Android kotlin 数据绑定(bind)与 fragment 错误 :cannot find symbol import com. example.***.databinding.FragmentUserBindingImpl;

Question

我对 kotlin 比较陌生，并尝试构建一个在某些 fragment 中进行数据绑定(bind)的项目。我有一个名为 UserFragment 的 fragment ，其中包含一个 Recyclerview，如下所示:

class UserFragment : Fragment() {
  private lateinit var binding: FragmentUserBinding
  private lateinit var viewModel: UserListViewModel
  override fun onCreateView(
      inflater: LayoutInflater, container: ViewGroup?,
      savedInstanceState: Bundle?
  ): View? {
      // Inflate the layout for this fragment
      binding=DataBindingUtil.inflate(inflater,R.layout.fragment_user, container, false)
      binding.userRecycler.layoutManager = LinearLayoutManager(context, RecyclerView.VERTICAL, false)

      viewModel = ViewModelProviders.of(this).get(UserListViewModel::class.java)
      viewModel.errorMessage.observe(this, Observer {
              errorMessage -> if(errorMessage != null) showError(errorMessage) else hideError()
      })
      binding.mViewModel=viewModel
      return binding.root
      }

  override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
      super.onViewCreated(view, savedInstanceState)
      //start add activity
      val i= Intent(activity,AddUserActivity::class.java)
      userFab.setOnClickListener(View.OnClickListener {
          startActivity(i)
      })
  }
  private fun showError(@StringRes errorMessage:Int){
      errorSnackbar = Snackbar.make(binding.root, errorMessage, Snackbar.LENGTH_INDEFINITE)
      errorSnackbar?.setAction(R.string.retry, viewModel.errorClickListener)
      errorSnackbar?.show()
  }

  private fun hideError(){
      errorSnackbar?.dismiss()
  }
  }

xml 布局文件 fragment_user.xml 如下所示:

<?xml version="1.0" encoding="utf-8"?>
<layout
        xmlns:android="http://schemas.android.com/apk/res/android"
        xmlns:app="http://schemas.android.com/apk/res-auto" xmlns:tools="http://schemas.android.com/tools">
    <data>
        <variable
                name="mViewModel"
                type="com.example.***.ui.User.UserListViewModel" />
    </data>
    <androidx.constraintlayout.widget.ConstraintLayout
            android:layout_width="match_parent"
            android:layout_height="match_parent">
        <EditText
                android:id="@+id/userDateEditText"
                android:layout_width="0dp"
                android:layout_height="wrap_content"
                android:hint="@string/pick_date"
                android:background="@drawable/roundededittext"
                android:layout_marginEnd="8dp"
                app:layout_constraintEnd_toEndOf="parent"
                android:layout_marginStart="8dp"
                app:layout_constraintStart_toStartOf="parent"
                android:layout_marginTop="8dp"
                app:layout_constraintTop_toTopOf="parent"/>

        <androidx.recyclerview.widget.RecyclerView
                android:id="@+id/userRecycler"
                android:layout_width="293dp"
                android:layout_height="475dp" android:layout_marginTop="8dp"
                app:layout_constraintTop_toBottomOf="@+id/userDateEditText"
                app:adapter="@{viewModel.getUserListAdapter()}"
                app:layout_constraintEnd_toEndOf="parent"
                android:layout_marginEnd="8dp" app:layout_constraintStart_toStartOf="parent"
                android:layout_marginStart="8dp" android:layout_marginBottom="8dp"
                app:layout_constraintBottom_toBottomOf="parent"/>

        <com.google.android.material.floatingactionbutton.FloatingActionButton
                android:id="@+id/userFab"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content" android:layout_marginBottom="48dp"
                app:layout_constraintBottom_toBottomOf="parent" app:layout_constraintEnd_toEndOf="parent"
                android:layout_marginEnd="8dp"/>
        <ProgressBar
                style="?android:attr/progressBarStyle"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                app:mutableVisibility="@{viewModel.getLoadingVisibility()}"
                android:id="@+id/userProgressBar" app:layout_constraintEnd_toEndOf="parent"
                android:layout_marginEnd="140dp" app:layout_constraintStart_toStartOf="parent"
                android:layout_marginStart="8dp" android:layout_marginTop="8dp"
                app:layout_constraintTop_toTopOf="parent" android:layout_marginBottom="8dp"
                app:layout_constraintBottom_toBottomOf="parent" app:layout_constraintHorizontal_bias="0.804"
                app:layout_constraintVertical_bias="0.499"/>
    </androidx.constraintlayout.widget.ConstraintLayout>
</layout>

还有一个类似的适配器类和item_user.xml:

class UserListAdapter : RecyclerView.Adapter<UserListAdapter.ViewHolder>() {
    private lateinit var userModelList:List<UserModel>

    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): UserListAdapter.ViewHolder {
        val binding: ItemUserBinding = DataBindingUtil.inflate(LayoutInflater.from(parent.context), R.layout.item_user, parent, false)
        return ViewHolder(binding)
    }

    override fun onBindViewHolder(holder: UserListAdapter.ViewHolder, position: Int) {
        holder.bind(userModelList[position])
    }

    override fun getItemCount(): Int {
        return if(::userModelList.isInitialized) userModelList.size else 0
    }

    fun updateUserList(userModelList:List<UserModel>){
        this.userModelList = userModelList
        notifyDataSetChanged()
    }

    class ViewHolder(private val binding: ItemUserBinding):RecyclerView.ViewHolder(binding.root){
        private val viewModel = UserViewModel()

        fun bind(userModel: UserModel){
            viewModel.bind(userModel)
            binding.viewModel =viewModel
        }
    }
}

item-user.xml:

<?xml version="1.0" encoding="utf-8"?>
<layout xmlns:android="http://schemas.android.com/apk/res/android"
        xmlns:app="http://schemas.android.com/apk/res-auto">

    <data>
        <variable
                name="viewModel"
                type="com.example.***.ui.MyUser.UserViewModel" />
    </data>

    <android.support.constraint.ConstraintLayout
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:paddingLeft="16dp"
            android:paddingRight="16dp">

        <TextView
                android:id="@+id/user_title"
                android:layout_width="0dp"
                android:layout_height="wrap_content"
                android:textStyle="bold"
                app:mutableText="@{viewModel.getUserTitle()}"
                app:layout_constraintEnd_toEndOf="parent"
                app:layout_constraintStart_toStartOf="parent"
                app:layout_constraintTop_toTopOf="parent" />

        <TextView
                android:id="@+id/user_description"
                android:layout_width="0dp"
                android:layout_height="wrap_content"
                android:layout_marginTop="8dp"
                app:mutableText="@{viewModel.getUserDesc()}"
                app:layout_constraintEnd_toEndOf="parent"
                app:layout_constraintStart_toStartOf="parent"
                app:layout_constraintTop_toBottomOf="@+id/user_title" />
    </android.support.constraint.ConstraintLayout>
</layout>

请注意，数据绑定(bind)已在 gradle 中启用这里非常重要的问题是在我的 fragment 和适配器中这一行:

binding.viewModel =viewModel

报告类型不匹配，如下所示:

Type mismatch.
Required:MyUser.UserListViewModel?
Found:com.example.***.ui.MyUser.UserListViewModel

当我构建项目时，错误如下:

error: cannot find symbol
import com.example.***.databinding.FragmentUserBindingImpl;

Answer 1

第一个错误非常明确:binding.viewModel 需要一个可为空的 UserListViewModel？并获取一个不可为 null 的 UserListViewModel(请参阅 kotlin null safety doc )。您可以尝试这样的方法来摆脱它:

将您的 viewModel 声明为

private var viewModel: UserListViewModel? = null

并以这种方式设置绑定(bind):

viewModel?.let{binding.viewModel = it}

关于第二个错误，您的声明似乎没问题，但有时 Android Studio 的缓存会损坏，请尝试无效缓存/重新启动，它可能会有所帮助。