Question

我使用NavigationDrawer并尝试了MVVM体系结构，所有布局都很好，然后我实现了一个RecyclerView，可在其中一个片段中显示Firebase的数据。

不会显示任何错误，但是RecyclerView不会填充数据。我向应用程序添加了日志记录，发现适配器返回了大小data != null，但有趣的是 recyclerView仅在设备配置更改（例如设备方向）上填充数据 < / p>

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

我尝试过：

使过程脱离对onCreateView
将数据源更改为虚拟集，并且有效
在终端上显示数据，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MutableList<Post>中的postList，而不更改MutableLiveData<MutableList<Post>>中的l_postList，因此RecyclerView永远不会通知更改。 MutableLiveData只能检测通过设置其value而发生的更改。

您应该更新retrievePosts方法以引用MutableLiveData，并在更改时设置该value的{{1}}。

RecyclerView仅在方向更改后才从ViewModel填充数据

1 个答案: