Fix - TopAppBar flickers when moving from destination to destination

[mahmed1987]

Issue currentTopLevelDestination is null momentarily , causing the TopAppBar to momentarily go out of the composition tree while navigating the top level destinations. In short the problem lies here

val currentTopLevelDestination: TopLevelDestination?
        @Composable get() {
            return TopLevelDestination.entries.firstOrNull { topLevelDestination ->
                Log.d("currentDestination","$currentDestination")
                currentDestination?.hasRoute(route = topLevelDestination.route) ?: false
            }
        }

The log statement above would confirm that while switching between the top level composables , the currentDestination is obtained as null at times , until it is not .

I dont have the exact reasoning as to why that is. My hypothesis is that since there is no "gate" between the currentDestination composable and the currentTopLevelDestination , currentTopLevelDestination reacts to everything that occurs to currentDestination and perhaps currentDestination goes through a null while going from one destination to another.

Solution

I just gated the currentTopLevelDestination with a remember .

Before After

[Songgyubin]

@mahmed1987 I have confirmed that the problem you mentioned has been resolved.

However, I think there is something that needs more thought about merging into the code.

1. What was the fundamental problem?
2. Is remember really a fundamental solution rather than a temporary measure?
3. Are there any problems such as memory leaks by using remember?

[mahmed1987]

@Songgyubin tbh I am not fully certain what is at play here.

Like I wrote in the PR description , this appears to be some situation that is occurring perhaps because of the compose state currentDestination is being read inside of a firstOrNull lambda ?

This appears to be a temporary fix until an explanation has been made available by the compose team .

[trambui09]

Closing this PR as we have a fix for this: https://github.com/android/nowinandroid/pull/1728