Why is TopAppBar navigationIcon's not a composable function?

Question

My IDE is showing that navigationIcon is not a composable function. Other people are doing the same thing. I'm getting this error

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

@Composable
fun AppBar(onClick: () -> Unit){
    TopAppBar(
        title = "Princess World", 
        navigationIcon = { 
            IconButton(onClick = onClick) {
                Icon(imageVector = Icons.Default.Menu, contentDescription = null)
            } 
        },
    ) {}
}

I'm unable to use composable functions inside of title and navigation icon {}

@Composable
fun AppBar(onClick: () -> Unit){
   TopAppBar(title = { }, navigationIcon = { }) {

   }
}

Answer 1

You have to remove the final {}:

TopAppBar(
    title = { Text("Princess World") },
    navigationIcon = {
        IconButton(onClick = onClick) {
            Icon(imageVector = Icons.Default.Menu, contentDescription = null)
        }
    }
)

Using the {} you are trying to use the constructor with the attribute content: @Composable RowScope.() -> Unit that doesn't have the title and the navigationIcon attributes.

Answer 2

Looks like there are 2 TopAppBar composable that you have to invoke properly with their corresponding parameters.

This one,

@Composable
fun AppBar(onClick: () -> Unit) {
    TopAppBar(
        title = { Text (text = "Princess World") },
        navigationIcon = {
            IconButton(onClick = onClick) {
                Icon(imageVector = Icons.Default.Menu, contentDescription = null)
            }
        }
    ) 
}

calling this one from the API,

@Composable
    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
    ) { … }

or this one,

TopAppBar {

}

calling this from the API

@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
) { … }