Android Studio Arctic Fox (Patch 3) flags, "The value true assigned to var isVisited: Boolean in the following composable is never used":
@Composable
fun MainView(navController: NavController) {
var isVisited by rememberSaveable { mutableStateOf(false) }
if (!isVisited) {
isVisited = true // never used?
navController.navigate("NextView")
}
Button(onClick = { navController.navigate("NextView") }) {
Text(text = "MainView")
}
}
while it is clearly working as intended: to prevent MainView from navigating to NextView on subsequent visits to MainView. Here's the definition of NextView:
@Composable
fun NextView(navController: NavController) {
Button(onClick = { navController.popBackStack() }) {
Text(text = "NextView")
}
}
Is Android Studio simply not recognizing variable usage across recompositions? Or is there a more idiomatic way to execute code conditionally upon recompositions? Thanks for any answer or pointer.
I'm building for API Level 31 with Kotlin 1.5.31, Compose 1.1.0-alpha06, navigation-compose 2.4.0-alpha04, lifecycle-runtime-ktx 2.4.0-rc01, though I've seen the same behavior on API Level 30, Kotlin 1.5.21, Compose 1.0.1, navigation-compose 2.4.0-alpha04, lifecycle-runtime-ktx 2.3.1. (I'd be happy to share my MainActivity where I set up NavHost with these two views or other dependency and system info if helpful.)
The studio code highlighter was designed with the traditional programming model in mind, i.e., the non-declarative View System. In that, a modern Composable would be treated as just a regular function. Now inside that function, you are creating a variable, reading it in a conditional, then updating its value. Then that value is never accessed in the scope of that function, and hence the warning "The updated value is never used".
However, we being Compose-Oriented now, know that it is a declarative paradigm and that recompositions are a thing, by the virtue of which, the function would be called over and over again and hence the updated value will, in fact, be used within that same conditional, which is why the rememberSaveable block is there to begin with. The same goes for the compiler, since you never "type the name of the variable" in the function scope after updating it, looking from the traditional way, the value is never used again, so the update is pointless to the compiler (as well as the highlighter), and hence the warning. It only makes sense to ignore it, but if you are really bugged by it, just type the name of the variable right after the whole loop, it should go away. God I was so dumb a year ago...
TOP OF THE LINE: You can safely ignore that warning, because the purpose of warnings is to prevent developers from using unnecessary system resources. Because of some internal highlighter logic error (perhaps?), it identifies a useful variable as an unused one, but since you know you are actively using it in code flow, you can just ditch that warning. Just suppress it with an annotation if it bugs you.
BODY CONTENT:-
Don't bother, it happens at times. There seems no error to me, just try deleting the line, and then re-adding it while using as much code completion as you can. If that does not help, just compile the project, and see the build log. If it does not show the warning: var 'isVisited' is never used, then you can relax about it, since it would then be a bug in the studio's code highlighter. Long as you don't receive any build time warnings, be sure that it does not pose any threat pertaining to performance or whatsoever.