我有 2 個可變狀態，ViewModel并希望他們觀察我的可組合中的文本欄位。
ViewModel：
@HiltViewModel
class AddEditTransactionViewModel @Inject constructor(
private val moneyManagerUseCases: MoneyManagerUseCases
) : ViewModel() {
private val _transactionDescription = mutableStateOf("")
val transactionDescription: State<String> = _transactionDescription
private val _transactionAmount = mutableStateOf("")
val transactionAmount: State<String> = _transactionAmount
Composable：
@Composable
fun AddEditTransactionScreen(
navController: NavController,
viewModel: AddEditTransactionViewModel = hiltViewModel(),
) {
// This works
var descriptionState by remember { mutableStateOf("") }
var amountState by remember { mutableStateOf("") }
// This doesn't work
var viewModelDescriptionState = viewModel.transactionDescription.value
var viewModelAmountState = viewModel.transactionAmount.value
Column(
modifier = Modifier
.fillMaxSize()
.padding(8.dp),
verticalArrangement = Arrangement.Center,
horizontalAlignment = Alignment.CenterHorizontally
) {
TextField(
value = viewModelDescriptionState,
onValueChange = { viewModelDescriptionState = it },
label = { Text(text = "Description") })
Spacer(modifier = Modifier.padding(20.dp))
TextField(
value = viewModelAmountState,
onValueChange = { viewModelAmountState = it },
label = { Text(text = "Amount") },
keyboardOptions = KeyboardOptions(keyboardType = KeyboardType.Number)
)
如果我使用我的可組合中的狀態，它會起作用，但這種效果會超出 ViewModel 的目的。
uj5u.com熱心網友回復：
第一行：
var descriptionState by remember { mutableStateOf(") }
它有效是因為您正在更新mutableStatevia 委托的值。如果你看看在沒有委托的情況下你會如何做，會更容易理解：
val descriptionState = remember { mutableStateOf("") }
//...
descriptionState.value = "new value"
remember將在重新組合之間保存相同的物件，但該物件的value欄位正在發生變化。
另一方面，這里：
var viewModelDescriptionState = viewModel.transactionDescription.value
您創建了一個區域變數，并且在重新組合之間不會保存任何更改。
這通常通過在視圖模型中創建一個 setter 函式來解決。你也可以在視圖模型中使用委托，如果你想限制它在沒有 setter 的情況下更新，你可以添加private set, 而不是有private _variableand public variable：
var transactionDescription by mutableStateOf("")
private set
fun updateTransactionDescription(newValue: String) {
transactionDescription = newValue
}
這被認為是最佳實踐，因為 setter 可以包含比簡單更新值更多的邏輯。在只需要存盤的情況下，可以去掉private部分直接更新，不需要setter函式。
您可以在Compose檔案中找到有關狀態的更多資訊，包括解釋基本原理的這個 youtube 視頻。
uj5u.com熱心網友回復：
解決方案
AddEditTransactionScreen：
@Composable
fun AddEditTransactionScreen(
navController: NavController,
viewModel: AddEditTransactionViewModel = hiltViewModel()
) {
val descriptionState = viewModel.transactionDescription
val amountState = viewModel.transactionAmount
Column(
modifier = Modifier
.fillMaxSize()
.padding(8.dp),
verticalArrangement = Arrangement.Center,
horizontalAlignment = Alignment.CenterHorizontally
) {
TextField(
value = descriptionState.value,
onValueChange = { viewModel.updateTransactionDescription(it) },
label = { Text(text = "Description") })
Spacer(modifier = Modifier.padding(20.dp))
TextField(
value = amountState.value,
onValueChange = { viewModel.updateTransactionAmount(it) },
label = { Text(text = "Amount") },
keyboardOptions = KeyboardOptions(keyboardType = KeyboardType.Number)
)
}
}
AddEditTransactionViewModel：
@HiltViewModel
class AddEditTransactionViewModel @Inject constructor() : ViewModel() {
private val _transactionDescription = mutableStateOf("")
val transactionDescription: State<String> = _transactionDescription
private val _transactionAmount = mutableStateOf("")
val transactionAmount: State<String> = _transactionAmount
fun updateTransactionDescription(value: String) {
_transactionDescription.value = value
}
fun updateTransactionAmount(value: String) {
_transactionAmount.value = value
}
}
解釋
它transactionDescription和transactionAmount屬性是State<String>型別，State型別不能改變它的值，與MutableState型別不同，一個簡單的解決方案只通過視圖模型維護更改保護，為每個要更改的屬性創建一個方法并進行此更改在屬性中MutableState。
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