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Jetpack Compose 架构MVI

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为什么是MVI而不是MVVM

MVVM作为流行的架构模式,应用在 Compose上,并没有大的问题或者设计缺陷。但是在使用期间,发现了并不适合我的地方,或者说是使用起来不顺手的地方:

单向数据流

单向数据流 (UDF) 是一种设计模式,在该模式下状态向下流动,事件向上流动。通过采用单向数据流,您可以将在界面中显示状态的可组合项与应用中存储和更改状态的部分分离开来。

使用单向数据流的应用的界面更新循环如下所示:

以上是官方对单向数据流的介绍。下面介绍适合单项数据流的架构 MVI。

MVI

MVI 包含三部分,ModelViewIntent

可以看出 MVI 完美的符合官方推荐架构 ,我们引用 React Redux 的概念分而治之:

实现

首先我们需要约束类型的接口:

interface UiState
interface UiEvent
interface UiEffect

然后创建抽象的 ViewModel :

abstract class BaseViewModel< S : UiState, E : UiEvent,F : UiEffect>  : ViewModel() {}

对于状态的处理,我们使用StateFlowStateFlow就像LiveData但具有初始值,所以需要一个初始状态。这也是一种SharedFlow.我们总是希望在 UI 变得可见时接收最后一个视图状态。为什么不使用MutableState,因为Flow 的api和操作符十分强大。

    private val initialState: S by lazy { initialState() }
    protected abstract fun initialState(): S
    private val _uiState: MutableStateFlow<S> by lazy { MutableStateFlow(initialState) }
    val uiState: StateFlow<S> by lazy { _uiState }

对于意图,即事件,我要接收和处理:

  private val _uiEvent: MutableSharedFlow<E> = MutableSharedFlow()
    init {
        subscribeEvents()
    }
    /**
     * 收集事件
     */
    private fun subscribeEvents() {
        viewModelScope.launch {
            _uiEvent.collect {
                // reduce event
            }
        }
    }
    fun sendEvent(event: E) {
        viewModelScope.launch {
            _uiEvent.emit(event)
        }
    }

然后 Reducer 处理事件,更新状态:

    /**
     * 处理事件,更新状态
     * @param state S
     * @param event E
     */
    private fun reduceEvent(state: S, event: E) {
        viewModelScope.launch {
            handleEvent(event, state)?.let { newState -> sendState { newState } }
        }
    }
    protected abstract suspend fun handleEvent(event: E, state: S): S?

单一的副作用:

    private val _uiEffect: MutableSharedFlow<F> = MutableSharedFlow()
    val uiEffect: Flow<F> = _uiEffect
    protected fun sendEffect(effect: F) {
        viewModelScope.launch { _uiEffect.emit(effect) }
    }

使用

接下来实现一个 Todo 应用,打开应用获取历史任务,点击加号增加一条新的任务,完成任务后后 Toast 提示。

首先分析都有哪些状态:

创建约束类:

internal data class TodoState(
    val isShowAddDialog: Boolean=false,
    val isLoading: Boolean = false,
    val goodsList: List<Todo> = listOf(),
) : UiState

然后分析有哪些意图:

internal sealed interface TodoEvent : UiEvent {
    data class ShowData(val items: List<Todo>) : TodoEvent
    data class OnChangeDialogState(val show: Boolean) : TodoEvent
    data class AddNewItem(val text: String) : TodoEvent
    data class OnItemCheckedChanged(val index: Int, val isChecked: Boolean) : TodoEvent
}

而单一事件就一种,完成任务时候的提示:

internal sealed interface TodoEffect : UiEffect {
    // 已完成
    data class Completed(val text: String) : TodoEffect
}

界面

@OptIn(ExperimentalLifecycleComposeApi::class)
@Composable
internal fun TodoScreen(
    viewModel: TodoViewModel = viewModel(),
) {
    val state by viewModel.uiState.collectAsStateWithLifecycle()
    val context = LocalContext.current
    viewModel.collectSideEffect { effect ->
        Log.e("", "TodoScreen: collectSideEffect")
        when (effect) {
            is TodoEffect.Completed -> Toast.makeText(context,
                "${effect.text}已完成",
                Toast.LENGTH_SHORT)
                .show()
        }
    }
//    LaunchedEffect(Unit) {
//        viewModel.uiEffect.collect { effect ->
//            Log.e("", "TodoScreen: LaunchedEffect")
//
//            when (effect) {
//                is TodoEffect.Completed -> Toast.makeText(context,
//                    "${effect.text}已完成",
//                    Toast.LENGTH_SHORT)
//                    .show()
//            }
//        }
//    }
    when {
        state.isLoading -> ContentWithProgress()
        state.goodsList.isNotEmpty() -> TodoListContent(
            state.goodsList,
            state.isShowAddDialog,
            onItemCheckedChanged = { index, isChecked ->
                viewModel.sendEvent(TodoEvent.OnItemCheckedChanged(index, isChecked))
            },
            onAddButtonClick = { viewModel.sendEvent(TodoEvent.OnChangeDialogState(true)) },
            onDialogDismissClick = { viewModel.sendEvent(TodoEvent.OnChangeDialogState(false)) },
            onDialogOkClick = { text -> viewModel.sendEvent(TodoEvent.AddNewItem(text)) },
        )
    }
}
@Composable
private fun TodoListContent(
    todos: List<Todo>,
    isShowAddDialog: Boolean,
    onItemCheckedChanged: (Int, Boolean) -> Unit,
    onAddButtonClick: () -> Unit,
    onDialogDismissClick: () -> Unit,
    onDialogOkClick: (String) -> Unit,
) {
    Box {
        LazyColumn(content = {
            itemsIndexed(todos) { index, item ->
                TodoListItem(item = item, onItemCheckedChanged, index)
                if (index == todos.size - 1)
                    AddButton(onAddButtonClick)
            }
        })
        if (isShowAddDialog) {
            AddNewItemDialog(onDialogDismissClick, onDialogOkClick)
        }
    }
}
@Composable
private fun AddButton(
    onAddButtonClick: () -> Unit,
) {
    Box(modifier = Modifier.fillMaxWidth()) {
        Icon(imageVector = Icons.Default.Add,
            contentDescription = null,
            modifier = Modifier
                .size(40.dp)
                .align(Alignment.Center)
                .clickable(
                    interactionSource = remember { MutableInteractionSource() },
                    indication = null,
                    onClick = onAddButtonClick
                ))
    }
}
@Composable
private fun AddNewItemDialog(
    onDialogDismissClick: () -> Unit,
    onDialogOkClick: (String) -> Unit,
) {
    var text by remember { mutableStateOf("") }
    AlertDialog(onDismissRequest = { },
        text = {
            TextField(
                value = text,
                onValueChange = { newText ->
                    text = newText
                },
                colors = TextFieldDefaults.textFieldColors(
                    focusedIndicatorColor = Color.Blue,
                    disabledIndicatorColor = Color.Blue,
                    unfocusedIndicatorColor = Color.Blue,
                    backgroundColor = Color.LightGray,
                )
            )
        },
        confirmButton = {
            Button(
                onClick = { onDialogOkClick(text) },
                colors = ButtonDefaults.buttonColors(backgroundColor = Color.Blue)
            ) {
                Text(text = "Ok", style = TextStyle(color = Color.White, fontSize = 12.sp))
            }
        }, dismissButton = {
            Button(
                onClick = onDialogDismissClick,
                colors = ButtonDefaults.buttonColors(backgroundColor = Color.Blue)
            ) {
                Text(text = "Cancel", style = TextStyle(color = Color.White, fontSize = 12.sp))
            }
        }
    )
}
@Composable
private fun TodoListItem(
    item: Todo,
    onItemCheckedChanged: (Int, Boolean) -> Unit,
    index: Int,
) {
    Row(
        modifier = Modifier.padding(16.dp),
        horizontalArrangement = Arrangement.Center,
        verticalAlignment = Alignment.CenterVertically
    ) {
        Checkbox(
            colors = CheckboxDefaults.colors(Color.Blue),
            checked = item.isChecked,
            onCheckedChange = {
                onItemCheckedChanged(index, !item.isChecked)
            }
        )
        Text(
            text = item.text,
            modifier = Modifier.padding(start = 16.dp),
            textDecoration = if (item.isChecked) TextDecoration.LineThrough else TextDecoration.None,
            style = TextStyle(
                color = Color.Black,
                fontSize = 14.sp
            )
        )
    }
}
@Composable
private fun ContentWithProgress() {
    Surface(color = Color.LightGray) {
        Box(
            modifier = Modifier.fillMaxSize(),
            contentAlignment = Alignment.Center
        ) {
            CircularProgressIndicator()
        }
    }
}

ViewModel

internal class TodoViewModel :
    BaseViewModel<TodoState, TodoEvent, TodoEffect>() {
    private val repository: TodoRepository = TodoRepository()
    init {
        getTodo()
    }
    private fun getTodo() {
        viewModelScope.launch {
            val goodsList = repository.getTodoList()
         sendEvent(TodoEvent.ShowData(goodsList))
      }
   }
   override fun initialState(): TodoState = TodoState(isLoading = true)
   override suspend fun handleEvent(event: TodoEvent, state: TodoState): TodoState? {
      return when (event) {
         is TodoEvent.AddNewItem -> {
            val newList = state.goodsList.toMutableList()
            newList.add(
               index = state.goodsList.size,
               element = Todo(false, event.text),
            )
            state.copy(
               goodsList = newList,
               isShowAddDialog = false
            )
         }
         is TodoEvent.OnChangeDialogState -> state.copy(
            isShowAddDialog = event.show
         )
         is TodoEvent.OnItemCheckedChanged -> {
                val newList = state.goodsList.toMutableList()
                newList[event.index] = newList[event.index].copy(isChecked = event.isChecked)
                if (event.isChecked) {
                    sendEffect(TodoEffect.Completed(newList[event.index].text))
                }
                state.copy(goodsList = newList)
            }
         is TodoEvent.ShowData -> state.copy(isLoading = false, goodsList = event.items)
      }
   }
}

优化

本来单次事件在LaunchedEffect里加载,但是会出现在 UI 在停止状态下依然收集新事件,并且每次写LaunchedEffect比较麻烦,所以写了一个扩展:

@Composable
fun <S : UiState, E : UiEvent, F : UiEffect> BaseViewModel<S, E, F>.collectSideEffect(
    lifecycleState: Lifecycle.State = Lifecycle.State.STARTED,
    sideEffect: (suspend (sideEffect: F) -> Unit),
) {
    val sideEffectFlow = this.uiEffect
    val lifecycleOwner = LocalLifecycleOwner.current
    LaunchedEffect(sideEffectFlow, lifecycleOwner) {
        lifecycleOwner.lifecycle.repeatOnLifecycle(lifecycleState) {
            sideEffectFlow.collect { sideEffect(it) }
        }
    }
}

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