RecyclerView，带有 DiffUtil 和 LiveData

Question

我已经坚持了几天，真的需要一些帮助和理解如何使用 DiffUtil 设置 RecylerView(RV)，似乎无论我尝试什么我都无法让 RV 更新。我会运行了解我所拥有的，希望有人能指出我做错了什么。

应用启动

创建视图模型，
- 然后在 ViewModel 中填充 LiveDataList。
RV实例化
- RV应用LayoutManager和ListAdapter
ItemTouchHelper 附加到 RecyclerView，允许在向左滑动时从 LiveDataList 中删除项目。
观察LiveDataList，我的理解是当列表发生变化时，如果原始提交的列表与修改后的列表有差异，观察者会将列表提交给AdapterList与旧列表进行比较列出 AdapterList 将更新 RecyclerView。

为什么在 LiveDataList 列表中添加或删除项目时不调用观察者进行更新。

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class：这是非常基本的，这里发生的事情不多。

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

ViewModel：MyViewModel 仅存储 MutableLiveDat

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.  
}

当从 LiveData List 中删除或添加项目时，观察者将修改后的列表传递给 ListAdapter 可以做什么？

包括 StackTrace 索引出站。

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$onCreate.onSwiped(MainActivity.kt:100)
        at androidx.recyclerview.widget.ItemTouchHelper.run(ItemTouchHelper.java:712)
        at android.os.Handler.handleCallback(Handler.java:883)
        at android.os.Handler.dispatchMessage(Handler.java:100)
        at android.os.Looper.loop(Looper.java:214)
        at android.app.ActivityThread.main(ActivityThread.java:7356)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$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，因为您已经对其进行了修改。这样 notifyDataChanged() 方法就不会被调用，您会在第二次滑动时得到 IndexOutOfBoundsException，因为该项目不再存在。

解决方法是修改并提交一份副本的列表：

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)
}

RecyclerView，带有 DiffUtil 和 LiveData

RecyclerView, with DiffUtil and LiveData

android

kotlin

android-recyclerview

android-diffutils