课题

1. 程序界面由3个文本编辑框和1个文本标签组成。
2. 要求文本标签实时显示3个文本编辑框所输入的数字之和。
3. 文本编辑框输入的不是合法数字时，将其值视为0。
4. 3个文本编辑框的初值分别为1，2，3。

创建工程

打开 Android Studio，File / New / Project...
在 New Project 向导的第1页，选择 Empty Compose Activity
在向导的第2页 Name 填上 Compose Example
在向导的第4页点击 Finish 按钮创建工程

ViewModel

在 com.example.composeexample 包中添加 NumbersViewModel 类

class NumbersViewModel : ViewModel() {
    val number1 = MutableStateFlow("1")
    val number2 = MutableStateFlow("2")
    val number3 = MutableStateFlow("3")
    val result = combine(number1, number2, number3) { n1, n2, n3 ->
        ((n1.toIntOrNull() ?: 0) + (n2.toIntOrNull() ?: 0) + (n3.toIntOrNull() ?: 0)).toString()
    }
}

配置 UI

打开 MainActivity.kt，将内容修改为

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

@Composable
fun LeftText(
    text: String = "",
) {
    Text(
        text = text,
        modifier = Modifier.width(40.dp),
        textAlign = TextAlign.Center,
    )
}

@Composable
fun RightTextField(
    value: String,
    onValueChange: (String) -> Unit,
    readOnly: Boolean = false,
) {
    TextField(
        value = value,
        onValueChange = onValueChange,
        readOnly = readOnly,
        modifier = Modifier.width(200.dp),
        textStyle = LocalTextStyle.current.copy(textAlign = TextAlign.End)
    )
}

@Composable
fun AddNumbers(vm: NumbersViewModel = NumbersViewModel()) {
    Column(
        modifier = Modifier.fillMaxSize(),
        horizontalAlignment = Alignment.CenterHorizontally,
        verticalArrangement = Arrangement.Center
    ) {
        Row {
            LeftText()
            RightTextField(
                value = vm.number1.collectAsState().value,
                onValueChange = { vm.number1.value = it },
            )
        }
        Row {
            LeftText()
            RightTextField(
                value = vm.number2.collectAsState().value,
                onValueChange = { vm.number2.value = it }
            )
        }
        Row(verticalAlignment = Alignment.CenterVertically) {
            LeftText("+")
            RightTextField(
                value = vm.number3.collectAsState().value,
                onValueChange = { vm.number3.value = it }
            )
        }
        Row {
            LeftText()
            RightTextField(
                value = vm.result.collectAsState("").value,
                onValueChange = {},
                readOnly = true
            )
        }
    }
}

@Preview(showBackground = true)
@Composable
fun DefaultPreview() {
    ComposeExampleTheme {
        AddNumbers()
    }
}