> I built an exposure notification app that solved the backgrounding issue by simply using the backend to reconstruct the missing Android -> iOS piece: https://medium.com/@dangish/a-solution-to-iphone-backgroundi....
An interesting approach here, although worth noting there are some privacy implications of this, since it requires "healthy and not ill" people to share their full list of contact events (or at least healthy iOS users). And Android users in your approach would need to check in for updates to "what they missed".
One thing I'm slightly unsure of is how well this would perform in reality - did you get this to work for two long-sleeping iPhones? That seemed to be the "edge case" that was the most challenging here, and I'm not sure what you're suggesting changes anything there?