I'm learning Compose, the following code is from the article.

The author use var toolbarTitle by remember { mutableStateOf("Home") } only for a title, is it necessary ?

I think var toolbarTitle= mutableStateOf("Home") is enough, right?

Source Code

class MainActivity : ComponentActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContent {
            JetpackComposeScaffoldLayoutTheme {
                // A surface container using the 'background' color from the theme
                Surface(color = MaterialTheme.colors.background) {
                    var toolbarTitle by remember {
                        mutableStateOf("Home")
                    }
                    val scaffoldState =
                        rememberScaffoldState(rememberDrawerState(initialValue = DrawerValue.Closed))
                    val scope = rememberCoroutineScope()

                    Scaffold(
                        modifier = Modifier.background(Color.White),
                        scaffoldState = scaffoldState,
                        topBar = {
                            AppToolbar(
                                scaffoldState = scaffoldState,
                                scope = scope,
                                toolbarTitle = toolbarTitle
                            )
                        }, drawerContent = {
                            DrawerContent(scaffoldState = scaffoldState, scope = scope)
                        },
                       ...
                    )
                }
            }
        }
    }
}

CodePudding user response：

If you don't use remember the value will reset again to Home on every recomposition by using remember the value will be persisted even after a recomposition

by the way recomposition means when the composable renders again which can happen a lot of times when something changes on the screen and needs to be rendered again

CodePudding user response：

I think, if the article is from a reputed source, the variable might have some further usage in the project. The very reason for initialising the variable as a MutableStateis that the developer wants recompositions to occur upon the change of this value.

If this was not the case, it could have been just var title = "Home", or better instead just use "Home" in the parameter, no need of a variable at all. You see, if you are creating a MutableState, in most scenarios, it is useless to declare it without using remember. In fact, the only scenario I can think of, to declare a MutableState without remeber is to trigger recompositions manually using the var as a trigger.

Anyway, most of the times, you want to read the value of the var that is declared MutableState. If any modifications are made to the value of the var, then a recomposition is triggered. Now, if you declare it without any rememberance, the value will be re-initlaised to whatever you provided as the initial value. The updated value is gone for good in this case.

Hence, in the latest versions of the compiler, I think it will not even allow you to create a MutableState var without using remember. If not a compile-time error, I'm sure it gives at least a warning (though I am almost certain it won't allow you to compile, which makes me think Compose Developers do not want us to trigger dummy recompositions!)

PS: The recompositions can be triggered manually by using remember too, so I guess that was not their motto.