iOS. ComposeUIViewController. Dispose composition on viewDidDisappear

[elijah-semyonov]

Proposed Changes

Revert memory leak fix PR
Implement eager dispose on viewDidDisappear, reconstruct scene on viewWillAppear

Testing

Test: same as reverted PR

API Changes

ComposeUIViewController now disposes the composition on viewDidDisappear and reconstructs it on viewWillAppear.
Clients should keep the important state outside of the composition and composition should subscribe to it:

class MyScreenState {
    var title by mutableStateOf("Title")
        private set
}

val state = MyScreenState()
val viewController = ComposeUIViewController {
    Text(state.title)
}

[igordmn]

I wanted to add saving/restoring state via rememberSaveable, but it appears that Android doesn't save the state on attach/detach:

import android.os.Bundle
import android.view.View
import androidx.activity.ComponentActivity
import androidx.activity.compose.setContent
import androidx.compose.foundation.layout.Column
import androidx.compose.material.Button
import androidx.compose.material.Text
import androidx.compose.runtime.getValue
import androidx.compose.runtime.mutableStateOf
import androidx.compose.runtime.remember
import androidx.compose.runtime.saveable.rememberSaveable
import androidx.compose.runtime.setValue
import androidx.compose.ui.platform.ComposeView
import kotlinx.coroutines.Dispatchers
import kotlinx.coroutines.GlobalScope
import kotlinx.coroutines.delay
import kotlinx.coroutines.launch

class MainActivity : ComponentActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)

        val view = ComposeView(this).apply {
            setContent {
                val savedState = rememberSaveable { Foo("Not saved") }
                var text by remember { mutableStateOf(savedState.text) }

                Column {
                    Text(text)

                    Button(
                        onClick = {
                            savedState.text = "Saved"
                            text = "Saved"
                        }
                    ) {
                        Text("Save")
                    }
                }
            }
        }
        setContentView(view)

        GlobalScope.launch(Dispatchers.Main) {
            delay(8000)
            setContentView(object : View(this@MainActivity) {})

            delay(2000)
            setContentView(view)
        }
    }
}

data class Foo(var text: String) : java.io.Serializable

[igordmn]

cc @dima-avdeev-jb , we now drop the Compose state when the UIViewController dissapears, and don't restore it when the controller appears. This is how Android behaves by default (see above). It also supports other cases via ViewCompositionStrategy, but we can support them later (if it is possible, there are complications).

Check also, the API Change section.

I will merge it now to build dev and beta02 faster. Let me know, if the change is needed to be explained.