I there a way to use Android Live-Data in Jetpack Compose Destkop?

# compose-desktop

[Frank]

I there a way to use Android Live-Data in Jetpack Compose Destkop?

[Bryan Herbst]

The only benefit of LiveData over other reactive libraries (Kotlin Flows, RxJava) is that it is aware of the Android Lifecycle.

For desktop, you probably don’t care about the Android Lifecycle and would be better off with a different library 😉

[Frank]

ok

[Frank]

I mean I have a channel which I’m sending data to

[Frank]

but this channel is not taken into consideration to update the UI

[Frank]

it is only done once

[Frank]

and then never again

[Frank]

I was looking for something like this observeAsState - function

[Frank]

but I this functionality is only in livedata

[Bryan Herbst]

Have you tried collectAsState()?
https://developer.android.com/jetpack/compose/libraries#streams

[Frank]

I’ve tried this :

vm.countChannel.consumeAsFlow().collectAsState(99)

But this works only once

[Frank]

Initially it displays 99

[Frank]

and then 1

[Frank]

but then it doesn’t change anymore

[Adam Powell]

does countChannel need to be a channel or could it be snapshot state? e.g.

var count by mutableStateOf(99)
  private set

accessing vm.count directly in your @Composable fun will observe updates to count made from elsewhere.

[Frank]

This doesn’t work

[Frank]

the value in the UI doesn’t change

[Frank]

Basically I want to observe a property in my viewmodel

[Frank]

modify it when a button is clicked and this change should reflect in the UI

[Frank]

It works with the channel the first time

[Frank]

maybe there is something wrong in my mode

[Adam Powell]

if snapshot state didn't update a compose UI when it changes then all of compose wouldn't work at all 🙂

[Frank]

In my constellation it doesn’t work. I have to check my code

[Frank]

How does this code observe my counter?

[Frank]

Ok I will check this

[Dominaezzz]

Don't forget to remember the right stuff.

[Frank]

Thanks - I will try that tomorrow. And after that I want the channel to work somehow.

[Michael Paus]

@Frank Don’t hit the Return key after every line of text here.
Use Shift-Return, like I just did, to go to a new line 😉
Otherwise you will post each line as a separate message.

[Frank]

ok

[Frank]

I’ve tried the Channel-Solution again. When I send the first value to the channel the UI get’s updated with this value. Successive values lead to an exception:

kotlinx.coroutines.JobCancellationException: StandaloneCoroutine was cancelled; job=StandaloneCoroutine{Cancelled}@47c42b90

I have no idea why this happens.

[Frank]

When I’m send a value to the channel using trySend. If I used send(value) the second call blocks forever.

I observe the channel in the composable using this line of code:

val cc: State<Int>  =   vm.countChannel.consumeAsFlow().collectAsState(0)

[Dominaezzz]

Try val cc: State<Int> = remember(vm.countChannel) { vm.countChannel.consumeAsFlow() }.collectAsState(0)

[Frank]

Holy sh… - it works! Thank you! I only need to understand your magic code now 😉 ,

[Frank]

Is there some kind of API-Documentation which explains all the different versions of “remember” - I didn’t know about this signature. I’ve never seen it before. I’ve only encountered the “simple” version like this:

val s = remember { mutableStateOf(0) }

[Frank]

So what I understood is:

The “key” in the remember-function saves the value of the current calculation function. In the next compose - cycle it checks if the value under this key is equal to the previous cycle. If so it takes the cached value - if not it executes the calculation function again,

[Dominaezzz]

That's about right

[Adam Powell]

now that you've got the mechanics up and running there are some conceptual issues to deal with in this setup that will continue to bite you down the line, so it's probably best to take a look at them sooner rather than later. 🙂 Using a Channel to funnel state from a viewmodel to a collectAsState in a composable means that observing your viewmodel is not idempotent and has side effects. Your composable isn't just observing the channel, it's consuming from it.

[Adam Powell]

Part of this is at the root of the trouble you were having before - by using Channel.consumeAsFlow you were not only consuming values from the viewmodel - changing the viewmodel as you receive those values from the channel - consumeAsFlow enforces a single collector and cancels the channel after that single collector cancels the collection of the converted flow. Recomposing without the remember meant collectAsState cancelled collection of the flow to start collection of a new one, but that cancellation also rendered the channel unusable for the future.

[Adam Powell]

and the trouble doesn't stop there; if you have two composables on the screen observing the same viewmodel, they can't both see updates to the count, since the channel is delivering values to only one of them.

[Adam Powell]

If you changed Channel.consumeAsFlow to Channel.receiveAsFlow then it won't enforce only a single collector or cancel the channel, but it will fan-out values instead, so if you have multiple composables observing that same viewmodel again, some values would go to composable A and some would go to composable B

[Adam Powell]

it's almost certain that you want to use mutableStateOf to store the count here instead. If you wanted to be more complicated about it you could use a StateFlow.

[Adam Powell]

Observers don't consume the value of observable state holders like these, making them safe for use as a source of truth in a viewmodel.

[Frank]

@Adam Powell The purpose of using a channel is to get experience in developing more complex solutions. I know that using a channel for a simple counter is a overkill. But these are the first prototyping steps to develop a MVVM-Architecture for a more complex application.

But thank you for the tip with the fan-out - I will very likely need that.

[Frank]

@Adam Powell And how can I emit a value to a StateFlow?

[Dominaezzz]

There's an emit method lol. The proper way is to set .value.

[Frank]

@Adam Powell Ok - I got the StateFlow ( MutableStateFlow ) working.

[Frank]

@Dominaezzz Thanks! I got so confused with all this stuff I didn’t see it.