Firestore写入正常但无法读取数据且无报错问题求助

问题现象 #

• Firestore数据写入功能正常运行
• 参考官方文档编写的读取代码全程无报错、不触发失败回调，但无法获取到任何数据
• 调用读取方法后立刻获取数据列表为空，RecyclerView无法展示内容

问题代码 #

Fragment 实现 #

override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
    super.onViewCreated(view, savedInstanceState)
    init()
}

private fun init() {
    db = FirebaseFirestore.getInstance()
    mFirestoreRef = db.collection("users").document(USER_ID).collection("deals")
    val viewModel = ViewModelProvider(this).get(DealsViewModel::class.java)
    viewModel.retriveData(mFirestoreRef)
    recyclerView = binding.rcView
    adapter = TradeNoteAdapter()
    recyclerView.adapter = adapter
    var notes = viewModel.getAllNotes()
    adapter.setList(notes)
}

ViewModel 实现 #

import android.widget.Toast
import androidx.lifecycle.ViewModel
import com.google.firebase.firestore.CollectionReference
import space.timur.tradenotefirebase.APP
import space.timur.tradenotefirebase.model.TradeNoteModel

class DealsViewModel : ViewModel() {

private var notes: ArrayList<TradeNoteModel> = ArrayList()

fun retriveData(firestoreReference: CollectionReference){
    firestoreReference.get()
        .addOnSuccessListener { documents ->
            for(document in documents){
                var note = document.toObject(TradeNoteModel::class.java)
                notes.add(note)
            }
        }.addOnFailureListener{ e->
            Toast.makeText(APP, e.message, Toast.LENGTH_SHORT).show()
        }
}

fun getAllNotes(): ArrayList<TradeNoteModel> {
    return notes
}

}

问题根因 #

Firestore的get()方法是异步网络请求，调用后不会阻塞当前线程等待结果返回。你在调用retriveData()后立刻执行getAllNotes()时，请求还在网络传输阶段，notes列表还没被写入数据，自然是空的。等后续addOnSuccessListener回调执行、往notes里添加数据时，你早就把空列表传给了适配器，也没有通知适配器数据更新，所以RecyclerView一直显示空白。

修复方案 #

使用LiveData持有加载完成的数据，让Fragment观察数据变化，数据拉取完成后自动刷新UI：

修正后的ViewModel #

import android.widget.Toast
import androidx.lifecycle.LiveData
import androidx.lifecycle.MutableLiveData
import androidx.lifecycle.ViewModel
import com.google.firebase.firestore.CollectionReference
import space.timur.tradenotefirebase.APP
import space.timur.tradenotefirebase.model.TradeNoteModel

class DealsViewModel : ViewModel() {

    private val _notes = MutableLiveData<List<TradeNoteModel>>()
    val notes: LiveData<List<TradeNoteModel>> = _notes

    fun retriveData(firestoreReference: CollectionReference){
        firestoreReference.get()
            .addOnSuccessListener { documents ->
                val noteList = arrayListOf<TradeNoteModel>()
                for(document in documents){
                    val note = document.toObject(TradeNoteModel::class.java)
                    noteList.add(note)
                }
                // 数据加载完成后发送到LiveData
                _notes.postValue(noteList)
            }.addOnFailureListener{ e->
                Toast.makeText(APP, e.message, Toast.LENGTH_SHORT).show()
            }
    }
}

修正后的Fragment初始化逻辑 #

private fun init() {
    db = FirebaseFirestore.getInstance()
    mFirestoreRef = db.collection("users").document(USER_ID).collection("deals")
    val viewModel = ViewModelProvider(this).get(DealsViewModel::class.java)
    recyclerView = binding.rcView
    adapter = TradeNoteAdapter()
    recyclerView.adapter = adapter

    // 观察数据变化，数据加载完成后自动刷新列表
    viewModel.notes.observe(viewLifecycleOwner) { noteList ->
        adapter.setList(noteList)
    }

    // 触发数据拉取
    viewModel.retriveData(mFirestoreRef)
}

注意：如果不使用LiveData，也可以直接在addOnSuccessListener回调中拿到数据后更新适配器，但绝对不能在异步请求刚发起时同步读取结果，Firestore所有网络操作默认都是异步执行的，不会立刻返回结果。

内容的提问来源于stack exchange，提问作者Timson01