RecyclerView 仅在方向更改后从 ViewModel 填充数据

Question

我正在使用 NavigationDrawer 并试用 MVVM 架构，所有布局都很好，然后我实现了一个 RecyclerView，它在其中一个片段中显示来自 Firebase 的数据。

未显示任何错误，但 RecyclerView 未填充数据。我向应用程序添加了日志记录，发现适配器 returns 大小，data != null，但有趣的是 recyclerView 仅在设备配置更改时填充数据，例如设备方向

在片段之间切换并返回到 RecyclerView 片段：再次出现同样的问题，直到我旋转设备

我试过：

将流程与对 onCreateView
将数据源更改为虚拟集，它起作用了
在终端显示数据，并打印数据

HomeFragment

...
class HomeFragment : Fragment() {

    private lateinit var homeViewModel: HomeViewModel

    lateinit var recyclerView: RecyclerView

    lateinit var swipeRefresh: SwipeRefreshLayout

    lateinit var postAdapter: PostAdapter

    val viewModelFactory = HomeViewModelFactory()



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

        val view = inflater.inflate(R.layout.fragment_home, container, false)
    homeViewModel = ViewModelProviders.of(this, viewModelFactory).get(HomeViewModel::class.java)


        recyclerView  = view.findViewById(R.id.post_recycler_view)
        recyclerView.setHasFixedSize(true)
        val linearLayoutManager = LinearLayoutManager(context, RecyclerView.VERTICAL, true)
        linearLayoutManager.stackFromEnd = true

        recyclerView.layoutManager = linearLayoutManager

        // Observe the model
        homeViewModel.postList.observe(this@HomeFragment, Observer { posts ->

            postAdapter = PostAdapter(context!!, posts)
            recyclerView.adapter = postAdapter
            postAdapter.notifyDataSetChanged()

            homeViewModel.myRef.keepSynced(true)
        })

        swipeRefresh = view.findViewById(R.id.swiperefresh)

        swipeRefresh.setOnRefreshListener {

            Toast.makeText(context, " Adapter size: ${postAdapter.itemCount}", Toast.LENGTH_LONG).show() // works : returns 12
            println("list:${homeViewmodel.postList.value} ") works : prints data

            swipeRefresh.isRefreshing = false
        }

        return view
    }

viewModel

class HomeViewModel() : ViewModel() {

var postList = loadPosts()

fun loadPosts(): MutableLiveData<MutableList<Post>> {

        val postList = mutableListOf<Post>()
        val l_postList = MutableLiveData<MutableList<Post>>()

        firebaseRepo.retrievePosts(postList)

       l_postList.value =  postList


        return l_postList
    }
}

firebaseRepo

       fun retrievePosts(postList: MutableList<Post>) {

        val postsRef = ref.child("Posts")

        postsRef.addValueEventListener(object : ValueEventListener{
            override fun onCancelled(p0: DatabaseError) {}

            override fun onDataChange(p0: DataSnapshot) {
                postList.clear()

                for(snapshot in p0.children){
                    val post = snapshot.getValue(Post::class.java)

                    postList.add(post!!)


                    }
                }
        })
    }

Answer 1

您的 retrievePosts 方法仅更改 postList 中的 MutableList<Post>，而不是 l_postList 中的 MutableLiveData<MutableList<Post>>，因此 RecyclerView永远不会被通知更改。 MutableLiveData 只能检测通过设置其 value.

发生的变化

您应该更新 retrievePosts 方法以引用 MutableLiveData 并在更改时设置 MutableLiveData 的 value。

RecyclerView 仅在方向更改后从 ViewModel 填充数据

RecyclerView only populates data from ViewModel after the orientation changes

android

kotlin

android-recyclerview

android-livedata

android-viewmodel