Question

我有一个Activity，我必须使用另一个类DataBase中的函数。在DataBase中，代码必须从我的 FireBase数据库获取信息，然后返回ArrayList作为结果。但是当我在takeDataFromFirebase中使用函数null时，它的大小为null。如何将带有FireBase信息的数组添加到我的活动中？

public class DataBase{    
public ArrayList<String> arraylist = new ArrayList<>();
public ArrayList<String> takeDataFromFirebase(DatabaseReference dRef) {
        usersRef = dRef.child(USERS_CHILD);
        ValueEventListener valueEventListener = new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {
                for(DataSnapshot ds : dataSnapshot.getChildren()) {
                    String userKey = ds.getKey();
                    DatabaseReference messagesRef = usersRef.child(userKey).child(USER_MESSAGES);
                    ValueEventListener eventListener = new ValueEventListener() {
                        @Override
                        public void onDataChange(DataSnapshot dataSnapshot) {
                            for(DataSnapshot dSnapshot : dataSnapshot.getChildren()) {
                                messageText = dSnapshot.child(USER_MESSAGE_TEXT).getValue(String.class);
                                arrayList.add(messageText);
                            }
                        }
                        @Override
                        public void onCancelled(DatabaseError databaseError) {}
                    };
                    messagesRef.addListenerForSingleValueEvent(eventListener);
                }
            }
            @Override
            public void onCancelled(DatabaseError databaseError) {}
        };
        usersRef.addListenerForSingleValueEvent(valueEventListener);

        return null;
    }
}

以及我使用此方法的活动：

public class Categories extends AppCompatActivity{


private static final String TAG = "CATEGORIES";
FirebaseAuth mAuth;

private ListView all_problems_list_view;
private Button add_problem_button, signOut;
private static long back_pressed;


FirebaseDatabase mDataBase;
DatabaseReference dRef;
DatabaseReference userRef;
DataBase dataBaseFunctions;

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

    dataBaseFunctions = new DataBase();
    mAuth = FirebaseAuth.getInstance();


    mDataBase = FirebaseDatabase.getInstance();
    dRef = mDataBase.getReference();
    userRef = dRef.child("Users");
    //region add all items

    all_problems_list_view = (ListView) findViewById(id.all_problems_list_view);
    add_problem_button = (Button) findViewById(id.add_problem_button);
    signOut = (Button) findViewById(id.SignOutBut);
    //endregion

    //region set onClick to buttons
    signOut.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            logout();
        }
    });

    add_problem_button.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            startActivity(new Intent(Categories.this, sendEmailActivity.class));
        }
    });


    //endregiond



    ArrayList<String> list = dataBaseFunctions.takeDataFromFirebase(dRef);
    ArrayAdapter<String> adapter = new ArrayAdapter<>(this,
            android.R.layout.simple_list_item_1, list);
    all_problems_list_view.setAdapter(adapter);


}

Answer 1

你的函数takeDataFromFirebase返回null，所以你的结果总是为null。用你想得到的数组替换这个null，我想是arrayList。

为什么我不能从另一个类中获取我的数组

1 个答案: