Hey everyone Sorry, if this is not the right channel to ask this. I'm facing a weird issue with Pag...

# coroutines

[Vaibhav Jaiswal]

Hey everyone
Sorry, if this is not the right channel to ask this.

I'm facing a weird issue with PagingData's cachedIn() flow operator
• If I have a cachedIn() operator before catch {} operator, catch operator does not work and the flow throws an exception and my app crashes
• If i have catch{} before cachedIn(), the flow never completes, and nothing is displayed in my UI
It all works fine, if I dont have cachedIn() at all

This is a small test I've written to verify it

test("Should not crash"){
    val flow = flow<PagingData<Int>> { throw IllegalStateException("Testing") }
    shouldNotThrowAny {
        flow.cachedIn(this).catch {  }.collect()
    }
}

It passes if I remove cachedIn() , but fails with it
and test never completes if I put cachedIn() after catch

[Ian Lake]

That all sounds WAI - Paging has its own error reporting mechanisms, so catch after cachedIn is never going to fire and cachedIn transforms your Flow into essentially a State flow, which never terminates, so it is expected that the collect would never end

[Ian Lake]

You'll actually never get any exception from a real Flow<PagingData> produced by an actual Pager, so what you're testing doesn't actually test any real scenario at all

[Vaibhav Jaiswal]

Pager wont ever throw an exception
But we can have exceptions in the PagingData transformers, or in some flow operator

[Vaibhav Jaiswal]

My app was crashing because there was an exception thrown inside a mapLatest {} operator

[Ian Lake]

Yeah, sounds like catch before cachedIn would be the right tool to avoid crashing the whole app, but tbh, I'd probably just fix your mapLatest

[Vaibhav Jaiswal]

Yeah, ideally there should not be any exception at all
But if there is for any reason or I intentionally throw some exception, catch should catch it

[Vaibhav Jaiswal]

Also, in case of multiple cachedIn(), if I have multiple .combine(anotherFlow) and cachedIn() before each (as app crashes if I dont add cachedIn)

How can i add a catch before, it will always be after some cachedIn()

[myanmarking]

forget the catch. If there is indeed a but in the map operator, the expected result would be to crash. its fine

[Vaibhav Jaiswal]

But i dont want app to crash, why would anyone want app to crash
I want to show a error UI or toggle a snackbar etc, enabled from the catch operator