无法创建 com.ma.todo.data.viewmodel.ToDoViewModel 类的实例

Question

我正在创建一个简单的待办事项应用程序。当尝试创建视图模型时。我遇到异常并且应用程序崩溃了。

java.lang.RuntimeException: Cannot create an instance of class com.ma.todo2.data.viewmodel.ToDoViewModel
                                                                                                        at androidx.lifecycle.ViewModelProvider$AndroidViewModelFactory.create(ViewModelProvider.kt:322)
                                                                                                        at androidx.lifecycle.ViewModelProvider$AndroidViewModelFactory.create(ViewModelProvider.kt:306)
                                                                                                        at androidx.lifecycle.ViewModelProvider$AndroidViewModelFactory.create(ViewModelProvider.kt:280)
                                                                                                        at androidx.lifecycle.SavedStateViewModelFactory.create(SavedStateViewModelFactory.kt:128)
                                                                                                        at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.kt:187)
                                                                                                        at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.kt:153)
                                                                                                        at androidx.lifecycle.ViewModelLazy.getValue(ViewModelLazy.kt:53)
                                                                                                        at androidx.lifecycle.ViewModelLazy.getValue(ViewModelLazy.kt:35)
                                                                                                        at com.mahi.todo2.fragments.add.AddFragment.getMToDoViewModel(AddFragment.kt:31)
                                                                                                        at com.mahi.todo2.fragments.add.AddFragment.insertDataToDb(AddFragment.kt:78)
                                                                                                        at com.mahi.todo2.fragments.add.AddFragment.onOptionsItemSelected(AddFragment.kt:61)
                                                                               

第 31 行，

    private val mToDoViewModel: ToDoViewModel by viewModels()

为什么会发生这种情况？解决办法是什么？

TodoViewModel

class ToDoViewModel(application: Application): AndroidViewModel(application)
{
    private val toDoDao = ToDoDatabase.getDatabase(application).toDoDao()
    private val repository: ToDoRepository

    private val getAllData: LiveData<List<ToDoData>>

    init {
        repository = ToDoRepository(toDoDao)
        getAllData = repository.getAllData
    }

    fun insertData(toDoData: ToDoData){
        viewModelScope.launch(Dispatchers.IO) {
            repository.insertData(toDoData)
        }
    }
}

添加片段

class AddFragment : Fragment() {

    private lateinit var title_et: EditText
    private lateinit var priorities_spinner: Spinner
    private lateinit var description_et : EditText

    private val mToDoViewModel: ToDoViewModel by viewModels()
    private val mSharedViewModel: SharedViewModel by viewModels()

    override fun onCreateView(
        inflater: LayoutInflater, container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View? {

        // Inflate the layout for this fragment
      val view = inflater.inflate(R.layout.fragment_add, container, false)

      setHasOptionsMenu(true)

      title_et =  view.findViewById<EditText>(R.id.title_et)
      priorities_spinner  = view.findViewById<Spinner>(R.id.priorities_spinner)
      description_et = view.findViewById<EditText>(R.id.description_et)

      priorities_spinner.onItemSelectedListener = mSharedViewModel.listener

      return view

    }

    override fun onCreateOptionsMenu(menu: Menu, inflater: MenuInflater) {
        inflater.inflate(R.menu.add_fragment_menu, menu)
        super.onCreateOptionsMenu(menu, inflater)
    }

    override fun onOptionsItemSelected(item: MenuItem): Boolean {
        if(item.itemId == R.id.menu_add){
            insertDataToDb()
        }
        return super.onOptionsItemSelected(item)
    }

    private fun insertDataToDb(){

        val mTitle = title_et.text.toString()
        val mPriority = priorities_spinner.selectedItem.toString()
        val mDescription = description_et.text.toString()

        val validation = mSharedViewModel.verifyDataFromUser(mTitle, mDescription)

        if(validation){
            //iinsert data to database
            val newData = ToDoData(0, mTitle, mSharedViewModel.parsePriority(mPriority), mDescription)

            mToDoViewModel.insertData(newData)
            Toast.makeText(requireContext(), "Successfully added!", Toast.LENGTH_SHORT).show()
            //Navigate back
            findNavController().navigate(R.id.action_addFragment_to_listFragment)
        }
        else{
            Toast.makeText(requireContext(), "Please fill out all fields.", Toast.LENGTH_SHORT).show() 
        }

    }


}

Answer 1

1。创建一个ViewModelFactory：

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

1. 更新您的 AddFragment 以使用 ViewModelFactory:

   类 AddFragment : Fragment() { // 其他成员变量...

   // Create a ViewModelFactory instance using the Application context
   private val viewModelFactory: ToDoViewModelFactory by lazy {
       ToDoViewModelFactory(requireActivity().application)
   }
   
   // Use ViewModelProvider with the factory to create ViewModel instances
   private val mToDoViewModel: ToDoViewModel by viewModels { viewModelFactory }
   private val mSharedViewModel: SharedViewModel by viewModels()
   
   // Fragment code...
   
   private fun insertDataToDb() {
       // Your insertDataToDb function...
   }
   
   // methods...
   

   }

   此方法允许您使用自定义 ViewModelFactory 创建 ToDoViewModel 的实例。需要注意的是，ToDoViewModelFactory 需要访问应用程序上下文，这就是我们将其作为参数传递的原因。

---

通过这些更改，ViewModelFactory 将用于创建 ToDoViewModel 的实例，您可以确保在 ViewModel 创建过程中正确提供任何必要的依赖项。

由于您没有使用任何依赖注入，因此您必须使视图模型工厂在视图模型中传递参数