无法在新项目中创建 class ViewModel 的实例

Question

我有两个不同的项目。在第一个中，我将 ViewModel 与我的 activity 和 Room 数据库一起使用。一切正常。然后我为不同的目的创建了新项目，并以完全相同的方式实例化了 ViewModel，但我的应用程序崩溃并出现此错误：

java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example/com.example.MainActivity}: java.lang.RuntimeException: Cannot create an instance of class com.example.MainViewModel
    at android.app.ActivityThread.performLaunchActivity(Unknown Source:591)
    at android.app.ActivityThread.handleLaunchActivity(Unknown Source:36)
    at android.app.servertransaction.LaunchActivityItem.execute(Unknown Source:57)
    at android.app.servertransaction.TransactionExecutor.executeCallbacks(Unknown Source:99)
    at android.app.servertransaction.TransactionExecutor.execute(Unknown Source:34)
    at android.app.ActivityThread$H.handleMessage(Unknown Source:36)
    at android.os.Handler.dispatchMessage(Unknown Source:21)
    at android.os.Looper.loop(Unknown Source:207)
    at android.app.ActivityThread.main(Unknown Source:107)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(Unknown Source:11)
    at com.android.internal.os.ZygoteInit.main(Unknown Source:274)
 Caused by: java.lang.RuntimeException: Cannot create an instance of class com.example.MainViewModel
    at androidx.lifecycle.ViewModelProvider$NewInstanceFactory.create(ViewModelProvider.java:221)
    at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.java:187)
    at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.java:150)
    at com.kazguiu.MainActivity.applyAdapterAndViewModel(MainActivity.kt:37)
    at com.kazguiu.MainActivity.onCreate(MainActivity.kt:25)
    at android.app.Activity.performCreate(Unknown Source:16)
    at android.app.Activity.performCreate(Unknown Source:1)
    at android.app.Instrumentation.callActivityOnCreate(Unknown Source:3)
    at android.app.ActivityThread.performLaunchActivity(Unknown Source:368)
    at android.app.ActivityThread.handleLaunchActivity(Unknown Source:36) 
    at android.app.servertransaction.LaunchActivityItem.execute(Unknown Source:57) 
    at android.app.servertransaction.TransactionExecutor.executeCallbacks(Unknown Source:99) 
    at android.app.servertransaction.TransactionExecutor.execute(Unknown Source:34) 
    at android.app.ActivityThread$H.handleMessage(Unknown Source:36) 
    at android.os.Handler.dispatchMessage(Unknown Source:21) 
    at android.os.Looper.loop(Unknown Source:207) 
    at android.app.ActivityThread.main(Unknown Source:107) 
    at java.lang.reflect.Method.invoke(Native Method) 
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(Unknown Source:11) 
    at com.android.internal.os.ZygoteInit.main(Unknown Source:274) 
 Caused by: java.lang.InstantiationException: java.lang.Class<com.example.MainViewModel> has no zero argument constructor
    at java.lang.Class.newInstance(Native Method)
    at androidx.lifecycle.ViewModelProvider$NewInstanceFactory.create(ViewModelProvider.java:219)

我在 activity 中像这样实例化 ViewModel：

private lateinit var mainViewModel: MainViewModel

override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    setContentView(R.layout.activity_main)

    applyAdapterAndViewModel()

}

private fun applyAdapterAndViewModel() {
    mainViewModel = ViewModelProvider(this).get(MainViewModel::class.java)
}

我的视图模型：

class MainViewModel(application: Application) : AndroidViewModel(application) {

    private val exampleDao = Database.getDatabase(application).exampleDao()
    private val repository = Repository(exampleDao)

    fun getSubjects() = repository.getSubjects()

    fun addSubjects(subject: Subjects) = viewModelScope.launch(Dispatchers.IO) {
        repository.addSubject(subject)
    }

}

这种ViewModel的使用方式和我之前的项目是一样的。唯一的区别是我当前的 gradle:

中有两个额外的依赖项

implementation 'androidx.lifecycle:lifecycle-livedata-ktx:2.2.0'
implementation 'androidx.lifecycle:lifecycle-viewmodel-ktx:2.2.0'

如果我删除它们，Android Studio 无法识别 ViewModelProvider，但我以前的项目中根本没有这些依赖项，我使用 LiveData 和 ViewModel 的方式完全相同（甚至导入是一样的）那里。那么，我该怎么办？

Answer 1

您的 ViewModel 需要一个参数 application: Application，它只能通过使用 ViewModelFactory 来提供，如下所示：

ViewModelFactory

class MainViewModelFactory(
    private val application: Application
) : ViewModelProvider.Factory {
    @Suppress("unchecked_cast")
    override fun <T : ViewModel?> create(modelClass: Class<T>): T {
        if (modelClass.isAssignableFrom(MainViewModel::class.java)) {
            return MainViewModel(application) as T
        }
        throw IllegalArgumentException("Unknown ViewModel class")
    }
}

MainActivity

private fun applyAdapterAndViewModel() {
// Not so kotlinic
mainViewModel = ViewModelProvider(this,MainViewModelFactory(application)).get(MainViewModel::class.java)
}

此外，在 MainActivity 中，替换 private lateinit var mainViewModel: MainViewModel 与

private val viewModel by viewModels<MainViewModel> { 
MainViewModelFactory(application) }

无法在新项目中创建 class ViewModel 的实例

Cannot create an instance of class ViewModel in the new project

android

kotlin

android-viewmodel