java - 尝试使用 Reference.orderByChild() 从 Firebase 中的实时数据库获取数据时出错

Question

在我的 Android Studio 项目中，我试图实现一个功能，在用户登录后，应用程序将在 firebase 实时数据库中获取用户的名称，并获取子节点所在的节点的名称，“nomeProf”，将用户的名称作为值，并将标签的文本设置为它找到的节点的名称。

my firebase realtimedatabase structure

package com.example.classmob.fragment;


import android.content.Context;
import android.content.Intent;
import android.os.Bundle;
import android.support.annotation.NonNull;
import android.support.v4.app.Fragment;
import android.support.v7.widget.DividerItemDecoration;
import android.support.v7.widget.LinearLayoutManager;
import android.support.v7.widget.RecyclerView;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.AdapterView;
import android.widget.Button;
import android.widget.LinearLayout;
import android.widget.TextView;
import android.widget.Toast;

import com.example.classmob.R;
import com.example.classmob.RecyclerItemClickListener;
import com.example.classmob.activity.alunoActivity;
import com.example.classmob.activity.professorActivity;
import com.example.classmob.adapter.adapterSalas;
import com.example.classmob.model.ContentMain;
import com.google.firebase.auth.FirebaseAuth;
import com.google.firebase.database.DataSnapshot;
import com.google.firebase.database.DatabaseError;
import com.google.firebase.database.DatabaseReference;
import com.google.firebase.database.FirebaseDatabase;
import com.google.firebase.database.ValueEventListener;

import java.util.ArrayList;
import java.util.List;

/**
 * A simple {@link Fragment} subclass.
 */
public class salasFragment extends Fragment {
    private RecyclerView RecyclerView1;
    private List<ContentMain> serie = new ArrayList<>();
    private TextView txtSalas;
    private String nomeProf, salasAula="";
   // private Button btnTentar;
    private DatabaseReference reference = FirebaseDatabase.getInstance().getReference("usuarios/");
    private DatabaseReference referenceSalas = FirebaseDatabase.getInstance().getReference("salas/");

    ArrayList<String> aulaDada = new ArrayList<String>();

    public salasFragment() {
        // Required empty public constructor
    }


    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        // Inflate the layout for this fragment
        View view = inflater.inflate(R.layout.fragment_salas, container, false);

        final Context context = view.getContext();
        txtSalas= view.findViewById(R.id.txtSalas);
        //btnTentar= view.findViewById(R.id.btnTeste);

        salas();

        return view; }

        public void salas(){
            FirebaseAuth autenticacao = FirebaseAuth.getInstance();
            String emailUsu= autenticacao.getCurrentUser().getEmail();
            reference.orderByChild("email").equalTo(emailUsu).addListenerForSingleValueEvent(new ValueEventListener() {
                @Override
                public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                    for(DataSnapshot datas: dataSnapshot.getChildren()){
                        nomeProf= datas.child("nome").getValue().toString();
                    }
                }
                @Override
                public void onCancelled(@NonNull DatabaseError databaseError) {
                    throw databaseError.toException();
                }
            });
            if(nomeProf != null) {
                referenceSalas.orderByChild("nomeProf").equalTo(nomeProf).addListenerForSingleValueEvent(new ValueEventListener() {
                    @Override
                    public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                        for (DataSnapshot datas : dataSnapshot.getChildren()) {
                            salasAula += datas.getKey();

                        }

                    }

                    @Override
                    public void onCancelled(@NonNull DatabaseError databaseError) {
                        throw databaseError.toException();
                    }
                });
                if (!salasAula.equals(""))
                    txtSalas.setText(salasAula);
                else
                    Toast.makeText(getActivity(), "you dont teach any class yet"/*"Você não dá aula a nenhuma sala ainda"*/, Toast.LENGTH_LONG).show();
            }else{
                Toast.makeText(getActivity(), "ERROR", Toast.LENGTH_SHORT).show();

            }
        }
}

我已尝试使用此代码来执行此操作，但它不起作用，我不知道为什么。它总是最终显示“错误”的 toast 消息。我该如何修复它？

Answer 1

您不是在等待响应，而是在查询得到实际结果之前检查nomeProf != null。这是你必须做的:

public void salas(){
            FirebaseAuth autenticacao = FirebaseAuth.getInstance();
            String emailUsu= autenticacao.getCurrentUser().getEmail();
            reference.orderByChild("email").equalTo(emailUsu).addListenerForSingleValueEvent(new ValueEventListener() {
                @Override
                public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                    for(DataSnapshot datas: dataSnapshot.getChildren()){
                        nomeProf= datas.child("nome").getValue().toString();


                    if(nomeProf != null) {
                referenceSalas.orderByChild("nomeProf").equalTo(nomeProf).addListenerForSingleValueEvent(new ValueEventListener() {
                    @Override
                    public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                        for (DataSnapshot datas : dataSnapshot.getChildren()) {
                            salasAula += datas.getKey();

                        }

                    }

                    @Override
                    public void onCancelled(@NonNull DatabaseError databaseError) {
                        throw databaseError.toException();
                    }
                });
                if (!salasAula.equals(""))
                    txtSalas.setText(salasAula);
                else
                    Toast.makeText(getActivity(), "you dont teach any class yet"/*"Você não dá aula a nenhuma sala ainda"*/, Toast.LENGTH_LONG).show();
            }else{
                Toast.makeText(getActivity(), "ERROR", Toast.LENGTH_SHORT).show();

            }
                    }
                }
                @Override
                public void onCancelled(@NonNull DatabaseError databaseError) {
                    throw databaseError.toException();
                }
            });

        }