如何实现日历。hour_of_day更改文本内容

Question

如何正确地进行参考，以使topappbar更改中的文本根据设备一天中的时间而变化？我似乎因文字声明而迷失了方向。

class MainActivity : ComponentActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContent {
            Demo1Theme {
                Surface(
                    modifier = Modifier.fillMaxSize(),
                    color = MaterialTheme.colors.background
                ) {
                    Scaffold() {
                        Column {
                            TW()
                        }
                    }
                }
            }
        }
    }
}

@Composable
fun Greeting(name: String) {
    Text(text = "Hello $name!")
}

@Composable
fun TimeOfDay(time: String) {
    Text(text = "The $time")

    val cal: Calendar = Calendar.getInstance()
    val hour: Int = cal.get(Calendar.HOUR_OF_DAY)
    var mainMessage: String? = null

    when (hour) {
        in 6..11 -> {
            mainMessage = "Morning"
        }
        in 12..16 -> {
            mainMessage = "Afternoon"
        }
        in 17..20 -> {
            mainMessage = "Evening"
        }
        in 21..23 -> {
            mainMessage = "Night"
        }
        else -> {
            mainMessage = "Morning"
        }
    }
}

@Composable
fun TW() {
    Scaffold(
        topBar = {
            TopAppBar(
                title = {
                        TimeOfDay(time = "?")
                },
                elevation = 12.dp
            )
        }, content = {
            Greeting("Android")
        }
    )
}

Answer 1

一次尝试一次。

 fun getGreetingMessage(): String {
        val c = Calendar.getInstance()
        return when (c.get(Calendar.HOUR_OF_DAY)) {
            in 6..11 -> "Good Morning"
            in 12..16 -> "Good Afternoon"
            in 17..20 -> "Good Evening"
            in 21..23 -> "Good Night"
            else -> "Good Morning"
        }

和Topappbar

 TopAppBar(
    title = {
        Text(
            text = getGreetingMessage(),

            )
    },
)

Answer 2

您可以使用这样的启动效果来实现这一目标

@Composable
fun Test() {
    var hour by remember { mutableStateOf(Calendar.getInstance().get(Calendar.HOUR_OF_DAY)) }

    LaunchedEffect(Unit) {
        while (true) {
            hour = Calendar.getInstance().get(Calendar.HOUR_OF_DAY)
            delay(1000) // Update hour every seconds
        }
    }

    Box(
        Modifier
            .fillMaxSize()
            .systemBarsPadding()) {
        Column {
            Text(text = "$hour h")
            Text(text = getPeriod(hour))
        }
    }
}

private fun getPeriod(hour: Int) = when (hour) {
    in 6..11 -> {
        "Morning"
    }
    in 12..16 -> {
        "Afternoon"
    }
    in 17..20 -> {
        "Evening"
    }
    in 21..23 -> {
        "Night"
    }
    else -> {
        "Morning"
    }
}