java - 如何从 Firebase 查询

Question

我做了很多搜索，但我不知道如何从 firebase 进行简单的查询。这是我的数据库的结构:

“locations”的父级是数据库的名称。我试图获取“位置”的所有子项的列表，以下是对我不起作用的内容:

 mFirebase = FirebaseDatabase.getInstance().getReference();
    //Create location list
    mFirebase.child("locations").addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            for(DataSnapshot entry: dataSnapshot.getChildren()) {
                Log.d("HERE: ", entry.toString());
            }
        }

        @Override
        public void onCancelled(DatabaseError databaseError) {

        }
    });

Log.d 方法从未运行过，因此它永远不会进入 forEach 循环。有没有更简单的方法从数据库查询，或者我做错了？谢谢

编辑:更改为 addListenerForSingleValueEvent() 并在 ForEach 循环之前添加一行，以确保实际调用 onDataChange() 方法(确实如此)

mFirebase.child("locations").addListenerForSingleValueEvent(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            Log.d("Entered: ", "onDataChange");
            for(DataSnapshot entry: dataSnapshot.getChildren()) {
                Log.d("HERE: ", entry.toString());
            }
        }

        @Override
        public void onCancelled(DatabaseError databaseError) {

        }
    });

这是完整的代码:(注意:我正在从 SQLite 切换到 Firebase，因此那里仍然有来自 SQLite 的代码，当我让 Firebase 工作时，这些代码就会消失。

package osu.mobile_apps.ohiostatetourchallenge;

import android.content.Intent;
import android.provider.ContactsContract;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.support.v7.widget.*;
import android.util.Log;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.TextView;

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.Collections;
import java.util.List;

import database.OsuTourDbSchema.DatabaseHelper;

public class ListActivity extends AppCompatActivity {

private RecyclerView mlocations;
private RecyclerView.Adapter mAdapter;
private List<ListItem> listItems;
private DatabaseHelper mDatabaseHelper = new DatabaseHelper(this);
private DatabaseReference mFirebase;

// Stops the back button
@Override
public void onBackPressed() {}

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    Log.d("TESTING","ON CREATE() FOR LIST");
    setContentView(R.layout.activity_list);

    //Get username from log in activity
    //String userName = getIntent().getExtras().getString("User");

    TextView welcome = (TextView) findViewById(R.id.welcome);
    if(welcome != null) {
        //welcome.setText("Welcome " + userName + "!");
    }
    mlocations = (RecyclerView) findViewById(R.id.locationList);
    //Every item has fixed size
    mlocations.setHasFixedSize(true);
    mFirebase = FirebaseDatabase.getInstance().getReference();
    //Create location list
    mFirebase.addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            Log.d("Before Loop: ", dataSnapshot.toString());
            for(DataSnapshot entry: dataSnapshot.getChildren()) {
                Log.d("HERE: ", entry.getValue().toString());
            }
        }

        @Override
        public void onCancelled(DatabaseError databaseError) {

        }
    });
    mlocations.setLayoutManager(new LinearLayoutManager(this));
    List<Location> locations = mDatabaseHelper.getLocations();
    Collections.sort(locations, Location.LocationNameComparator);
    listItems = new ArrayList<>();

    String description;
    String smallDescription;

    for(Location entry: locations) {
        description = entry.getDescription();
        //TODO Check if location is unlocked or not
        if (description.length()>110){
            smallDescription = description.substring(0,110) + "...";
        } else {
            smallDescription = description;
        }
        ListItem item = new ListItem(entry.getName(), smallDescription);
        listItems.add(item);
    }

    mAdapter = new myAdpater(listItems);

    mlocations.setAdapter(mAdapter);

}
@Override
public boolean onCreateOptionsMenu(Menu menu){
    getMenuInflater().inflate(R.menu.menu_list,menu);
    return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item){
    switch(item.getItemId()){
        case R.id.map:
            //Start map activity

            return true;

        case R.id.about:
            //Start about activity
            Intent intent = new Intent(ListActivity.this, AboutActivity.class);
            startActivity(intent);
            return true;

    }
    return super.onOptionsItemSelected(item);
}

public void onClick(View v){
    Intent intent = new Intent(ListActivity.this, InformationActivity.class);
    if(v.getId()==R.id.textViewHead){
        TextView TestView = (TextView) v.findViewById(R.id.textViewHead);
        intent.putExtra("Header", TestView.getText());
    }
    startActivity(intent);
}

}

Answer 1

代码中的所有内容都是正确的，但它不起作用，因为您需要更改 firebase 控制台中的规则。

因此，转到 Firebase 数据库，然后单击规则并将规则更改为:

{
"rules": {
   ".read": "true",
   ".write": "true"
 }
}

您将在日志中得到以下输出:-

D/HERE:: DataSnapshot { key = 1, value = {description=sadasd, latitude=dssdfds} }

D/HERE:: DataSnapshot { key = 2, value = {description=asdasdas, latitude=randomwords} }

为 future 的观看者进行编辑

在 Frank van Puffelen 评论后，问题出在关键节点的引文上