TabView and long-running effects

[vykut]

Hi,

In my application, I have a tab view with 3 screens, each one of them and the TabView having their own Reducer.
In 2 of those reducers, I'm starting long-running effects via the View.onAppear, one which is an observation of database changes via AsyncStream (i.e for recipe in database.recipes { await send(.updateRecipes(recipes) }), while the other is a network request for a random recipe, at an interval of 30 seconds.

I'm noticing that every time I'm changing between tabs, say going from tab 1 to tab 3 and back to tab 1, these "onAppear" effects are canceled every time I leave their screen, i.e tab 1 -> tab 3 => reducer 1 cancels its currently running "onAppear" effect.

Is there a way to start an effect in a reducer, for its entire lifetime? i.e start an effect in the reducer's init
If there is not, how else should I handle these long-running effects?

I was thinking I could spawn a long-running effect for each tab, in the Tab View reducer (which doesn't trigger onAppear more than once) and send actions to each child reducer.

At the same time, I think the current behavior of reducers may actually be the correct one and in fact, these observations should be canceled once the user moves to a different tab and restarted when the user comes back. In that case, I'm not sure how I should handle a long-running effect that makes a network request based on a timer (i.e fetch a random recipe every 30 seconds and display it in the UI). In this case, every time the user switches back and forth between tabs, a random recipe is shown less than 30 seconds at a time.

[mbrandonw]

I'm noticing that every time I'm changing between tabs, say going from tab 1 to tab 3 and back to tab 1, these "onAppear" effects are canceled every time I leave their screen, i.e tab 1 -> tab 3 => reducer 1 cancels its currently running "onAppear" effect.

This should not be the case if you are using onAppear. Any effect created from sending an action in onAppear is untethered from the presentation of the tab, and should continue executing regardless if it's visible or not.

Could you share a minimal demo project that shows the problem you are seeing?

[vykut]

Hi @mbrandonw,

You are right, in my app I was using the .task modifier, instead of the .onAppear, which gets canceled once the view "disappears" (i.e user switches to another tab).

However, this doesn't address my root issue, which is how can I make only one long-running effect, regardless of the presentation state of the view, or would this be an anti-pattern?
I could store a boolean in the Reducer.State i.e. isRunningOnAppearEffect and base the logic on it, combined with the "non-cancellable" .onAppear modifier. Are there better alternatives?

[mbrandonw]

Yes, you probably do not want to use .task.

I personally would recommend creating your own kind of onFirstAppear view modifier that that acts like onAppear, but fires its closure only once. You would use some local @State in the modifier to determine that condition. You can also find versions of it online if you do not want to make your own.

I would not recommend storing that state in your feature. It just isn't that important for your feature's logic, and so is fine to let the view take care of it.