Jetpack Compose中ViewModel出现NoSuchMethodException问题求助

问题描述 #

运行Jetpack Compose项目时触发FATAL EXCEPTION: main，报错信息为：

java.lang.NoSuchMethodException: com.mydummy.viewmodeltesting.DummyViewModel.<init> []

已在Android Studio全新EmptyActivity项目中复现该问题，相关代码如下：

MainActivity.kt #

class MainActivity : ComponentActivity() {
    val viewModel: DummyViewModel by viewModels()
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContent {
            ViewModelTestingTheme {
                val data by viewModel.dataState.collectAsState()
                Surface(modifier = Modifier.fillMaxSize(), color = MaterialTheme.colorScheme.background) {
                    ShowDataList(dataList = data)
                }
            }
        }
    }
}

@Composable
fun ShowDataList(dataList: List<DummyMetadata>, modifier: Modifier = Modifier) {
    Box(modifier = Modifier.fillMaxSize()) {
        LazyColumn(modifier = Modifier) {
            items(dataList) { data -> ShowDataItem(model = data)}
        }
    }
}

@Composable
fun ShowDataItem(model: DummyMetadata) {
    Card(shape = RoundedCornerShape(8.dp), modifier = Modifier) {
        Text(text = model.title)
    }
}

DummyViewModel.kt #

class DummyViewModel(private val _repository: DummyRepository): ViewModel() {
    private val _dataState = MutableStateFlow<List<DummyMetadata>>(emptyList())
    val dataState: StateFlow<List<DummyMetadata>> = _dataState

    init {
        fetchData()
    }

    private fun fetchData() {
        viewModelScope.launch {
            try {
                val data = _repository.collectDummyData()
                _dataState.value = data
            } catch (e: Exception) {
                // TODO Error handling or at least better logging
                e.printStackTrace()
            }
        }
    }
}

DummyRepository.kt #

class DummyRepository {
    suspend fun collectDummyData(): List<DummyMetadata> {
        return listOf(
            DummyMetadata("First dummy"),
            DummyMetadata("Second dummy"),
            DummyMetadata("Last dummy")
        )
    }
}

DummyMetadata.kt #

data class DummyMetadata(
    val title: String
)

问题原因 #

ViewModel的实例化由ViewModelProvider负责，默认的ViewModelProvider.Factory会尝试调用ViewModel的无参构造函数。但当前DummyViewModel仅提供了带DummyRepository参数的构造函数，系统找不到无参构造方法，因此抛出NoSuchMethodException。

解决方法 #

提供以下两种可行方案：

方案1：为ViewModel添加无参构造函数 #

修改DummyViewModel，给构造函数的参数设置默认值，自动生成无参构造逻辑：

class DummyViewModel(private val _repository: DummyRepository = DummyRepository()): ViewModel() {
    // 原有代码保持不变
}

方案2：自定义ViewModelFactory #

通过自定义Factory来控制ViewModel的实例化过程，手动传入Repository依赖：

1. 创建Factory类：

class DummyViewModelFactory : ViewModelProvider.Factory {
    override fun <T : ViewModel> create(modelClass: Class<T>): T {
        if (modelClass.isAssignableFrom(DummyViewModel::class.java)) {
            @Suppress("UNCHECKED_CAST")
            return DummyViewModel(DummyRepository()) as T
        }
        throw IllegalArgumentException("Unknown ViewModel class")
    }
}

2. 在MainActivity中指定Factory初始化ViewModel：

val viewModel: DummyViewModel by viewModels { DummyViewModelFactory() }

内容的提问来源于stack exchange，提问作者Windowlicker