我正在尝试将Firebase数据库中的数据打印到android studio logcat中。 我能够将数据存储到列表中但我无法在logcat中打印列表。我认为这是因为在获取来自firebase的数据之前打印了数组。我不确定如何在获取来自firebase的数据后进行打印。
任何帮助将不胜感激,谢谢!
// Main activity
package com.imaginationcreators.bvnlibrary;
import android.content.Intent;
import android.support.v4.widget.DrawerLayout;
import android.support.v7.app.ActionBarDrawerToggle;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;import android.view.MenuItem;
import android.widget.Button;
import android.widget.TextView;
import com.google.firebase.database.ChildEventListener;
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 java.util.ArrayList;
public class MainActivity extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.home_screen);
Search search = new Search();
search.setLocalDatabase();
for(int i = 0; i < search.genre.size(); i++ ) {
Log.d("String", search.genre.get(i));
}
}
}
package com.imaginationcreators.bvnlibrary;
import android.util.Log;
import com.google.firebase.database.ChildEventListener;
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 java.util.ArrayList;
public class Search {
private ArrayList<Books> searchResults = new ArrayList<>();
private int numberOfResults;
private int childsAdded = 0;
private DatabaseReference databaseReference = FirebaseDatabase.getInstance().getReference();
public ArrayList<String> genre = new ArrayList<>();
public Search()
{
Log.d("String", "afterdatabasesearchclass");
}
public void setLocalDatabase()
{
Log.d("String", "notworking");
databaseReference.addChildEventListener(new ChildEventListener() {
@Override
public void onChildAdded(DataSnapshot dataSnapshot, String s) {
for(DataSnapshot dataSnapshot1 : dataSnapshot.getChildren())
{
genre.add(dataSnapshot1.getKey());
}
}
@Override
public void onChildChanged(DataSnapshot dataSnapshot, String s) {
}
@Override
public void onChildRemoved(DataSnapshot dataSnapshot) {
}
@Override
public void onChildMoved(DataSnapshot dataSnapshot, String s) {
}
@Override
public void onCancelled(DatabaseError databaseError) {
Log.d("pogo", databaseError.getDetails());
}
});
}
}
答案 0 :(得分:1)
Firebase API是异步的,这意味着它们会立即返回。运行以下代码时:
databaseReference.addChildEventListener(...)
addChildEventListener立即返回,查询结果将在稍后的某个时间传递给您的回调。只有在调用回调后才能访问快照。您的代码在结果准备好之前尝试访问它,因为setLocalDatabase()会在结果准备好之前立即返回。
Please read this blog更好地了解Firebase API异步的原因以及期望的内容。