So I have a Composable with a VM and I would like to send a

Question

So I have a Composable with a VM and I would like to send a callback to the parent composable when something happens in the VM something like ``` Composable fun FunA onResult Result gt Unit val vm by viewModels val resultState by viewModel onResult observeAsState val result = resultState if result = null onResult result val state = vm state observeAsState ``` but I don t want to call `onResult` at every composition only when `onResult` changes Any idea of how to implement that

Colton Idle · Answer

I think this is maybe a good candidate for something to be modelled as state see <https developer android com topic architecture ui layer events>

Colton Idle · Answer

2023 04 25 at 08 55 46 png

Colton Idle · Answer

but i dont understand your use case 100 so maybe its not the right answer but when my VM typically does some operation i want to update some state to signify that that operation succeeded etc and the compose layer just observes that and draws it or uses a LaunchedEffect to do something like navigate thats actually the example they show in the docs above on successful login you dont just send an event back to the composable you instead update some state and the composable observes that

Zun · Answer

I feel like you re putting too much business logic in the compose world You can have the parent compose be notified through your ViewModel as well Hoisting state out of this child composable which uses a magic ViewModel makes little sense

mattinger · Answer

Also you don t want the onResult out in open like that It s a side effect that belongs in some sort of side effect block To be honest i would use a LaunchedEffect Otherwise you re going to be sending the callback on every recomposition ```LaunchedEffect viewModel onResult observeAsState result let onResult it ```

juliocbcotta · Answer

Thank you all I learned a lot grin

juliocbcotta · Answer

I sent a null value to the LiveData so the next state change will not trigger the callback But I still don t understand the need of using LaunchedEffect

Colton Idle · Answer

long but good <https www youtube com watch v=CQzuFZjiUMs>

Colton Idle · Answer

manuel vivo talks about how events should be treated he wrote helped write the guide i linked to earlier