Question

我正在使用Android Jetpack的Compose，并且一直在尝试弄清楚如何保存方向更改状态。

我的思路是使类成为ViewModel。当我使用Android的传统API时，这通常可以正常工作。

当信息更改时，我使用了记住{}和mutableState {}来更新UI。请验证我的理解是否正确...

remember =保存变量并允许通过.value进行访问，这允许对值进行缓存。但是它的主要用途是在更改时不重新分配变量。

mutableState =更改某些内容时更新变量。

许多博客文章都说使用@Model，但是，尝试该方法时导入会出错。因此，我添加了一个：ViewModel（）

但是，我相信我记得{}阻止了此操作按预期进行？

我可以向正确的方向求点吗？

@Composable
fun DefaultFlashCard() {

    val flashCards = remember { mutableStateOf(FlashCards())}
    

    Spacer(modifier = Modifier.height(30.dp))

    MaterialTheme {


        val typography = MaterialTheme.typography
        var question = remember { mutableStateOf(flashCards.value.currentFlashCards.question) }



        Column(modifier = Modifier.padding(30.dp).then(Modifier.fillMaxWidth())
                .then(Modifier.wrapContentSize(Alignment.Center))
                .clip(shape = RoundedCornerShape(16.dp))) {
            Box(modifier = Modifier.preferredSize(350.dp)
                    .border(width = 4.dp,
                            color = Gray,
                            shape = RoundedCornerShape(16.dp))
                    .clickable(
                            onClick = {
                                question.value = flashCards.value.currentFlashCards.answer })
                    .gravity(align = Alignment.CenterHorizontally),
                    shape = RoundedCornerShape(2.dp),
                    backgroundColor = DarkGray,
                    gravity = Alignment.Center) {
                Text("${question.value}",
                        style = typography.h4, textAlign = TextAlign.Center, color = White
                )
            }
        }

        Column(modifier = Modifier.padding(16.dp),
                horizontalGravity = Alignment.CenterHorizontally) {

            Text("Flash Card application",
                    style = typography.h6,
                    color = Black)

            Text("The following is a demonstration of using " +
                    "Android Compose to create a Flash Card",
                    style = typography.body2,
                    color = Black,
                    textAlign = TextAlign.Center)

            Spacer(modifier = Modifier.height(30.dp))
            Button(onClick = {
                flashCards.value.incrementQuestion();
                question.value = flashCards.value.currentFlashCards.question },
                    shape = RoundedCornerShape(10.dp),
                    content = { Text("Next Card") },
                    backgroundColor = Cyan)
        }
    }
}


data class Question(val question: String, val answer: String) {
}


class FlashCards: ViewModel() {

    var flashCards = mutableStateOf( listOf(
            Question("How many Bananas should go in a Smoothie?", "3 Bananas"),
            Question("How many Eggs does it take to make an Omellete?", "8 Eggs"),
            Question("How do you say Hello in Japenese?", "Konichiwa"),
            Question("What is Korea's currency?", "Won")
    ))

    var currentQuestion = 0

    val currentFlashCards
        get() = flashCards.value[currentQuestion]

    fun incrementQuestion() {
        if (currentQuestion + 1 >= flashCards.value.size) currentQuestion = 0 else currentQuestion++
    }
}

已解决：

我的问题是在错误的位置实例化了ViewModel。它需要在Activity中，并需要ViewModelProvider（this）.get（x :: class.java）

一个简单的错误，我想Compose毕竟不一样！

Answer 1

在 Compose 中有另一种处理配置更改的方法，它是 rememberSaveable。作为docs says：

<块引用>

虽然 remember 可以帮助您在重新组合时保留状态，但不会在配置更改时保留状态。为此，您必须使用 rememberSaveable。 rememberSaveable 会自动保存可以保存在 Bundle 中的任何值。对于其他值，您可以传入自定义保护程序对象。

似乎 Mohammad's solution 更健壮，但这个似乎更简单。

Answer 2

与以前一样，您可以使用体系结构组件ViewModel来保留配置更改。

您应该在Activity / Fragment中初始化ViewModel，然后将其传递给Composable函数。

class UserDetailFragment : Fragment() {

    private val viewModel: UserDetailViewModel by viewModels()

    override fun onCreateView(
        inflater: LayoutInflater,
        container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View? {

        return ComposeView(context = requireContext()).apply {
            setContent {
                AppTheme {
                    UserDetailScreen(
                        viewModel = viewModel
                    )
                }
            }
        }
    }
}

然后，您的ViewModel应该通过LiveData或Flow之类的方法公开ViewState

UserDetailViewModel：

class UserDetailViewModel : ViewModel() {
    private val _userData = MutableLiveData<UserDetailViewState>()
    val userData: LiveData<UserDetailViewState> = _userData


    // or

    private val _state = MutableStateFlow<UserDetailViewState>()
    val state: StateFlow<UserDetailViewState>
        get() = _state

}

现在您可以在可组合函数中观察此状态：

@Composable
fun UserDetailScreen(
    viewModel:UserDetailViewModel
) {
    val state by viewModel.userData.observeAsState()
    // or
    val viewState by viewModel.state.collectAsState()

}

Jetpack在方向改变时撰写保存状态

2 个答案: