问题描述
我已经坚持了几天,可以在使用DiffUtil设置RecylerView(RV)方面真的可以使用一些帮助和理解,似乎我尝试无法使RV更新。我将回顾我所拥有的一切,希望有人可以阐明我在做错什么。
应用启动
- 创建ViewModel,
- 然后,将LiveDataList填充到ViewModel中。
- RV已实例化
- 将LayoutManager和ListAdapter应用于RV。
- ItemTouchHelper附加到RecyclerView,允许向左滑动时从LiveDataList中删除项目。
- 观察到LiveDataList,我的理解是,当列表发生更改时,如果原始提交的列表与修改后的列表之间存在差异,则观察者会将列表提交到AdapterList并将其与旧列表进行比较。 AdapterList将更新RecyclerView。
为什么在从LiveDataList列表中添加或删除项目时未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)
}
数据类:这是非常基本的,这里没有发生太多事情。
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.
}
从LiveData列表中删除或添加项目时,观察者会将修改后的列表传递给ListAdapter怎么办?
包含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$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)
解决方法
ListAdapter
未更新的原因是,由于您已对其进行了修改,因此无法在您提交的列表中找到差异。这样,notifyDataChanged()
方法不会被调用,并且您再次滑过IndexOutOfBoundsException
,因为该项目已不存在。
解决方法是修改并提交列表的副本:
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)
}