can anyone advise why my LaunchedEffect code block...

# compose

[james]

can anyone advise why my LaunchedEffect code block keeps triggering when coming back to my Composable, despite the key not changing? so my screen is composed, user navigates deeper, then comes back to that first screen using navigateUp().. at this point the LaunchedEffect block is triggered again. am I misunderstanding it?

code in thread ➡️

[james]

@Composable
fun MyScreen(dataId: String, viewModel: MyViewModel = hiltViewModel()) {
    ...
    LaunchedEffect(key1 = dataId) {
        viewModel.load(dataId)
    }
    ...
}

[james]

I thought that if I come back to this screen using navigateUp() that nothing has changed (and I've confirmed that dataId has not changed) and so my viewModel.load() wouldn't be called again?

[Ian Lake]

You are in a totally new composition when you come back (the previous composition was removed from the hierarchy as soon as the animation of it leaving finished)

[Ian Lake]

It sounds like you should be doing one time work in your ViewModel's init, using the arguments that are automatically available to your ViewModel when it takes a SavedStateHandle as a parameter

[Ian Lake]

https://kotlinlang.slack.com/archives/CJLTWPH7S/p1619730785154400?thread_ts=1619718599.133900&cid=CJLTWPH7S

[james]

ahh okay.. follow up question on your suggested method: that requires me to always know the value upfront, but in my UX there is a way for the user to trigger a change to that value (for the same screen) and I want to reload at that point, rather than instantiate a new viewmodel altogether

so let's say I change to your suggested method, can I do something like the following?:

class MyViewModel(savedStateHandle: SavedStateHandle) {

  var dataId: String = ""

  init {
    dataId = savedStateHandle.get("dataId")
    // call load/refresh func
  }

  fun userTriggeredUpdate(newId: String) {
    dataId = newId
    savedStateHandle.set("dataId", dataId)
    // call load/refresh func
  }
}

[james]

or is there more I need to do within that userTriggeredUpdate() method to update the data inside savedStateHandle?

[Ian Lake]

Yep, that'd work great

[Ian Lake]

You could get more fancy (having the SavedStateHandle fill in a MutableStateFlow that you then transform into a flow that you actually fill the UI from), but the idea is the same

[james]

awesome! this is really helpful, thanks a lot mate 🙏

[james]

alright I've moved it all across to what we discussed above and it works exactly as I'd hoped, thanks again Ian! 🎉