Is there an easy way to call a function just when the screen is opened? :thinking_face: Detailed e...

# compose

[nglauber]

Is there an easy way to call a function just when the screen is opened? 🤔

Detailed explanation:
Using Navigation, I’m opening ScreenA which I’m using

LaunchEffect(viewModel) {
    viewModel.loadSomeData()
}

In this screen, somewhere in the future I call ScreenB. But when I go back to ScreenA, the loadSomeData() is called again…

Is there a way to avoid that?

[Luke]

As far as I know, and I'm not an expert, I don't think you can. Though, I would argue that it's the viewModel's responsibility to not trigger another load, not the UI's.

[nglauber]

But the viewModel needs some kind of signal to this… This is what I want… In the old framework, it was onCreate method.

[KamilH]

Are you getting a new instance of the ViewModel? If not (and I think you shouldn't get) then ViewModel's constructor should solve the problem

[Adam Powell]

I agree with @Luke, this sounds like a ViewModel responsibility.

private val alreadyLoaded = false

fun loadSomeData() {
  if (alreadyLoaded) return

  // ...
  alreadyLoaded = true
}

With activities and fragments you still get multiple onCreates too under the same circumstances and need to take the same approach if you really want it to only happen once.

[nglauber]

Thanks @Adam Powell. But this solution sounds like more a workaround imho… 😕

[nglauber]

In navigation using Fragments I can request the data loading using onCreate because it will not be called again when I go back to it.

[Nabeel]

Does viewmodels init{} function solves the issue?

[nglauber]

No… let me put the real situation here…
When I open the Form, I set the data.
Then, when I navigate to Country to update the country field in Form the setMyData is called again and reset the data… 😕

[nglauber]

Oh! And just to clarify… the original question I asked because the myData set to Form is loaded in Home.
So, when I close the Form and go back to Home , the data is loaded again 😞

[Ian Lake]

If you want something to be done once for the lifetime of the entire screen's existence on the back stack, that's your ViewModel for you - just use the init block

[nglauber]

thanks @Ian Lake. But as you can see in the picture above, I can’t do it using init, because HomeViewModel load some data and it needs to pass some of this data to the FormViewModel just once.

[nglauber]

For now, I did this ugly workaround which @Adam Powell mentioned… 😞

private mydata: MyData? = null

fun setMyData(d: MyData) {
   if (myData == null) {
       mydata = d
   }
}

[Ian Lake]

Ah, it all comes back to trying to pass whole objects rather than actually have a single source of truth. Right, yep, then everything is wrong and hard. Best of luck

[nglauber]

@Ian Lake I have to say that after read your message I was a little upset, because I think you could’ve expressed your idea in a more educational way.
But I want to say thank you very much (again). You made me rethink the way I was doing my implementation. Now it’s much better thanks to you 👏

[Ian Lake]

Glad you were able to come up with a better structure 👍 Do you mind sharing what you came up with here? Maybe that would help others also avoid the same stumbling blocks

[nglauber]

Sure… Basically I tried to follow the single source of truth.
• My repository becomes a singleton where I’m caching the data.
• HomeViewModel requests the data to be loaded (which is cached by the repository)
• In the FormViewModel, in the init block, I asked for the data again (which cached) and call setMyData
That was it… An elegant solution 🙂

[Ian Lake]

🎉 that does sound much better

[Shakil Karim]

@nglauber What about case when Process recreated, isn't Singleton cache will be cleared?