java.lang.RuntimeException:无法在koltlin中创建com.example.mvvmapp.

Question

错误：无法创建视图模型类的实例

以下是我创建的方法

class MainActivity : AppCompatActivity() {
lateinit var noteViewModel: NoteViewModel
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
noteViewModel = ViewModelProvider(
this,
ViewModelProvider.AndroidViewModelFactory.getInstance(this.application)
).get(
NoteViewModel::class.java
)
noteViewModel.getAllNotes().observe(this, object : Observer<List<Note>> {
override fun onChanged(t: List<Note>?) {
Toast.makeText(this@MainActivity, t.toString(), Toast.LENGTH_LONG).show()
}
})
}
}

这是我的视图模型类

class NoteViewModel(application: Application) : AndroidViewModel(application) {
private val repository: NoteRepository = NoteRepository(application)
private val allNotes: LiveData<List<Note>> = repository.getAllNotes()
fun insert(note: Note) {
repository.insert(note)
}
fun delete(note: Note) {
repository.delete(note)
}
fun update(note: Note) {
repository.update(note)
}
fun deleteAll() {
repository.deleteAllNotes()
}
fun getAllNotes(): LiveData<List<Note>> = allNotes
}

一切看起来都很好，我不知道是什么原因导致错误

Answer 1

您可以使用kotlin属性delegate"viewModels(("实例化您的视图模型

class MainActivity : AppCompatActivity() {
var noteViewModel: NoteViewModel by viewModels()
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
noteViewModel.getAllNotes().observe(this, object : Observer<List<Note>> {
override fun onChanged(t: List<Note>?) {
Toast.makeText(this@MainActivity, t.toString(), Toast.LENGTH_LONG).show()
}
})
}

}

Answer 2

尝试以下依赖项，然后您将能够获得kotlin属性委托"viewModels(("在你的活动中。

dependencies {
val lifecycle_version = "2.3.1"
val arch_version = "2.1.0"
// ViewModel
implementation("androidx.lifecycle:lifecycle-viewmodel-ktx:$lifecycle_version")
// LiveData
implementation("androidx.lifecycle:lifecycle-livedata-ktx:$lifecycle_version")
// Lifecycles only (without ViewModel or LiveData)
implementation("androidx.lifecycle:lifecycle-runtime-ktx:$lifecycle_version")
// Saved state module for ViewModel
implementation("androidx.lifecycle:lifecycle-viewmodel-savedstate:$lifecycle_version")
implementation "androidx.activity:activity-ktx:1.3.0-beta01"
}

//write below code in your activity for instantiate viewModel
private val noteViewModel: NoteViewModel by viewModels()

//For more detail go through below link
https://developer.android.com/jetpack/androidx/releases/lifecycle#declaring_dependencies

Answer 3

我(在java中(遇到了同样的问题。这解决了问题：

//旧代码

mWordViewModel = new ViewModelProvider(this).get(WordViewModel.class);

//新代码

mWordViewModel = new ViewModelProvider(this, ViewModelProvider.AndroidViewModelFactory.getInstance(getApplication())).get(WordViewModel.class);

Answer 4

如果有人遇到这个问题，一个解决方案就是将ViewModel构造函数公开(扩展AndroidViewModel的类(