Android compose(2) 状态State

TextFiled 演示输入数字转换金额
mutableState 使用
remember 委托记住输入

class MainActivity : ComponentActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContent {
            GreetingCardTheme {
                // A surface container using the 'background' color from the theme
                Surface(
                    modifier = Modifier.fillMaxSize(),
                    color = MaterialTheme.colors.background
                ) {
                    TipTimeScreen()
                }
            }
        }
    }
}


@Preview(showBackground = true, showSystemUi = true)
@Composable
fun DefaultPreview() {
    GreetingCardTheme {
//        BirthdayGreetingWithImage(message = "Happy Birthdy David", from = " - Weimiao")
        Surface(
            modifier = Modifier.fillMaxSize(),
            color = MaterialTheme.colors.background
        ) {
            TipTimeScreen()
        }
    }
}





@Composable
fun TipTimeScreen() {
    var amountInput by remember {
        mutableStateOf("")
    }

    val amount = amountInput.toDoubleOrNull() ?: 0.0
    val tip = calculateTIp(amount)
    Column(
        modifier = Modifier.padding(32.dp),
        verticalArrangement = Arrangement.spacedBy(8.dp)
    ) {
        Text(
            text = stringResource(id = R.string.calculate_tip),
            fontSize = 24.sp,
            modifier = Modifier.align(Alignment.CenterHorizontally)
        )
        Spacer(modifier = Modifier.height(16.dp))
        EditNumberField(value = amountInput, onValueChange = {amountInput = it})
        Spacer(Modifier.height(24.dp))
        Text(
            text = stringResource(R.string.tip_amount, tip),
            modifier = Modifier.align(Alignment.CenterHorizontally),
            fontSize = 20.sp,
            fontWeight = FontWeight.Bold
        )
    }
}

@Composable
fun EditNumberField(value: String, onValueChange: (String) -> Unit) {
    TextField(
        value = value,
        onValueChange = onValueChange,
        modifier = Modifier.fillMaxWidth(),
        label = { Text(text = stringResource(id = R.string.cost_of_service)) },
        singleLine = true,
        keyboardOptions = KeyboardOptions(keyboardType = KeyboardType.Number)
    )
}

private fun calculateTIp(amount: Double, tipPercent: Double = 15.0):String {
    val tip = tipPercent / 100 * amount
    return NumberFormat.getCurrencyInstance().format(tip)
}

}