将 ViewModel 类作为参数传递给可组合项时，为什么会出现“无法实例化 ViewModel”？

Question

遵循本教程。我创建了 ViewModel 并希望将其公开给 GameScreen () 可组合项。我不断收到错误“无法实例化 ViewModel”，预览无法渲染，并且应用程序在模拟器中不断崩溃。

我知道这是合法的，因为解决方案代码有效！ 解决方案代码

可组合：

@Composable
fun GameScreen(
    gameViewModel: GameViewModel = viewModel()
) {
    val gameUiState by gameViewModel.uiState.collectAsState()
    /*
rest of composable
*/

这是 ViewModel 类（GameViewModel.kt）：

package com.example.unscramble.ui

import androidx.compose.runtime.getValue
import androidx.compose.runtime.mutableStateOf
import androidx.compose.runtime.setValue
import androidx.lifecycle.ViewModel
import kotlinx.coroutines.flow.MutableStateFlow
import kotlinx.coroutines.flow.StateFlow
import kotlinx.coroutines.flow.asStateFlow
import com.example.unscramble.data.allWords

/*viewmodel contains functionality of the app picking words, shuffling, resetting
* is immutable to other classes (read-only)
* */
class GameViewModel : ViewModel() {

    // Game UI state
    private val _uiState = MutableStateFlow(GameUiState())
    var userGuess by mutableStateOf("")
        private set
    // Backing property to avoid state updates from other classes
    val uiState: StateFlow<GameUiState> = _uiState.asStateFlow()
    init {
        resetGame()
    }
    fun resetGame() {
        usedWords.clear()
        _uiState.value = GameUiState(currentScrambledWord = pickRandomWordAndShuffle())
    }


    private lateinit var currentWord: String

    // Set of words used in the game
    private var usedWords: MutableSet<String> = mutableSetOf()
    private fun pickRandomWordAndShuffle(): String {
        // Continue picking up a new random word until you get one that hasn't been used before
        currentWord = allWords.random()
        if (usedWords.contains(currentWord)) {
            return pickRandomWordAndShuffle()
        } else {
            usedWords.add(currentWord)
            return shuffleCurrentWord(currentWord)
        }
    }
    private fun shuffleCurrentWord(word: String): String {
        val tempWord = word.toCharArray()
        // Scramble the word
        tempWord.shuffle()
        while (String(tempWord).equals(word)) {
            tempWord.shuffle()
        }
        return String(tempWord)
    }





}

我已经编辑了使用 ViewModel 的项目的依赖关系，如下所示

当前配置

dependencies {
    implementation(platform("androidx.compose:compose-bom:2023.08.00"))
    implementation("androidx.activity:activity-compose:1.8.0")
    implementation("androidx.compose.material3:material3")
    implementation("androidx.compose.ui:ui")
    implementation("androidx.compose.ui:ui-graphics")
    implementation("androidx.compose.ui:ui-tooling-preview")
    implementation("androidx.core:core-ktx:1.12.0")
    implementation("androidx.lifecycle:lifecycle-runtime-ktx:2.7.0")
    implementation("androidx.lifecycle:lifecycle-viewmodel-compose:2.7.0")
    implementation("androidx.lifecycle:lifecycle-viewmodel-ktx:2.7.0")
    implementation("androidx.lifecycle:lifecycle-viewmodel-savedstate:2.7.0")

我已将问题范围缩小到 GameViewModel.kt 中的这两行

init {
        resetGame()
    }
    fun resetGame() {
        usedWords.clear()
        _uiState.value = GameUiState(currentScrambledWord = pickRandomWordAndShuffle())
    }

如果我注释掉resetGame()中的行。视图模型被实例化，即工作并呈现预览。如果我取消注释“usedWords.clear()”或“_uiState.value = GameUiState(currentScrambledWord = pickRandomWordAndShuffle())”，则视图模型无法实例化。

为什么这两行会阻止视图模型的实例化？继续注释掉的行，但最终需要添加此功能。

Answer 1

非常愚蠢，移动“usedWords”的 var 声明上面函数定义消除了我的错误。必须打破一些语法规则，使“GameViewModel.kt”类非法/损坏。

改变

类 GameViewModel : ViewModel() {

// Game UI state
private val _uiState = MutableStateFlow(GameUiState())
var userGuess by mutableStateOf("")
    private set
// Backing property to avoid state updates from other classes
val uiState: StateFlow<GameUiState> = _uiState.asStateFlow()
init {
    resetGame()
}
fun resetGame() {
    usedWords.clear()
    _uiState.value = GameUiState(currentScrambledWord = pickRandomWordAndShuffle())
}


private lateinit var currentWord: String

// Set of words used in the game
private var usedWords: MutableSet<String> = mutableSetOf()

至

class GameViewModel : ViewModel() {

// Game UI state
private val _uiState = MutableStateFlow(GameUiState())
// Set of words used in the game
private var usedWords: MutableSet<String> = mutableSetOf()
var userGuess by mutableStateOf("")
    private set
// Backing property to avoid state updates from other classes
val uiState: StateFlow<GameUiState> = _uiState.asStateFlow()
init {
    resetGame()
}
fun resetGame() {
    usedWords.clear()
    _uiState.value = GameUiState(currentScrambledWord = pickRandomWordAndShuffle())
}

成功了。