回收人员视图，与DiffUtil和LiveData

Question

我已经被困在这个问题上几天了，真的需要一些帮助和理解如何用DiffUtil设置RecylerView（RV），似乎无论我尝试什么，我都无法让RV更新。我会浏览我所拥有的，希望有人能解释一下我做错了什么。

应用程序启动

• 创建视图模型，
  • 然后在ViewModel中填充LiveDataList。
  • LayoutManager和ListAdapter应用于RV

  为什么在LiveDataList中添加或删除一项时，观察者不被调用进行更新。

  class MainActivity : AppCompatActivity() {
      var myAdapter = RVAdapter()
      private lateinit var viewModel: MyViewModel()
  
      override fun onCreateView(savedInstanceState: Bundle? ) {
          super.onCreate(savedInstanceState)
          // Initialize the ViewModel, that stores the userList. 
          viewModel = ViewModelProvider(this).get(MyViewModel::class.java)
          // Creates list of users
          addUser()
  
          // Link an RV object to view
          val rv = findViewById<RecyclerView>(R.id.recycler_view)
          // apply RV Settings. 
          rv.apply {
              layoutManager = LinearLayoutManager(baseContext, LinearLayoutManager.VERTICAL, false)
              adapter= myAdapter
          }
  
          ItemTouchHelper(object : ItemTouchHelper.SimpleCallback(0, ItemTouchHelper.LEFT) {
               override fun onMove(...) {
                   // move items isn't used. 
               }
  
               override fun onSwipe(viewHolder: ViewHolder, direction: Int) {
                   // When I swipe a second item, the app crashes.
                   val editList: MutableList<User>? = viewModel.usersList?.value as MutableList<User>?
                   editList?.removeAt(viewHolder.adapterPosition)
                   viewModel.usersList?.postValue(userList)
               }
          }).attachtoRecyclerView(rv)
       
          // The observer will run after swiping an item but the UI doesn't update. 
          viewModel.usersList?observe(this, Observer {
              it?.let {
                  // Should update list but it doesn't
                  myAdapter.submitList(it)
              }
      }
  
      private fun addUser() {
          // Shouldn't there be away to add Items directly to list in ViewModel.
          val newList: MutableList<User> = mutableListOf()
          newList.add(User("Shawn", 1)
          newList.add(User("Shannon", 2)
          newList.add(User("Don", 3)
  
          viewModel.userList?.postValue(newList)
  }
  
  

  数据类：这是非常基本的，在这里很少发生。

  data class User(val name: String, val accountNumber: Int) {
  }
  

  ViewModel:MyViewModel只存储可变的LiveDat

  class MyViewModel : ViewModel() {
      val usersList: MutableLiveData<List<User>>? = MutableLiveData() 
  }
  

  RecycleView适配器

  class RVAdapter : ListAdapter<User, RVAdapter.ViewHolder>(MyDiffCallback() {
      
      // This DiffUtil class never gets called not even on startup.
      class MyDiffCallback: DiffUtil.ItemCallback<User>() {
          override fun areItemsTheSame(oldItem: User, newItem: User): Boolean {
              return oldItem.name == newItem.name
          }
  
          override fun areContentsTheSame(oldItem: User, newItem: User): Boolean {
              return oldItem == newItem
          }
  
      }
  
      ... Rest of RV is working fine but can be included if it's helpful.  
  }
  

  当一个项目从LiveData列表中删除或添加，观察者将把修改后的列表传递给列表适配器时，可以做些什么？

  StackTrace包含索引出界。

  2020-10-05 20:14:16.538 19042-19042/com.example.practicerecyclerview2 E/AndroidRuntime: FATAL EXCEPTION: main
      Process: com.example.practicerecyclerview2, PID: 19042
      java.lang.IndexOutOfBoundsException: Index: 2, Size: 2
          at java.util.ArrayList.remove(ArrayList.java:503)
          at com.example.practicerecyclerview2.MainActivity$onCreate$2.onSwiped(MainActivity.kt:100)
          at androidx.recyclerview.widget.ItemTouchHelper$4.run(ItemTouchHelper.java:712)
          at android.os.Handler.handleCallback(Handler.java:883)
          at android.os.Handler.dispatchMessage(Handler.java:100)
          at android.os.Looper.loop(Looper.java:214)
          at android.app.ActivityThread.main(ActivityThread.java:7356)
          at java.lang.reflect.Method.invoke(Native Method)
          at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492)
          at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:930)
  

Answer 1

您的ListAdapter没有更新的原因是它无法在您提交的列表中找到差异，因为您已经修改了它。这样一来，就不会调用notifyDataChanged（）方法，第二次滑动就会得到IndexOutOfBoundsException，因为该项不再存在。

解决方法是修改并提交列表副本：

override fun onSwipe(viewHolder: ViewHolder, direction: Int) {
    val editList: MutableList<User>? = viewModel.usersList?.value as MutableList<User>?
    val editListCopy = editList?.toMutableList()
    editListCopy?.removeAt(viewHolder.adapterPosition)
    viewModel.users?.postValue(editListCopy)
}