TopAppBar @composable invocations can only happen from the context of an @composable function-CodePudding

I was trying to add TopAppBar. When I tried to add the following code, I got

@composable invocations can only happen from the context of an @composable function** error.

@Composable
fun appTopBar(name: String) {
    val scaffoldState = rememberScaffoldState(rememberDrawerState(DrawerValue.Closed))
    Scaffold(
        scaffoldState = scaffoldState,
        topBar = {
            TopAppBar(
                title = { Text(text = name) },
                backgroundColor = MaterialTheme.colors.primary
            ) {
                
            }
        }, content = {

        })
}

But when I tried to delete the last {} of TopAppBar, the error was resolved. Like the following code.

@Composable
fun appTopBar(name: String) {
    val scaffoldState = rememberScaffoldState(rememberDrawerState(DrawerValue.Closed))
    Scaffold(
        scaffoldState = scaffoldState,
        topBar = {
            TopAppBar(
                title = { Text(text = name) },
                backgroundColor = MaterialTheme.colors.primary
            )
        }, content = {

        })
}

My question is what is the difference between TopAppBar(){} and TopAppBar(). {} is generated automatically by the Android Studio.

CodePudding user response：

Kotlin trailing lambda can only be used with the last lambda argument.

When you're specifying it for TopAppBar, the only possible variant doesn't have title argument:

@Composable
fun TopAppBar(
    modifier: Modifier = Modifier,
    backgroundColor: Color = MaterialTheme.colors.primarySurface,
    contentColor: Color = contentColorFor(backgroundColor),
    elevation: Dp = AppBarDefaults.TopAppBarElevation,
    contentPadding: PaddingValues = AppBarDefaults.ContentPadding,
    content: @Composable RowScope.() -> Unit
) ...

On the other hand, TopAppBar which accepts title doesn't have a last lambda argument:

fun TopAppBar(
    title: @Composable () -> Unit,
    modifier: Modifier = Modifier,
    navigationIcon: @Composable (() -> Unit)? = null,
    actions: @Composable RowScope.() -> Unit = {},
    backgroundColor: Color = MaterialTheme.colors.primarySurface,
    contentColor: Color = contentColorFor(backgroundColor),
    elevation: Dp = AppBarDefaults.TopAppBarElevation
) ...

It was generated by AS, because of a bug. I've reported it.