Fix handling removal of transitioning views

[kkafar]

Summary:

Related PR in react-native-screens:

• https://github.com/software-mansion/react-native-screens/pull/2495

Additional context:

• my detailed explanation of one of the issues
• Android Developer: ViewGroup.startViewTransition docs

Background

On Android view groups can be marked as "transitioning" with a ViewGroup.startViewTransition call. This effectively ensures, that in case a view group is marked with this call and its children are removed, they will be still drawn until endViewTransition is not called.

This mechanism is implemented in Android by keeping track of "transitioning" children in auxiliary mTransitioningViews array. Then when such "transitioning" child is removed, it is removed from children array but it's parent-child relationship is not cleared and it is still retained in the auxiliary array.

Having that established we can proceed with problem description.

Problem

https://github.com/user-attachments/assets/d0356bf5-2f17-4b06-ba53-bfca659a1071

Full code

```javascript import { NavigationContainer } from '@react-navigation/native'; import React from 'react'; import { createNativeStackNavigator } from '@react-navigation/native-stack'; import { enableScreens } from 'react-native-screens'; import { StyleSheet, Text, View, FlatList, Button, ViewProps, Image, FlatListProps, findNodeHandle, } from 'react-native'; enableScreens(true); function Item({ children, ...props }: ViewProps) { return ( {children} ); } function Home({ navigation }: any) { return ( navigation.navigate('List')} /> ); } function ListScreenSimplified({secondVisible}: {secondVisible?: (visible: boolean) => void}) { const containerRef = React.useRef(null); const innerViewRef = React.useRef(null); const childViewRef = React.useRef(null); React.useEffect(() => { if (containerRef.current != null) { const tag = findNodeHandle(containerRef.current); console.log(`Container has tag [${tag}]`); } if (innerViewRef.current != null) { const tag = findNodeHandle(innerViewRef.current); console.log(`InnerView has tag [${tag}]`); } if (childViewRef.current != null) { const tag = findNodeHandle(childViewRef.current); console.log(`ChildView has tag [${tag}]`); } }, [containerRef.current, innerViewRef.current, childViewRef.current]); return ( {secondVisible && ( secondVisible(false)} />)} ); } function ParentFlatlist(props: Partial>) { return ( { if (index === 10) { return ; } else if (index === 15) { return ; } else if (index === 20) { return ; } else if (index === 25) { return ; } else { return List item {index + 1}; } }} {...props} /> ); } function NestedFlatlist(props: Partial>) { return ( ( Nested list item {index + 1} )} {...props} /> ); } function ExtraNestedFlatlist(props: Partial>) { return ( index === 4 ? ( ) : ( Nested list item {index + 1} ) } {...props} /> ); } const Stack = createNativeStackNavigator(); export default function App(): React.JSX.Element { return ( ); } export function AppSimple(): React.JSX.Element { const [secondVisible, setSecondVisible] = React.useState(false); return ( {!secondVisible && ( setSecondVisible(true)} /> )} {secondVisible && ( )} ); } const styles = StyleSheet.create({ container: { flex: 1, alignItems: 'center', justifyContent: 'center', }, nestedList: { backgroundColor: '#FFA07A', }, item: { flexDirection: 'row', alignItems: 'center', padding: 10, gap: 10, }, text: { fontSize: 24, fontWeight: 'bold', color: 'black', }, image: { width: 50, height: 50, }, }); ```

Explanation (copied from here):

I've debugged this for a while now & I have good understanding of what's going on. This bug is caused by our usage of startViewTransition and its implications. We use it well, however React does not account for case that some view might be in transition. Error mechanism is as follows:

1. Let's have initially simple stack with two screens: "A, B". This is component rendered under "B":

    <View //<-- ContainerView (CV)
      removeClippedSubviews={false}
      style={{ flex: 1, backgroundColor: 'slateblue', overflow: 'hidden' }}>
      <View removeClippedSubviews style={{ height: '100%' }}> // <--- IntermediateView (IV)
        <View removeClippedSubviews={false} style={{ backgroundColor: 'pink', width: '100%', height: 50 }} /> // <--- ChildView (ChV)
      </View>
    </View>

2. We press the back button.
3. We're on Fabric, therefore subtree of B gets destroyed before B itself is unmounted -> in our commit hook we detect that the screen B will be unmounted & we mark every node under B as transitioning by calling startViewTransition.
4. React Mounting stage starts, view hierarchy is disassembled in bottom-up fashion (leafs first).
5. ReactViewGroupManager receives MountItem to detach ChV from IV.
6. A call to IV.removeView(ChV) is made, which effectively removes ChV from IV.children, HOWEVER it does not clear ChV.parent, meaning that after the call, ChV.parent == IV. This happens, due to view being marked as in-transition by our call to startViewTransition. If the view is not marked as in-transition this parent-child relationship is removed.
7. IV has removeClippedSubviews enabled, therefore a call to IV.removeViewWithSubviewsClippingEnabled(ChV) is made. This function does effectively two things:
   1. if the ChV has parent (interpretation: it has not yet been detached from parent), we compute it's index in IV.children (Android.ViewGroup's state) and remove it from the array,
   2. remove the ChV from mAllChildren array (this is state maintained by ReactViewGroup for purposes of implementing the "subview clipping" mechanism".

The crash happens in 7.1, because ChV has been removed from IV.children in step 6, but the parent-child relationship has not been broken up there. Under usual circumstances (this is my hypothesis now, yet unconfirmed) 7.1 does not execute, because ChV.parent is nulled in step no. 6.

Rationale for startViewTransition usage

Transitions. On Fabric, when some subtree is unmounted, views in the subtree are unmounted in bottom-up order. This leads to uncomfortable situation, where our components (react-native-screens), who want to drive & manage transitions are notified that their children will be removed after the subtrees mounted in screen subviews are already disassembled. If we start animation in this very moment we will have staggering effect of white flash (issue) (we animate just the screen with white background without it's children). This was not a problem on Paper, because the order of subtree disassembling was opposite - top-down. While we've managed to workaround this issue on Fabric using MountingTransactionObserving protocol on iOS and a commit hook on Android (we can inspect mutations in incoming transaction before it starts being applied) we still need to prevent view hierarchy from being disassembled in the middle of transition (on Paper this has also been less of an issue) - and this is where startViewTransition comes in. It allows us to draw views throughout transition after React Native removes them from HostTree model. On iOS we exchange subtree for its snapshot for transition time, however this approach isn't feasible on Android, because snapshots do not capture shadows.

Possible solutions

Android does not expose a method to verify whether a view is in transition (it has package visibility), therefore we need to retrieve this information with some workaround. I see two posibilities:

• first approach would be to override startViewTransition & endViewTransition in ReactViewGroup and keep the state on whether the view is transitioning there,
• second possible approach would be as follows: we can check for "transitioning" view by checking whether a view has parent but is not it's parent child (this should be reliable),

Having information on whether the view is in transition or not, we can prevent multiple removals of the same view in every call site (currently only in removeViewAt if parent.removeClippingSubviews == true).

Another option would be to do just as this PR does: having in mind this "transitioning" state we can pass a flag to removeViewWithSubviewClippingEnabled and prevent duplicated removal from parent if we already know that this has been requested.

I can also add override of this method:

  /*package*/ void removeViewWithSubviewClippingEnabled(View view) {
    this.removeViewWithSubviewClippingEnabled(view, false);
  }

to make this parameter optional.

Changelog:

[ANDROID] [FIXED] - Handle removal of in-transition views.

Test Plan:

WIP WIP

[tdn120]

This is interesting! It looks like there could certainly be issues with the current code largely ignoring transitions.

The current commit looks a little too simple, since this is the only call to removeViewWithSubviewClippingEnabled() in the library. So we'd effectively never be running the subview clipping logic on view removal.

The "first approach" seems like it should work, as long as the implementation is careful not to hold onto views after they're otherwise removed.

I'm not quite clear on how you envision the second approach - where/how would you be checking parent vs. child?