`UIApplication.willEnterForegroundNotification` is called each time an app enters the foreground on iOS, but only once on macCatalyst.

[michaellenaghan]

This isn't necessarily a bug, but it may be unexpected: UIApplication.willEnterForegroundNotification is called each time an app enters the foreground on iOS, but only once on macCatalyst.

https://github.com/TelemetryDeck/SwiftClient/blob/3f8bd438c8681ce7ccdf035b529cb5c4cd82cd7b/Sources/TelemetryClient/TelemetryClient.swift#L145

That means that an iOS client will generate a new session each time the app is activated, while a macCatalyst client will generate a new session each time the app is launched — even if it's left running for days, weeks or months. Again, not necessarily a bug, but someone who's focusing on session stats might think there's a lot less Mac usage than there really is.

It might also throw off TelemetryDeck's Daily Active Users and User Retention stats?

Here's a suggestion:

• On macCatalyst, observe UIScene.willEnterForegroundNotification rather than UIApplication.willEnterForegroundNotification
• Generate a new session whenever a) the current Y/M/D is different from the previous Y/M/D; or b) there's no previous Y/M/D

That would treat each launch and each additional day as a new session (if the app is activated by the user on that day).

[winsmith]

Love that idea!

[michaellenaghan]

TL;DR

On macCatalyst, observe NSWindowDidBecomeMainNotification rather than UIScene.willEnterForegroundNotification. Since that's an NSWindow notification name, and since NSWindow notification names aren't available in macCatalyst without special effort, subscribe using the stringified name, like this:

        self.token = NotificationCenter.default.addObserver(
            forName: Notification.Name("NSWindowDidBecomeMainNotification"), 
            object: nil, 
            queue: nil
        ) {
            notification in
            ...
        }

Background

macCatalyst sends out a number of UIScene.willEnterForegroundNotification notifications — all undocumented. Here's just one example: a popover sends a UIScene.willEnterForegroundNotification notification with a _UIPopoverScene object when it's presented. Observing NSWindowDidBecomeMainNotification is much simpler (and more reliable) than figuring out which notifications to pay attention to, and which to ignore.

[winsmith]

This is very compatible, doesn't break on other systems, and does what is needed, fantastic! I'm adding this to the roadmap – unless you'd like to submit a PR?

[michaellenaghan]

Well, it depends what you mean by "break." Stats would change. Since it takes time for people to update, that change would happen over a period of time rather than all at once, making it a bit harder to "see" and reason about.

If you don't think that's a problem I think I could submit a PR? If you do, we should talk it through a bit.

The "breaking" thing is more obvious with something like #106 where old data will have the wrong values, etc. (I'm aware of yet another wrong value that I haven't reported yet.) One option: bundle this change and those changes into a bigger release with a new major version number. The question is: would there be any effort to continue to support the old behaviour, under a flag of some kind, or would there just be a clean break…?

[winsmith]

I think that's fine. With "breaking" I meant more something like the app crashing because a notification type is not supported on macOS.

I know its been a minute, but if you're up for it I'd love the PR.