there really won't be a simpler way to just get at some default/root view controller and I will have to add this noise.

I ended up not trying this yet and instead just updating the activity instance. I still need access to the application delegate in the macOS application to be able to listen to the Handoff messages there, so I looked into how to access it in a SwiftUI application:

https://www.hackingwithswift.com/quick-start/swiftui/how-to-add-an-appdelegate-to-a-swiftui-app

This article is for iOS but the concept is the same for macOS, just using the NS prefix instead of UI. https://stackoverflow.com/a/71292001/2715716

This alone has not enabled me to receive the Handoff messages in the macOS app sent from the iOS app. It looks like I will have to figure out the view controller stuff on the iOS side.

I found one more link that looks very promising for this:

https://developer.apple.com/tutorials/swiftui/interfacing-with-uikit

• Access the view controller and override its updateUserActivityState
• Validate my updateUserActivityState override is being called
• Find out by what mechanism I should update the activity (push/pull):
  • Find out if I should pro-actively update the activity in the binding
  • Find out if I should re-actively update in updateUserActivityState
• Implement the Handoff handler in the macOS app and see if it gets called

I have also found out about Continuation Streams which seem more along the lines of what I need for a full two way communication between the iOS app and the macOS app:

https://developer.apple.com/documentation/foundation/nsuseractivity