Pass data to previous composable in Android Compose

Question

I will take a simple sample.

I have 2 Screens: Screen A and Screen B. From Screen A, I open Screen B. And when I return Screen B to Screen A, I want to transfer data back to Screen A.

With Android Fragment, I can use Shared ViewModel or Fragment Result API to do this. But with Android Compose, the Fragment Result Api is not in Compose. With using Shard ViewModel, what lifecycle do I have to attach Shared ViewModel so it can keep alive? Activity, ... or something else.

Or is there another way to do this?

Answer 1

If you use jetpack navigation, you can pass back data by adding it to the previous back stack entry's savedStateHandle. (Documentation)

Screen B passes data back:

composable("B") {
  ComposableB(
    popBackStack = { data ->
      // Pass data back to A
      navController.previousBackStackEntry
        ?.savedStateHandle
        ?.set("key", data)

      navController.popBackStack()
    }
  )
}

Screen A Receives data:

composable("A") { backStackEntry ->
  // get data passed back from B
  val data: T by backStackEntry
    .savedStateHandle
    .getLiveData<T>("key")
    .observeAsState()

  ComposableA(
    data = data,
    navToB = {
      // optional: clear data so LiveData emits
      // even if same value is passed again
      backStackEntry.savedStateHandle.remove("key")
      // navigate ...
    }
  )
}

Replace "key" with a unique string, T with the type of your data and data with your data.