从Firebase数据库检索时未显示数据

Question

App not retrieving any data from firebase database. App runs nicely but shows nothing inside the app, just shows blank. Here is my code

public class profilemain extends AppCompatActivity {

    private FirebaseAuth mAuth;
    private FirebaseAuth.AuthStateListener mAuthListener;

    private Firebase url;

    private ListView listview;
    ArrayList<String> arrayList=new ArrayList<>();
    ArrayAdapter<String> arrayAdapter;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_profilemain);

        listview=(ListView) findViewById(R.id.listview);

        Firebase.setAndroidContext(this);
        url=new Firebase("https://-------.firebaseio.com/users");
        arrayAdapter=new ArrayAdapter<String>(this,android.R.layout.simple_list_item_1,arrayList);
        listview.setAdapter(arrayAdapter);
        url.addChildEventListener(new com.firebase.client.ChildEventListener() {
            @Override
            public void onChildAdded(com.firebase.client.DataSnapshot dataSnapshot, String s) {
                String value=dataSnapshot.getValue(String.class);
                arrayList.add(value);
                arrayAdapter.notifyDataSetChanged();
            }

            // .. Other Overriden functions
        });

        mAuth=FirebaseAuth.getInstance();
        mAuthListener =new FirebaseAuth.AuthStateListener() {
            @Override
            public void onAuthStateChanged(@NonNull FirebaseAuth firebaseAuth) {
                if (firebaseAuth.getCurrentUser()==null){
                    startActivity(new Intent(profilemain.this,MainActivity.class));

                }
            }
        };
    }

    @Override
    protected void onStart() {
        super.onStart();
        mAuth.addAuthStateListener(mAuthListener);
    }
}

And here is my xml file below. The app is running nicely but it is not showing the data.

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/container"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    tools:context="com.food.sheenishere.stark.profilemain">

    <FrameLayout
        android:id="@+id/content"
        android:layout_width="match_parent"
        android:layout_height="0dp"
        android:layout_weight="1">

        <ListView
            android:id="@+id/listview"
            style="@style/Widget.AppCompat.ListView"
            android:layout_width="wrap_content"
            android:layout_height="337dp" />
    </FrameLayout>

    <android.support.design.widget.BottomNavigationView
        android:id="@+id/navigation"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_gravity="bottom"
        android:background="?android:attr/windowBackground"
        app:menu="@menu/navigation" />

</LinearLayout>

Here's the schema of my Firebase database.

Answer 1

You need to have a class like this

public class Users {
    List<String> users;
}

Now get the users from your Firebase database like this.

url.addValueEventListener(new ValueEventListener() {
  @Override
  public void onDataChange(DataSnapshot snapshot) {
      Users userList = dataSnapshot.getValue(Users.class);

      // Add all the users fetch in the ArrayList. 
      for (String user : userList.users)
          arrayList.add(user);

      arrayAdapter.notifyDataSetChanged();
  }

  @Override
  public void onCancelled(DatabaseError databaseError) {        
  }
});

Answer 2

You likely are not signed in, while the default security rules require that you're authenticated when you attach the listener . A quick fix:

    mAuth=FirebaseAuth.getInstance();
    mAuthListener =new FirebaseAuth.AuthStateListener() {
        @Override
        public void onAuthStateChanged(@NonNull FirebaseAuth firebaseAuth) {
            if (firebaseAuth.getCurrentUser()==null){

                startActivity(new Intent(profilemain.this,MainActivity.class));

                arrayAdapter=new ArrayAdapter<String>(this,android.R.layout.simple_list_item_1,arrayList);
                listview.setAdapter(arrayAdapter);
                url.addChildEventListener(new com.firebase.client.ChildEventListener() {
                    @Override
                    public void onChildAdded(com.firebase.client.DataSnapshot dataSnapshot, String s) {
                        String value=dataSnapshot.getValue(String.class);
                        arrayList.add(value);
                        arrayAdapter.notifyDataSetChanged();
                    }

                    // .. Other Overriden functions
                });
            }
        }
    };

But there are too many other things that could go wrong here. For one: you're on a very old Firebase version. I recommend starting fresh with the latest documentation and this codelab .