在方法上访问arraylist

Question

我是一个android开发的初学者，我遇到了arraylist.的问题，我声明arraylist城市为全局变量，并从firebase.But添加元素到其中。之后，当我在addchildevent监听器方法之外访问这些元素时，数组列表变成了empty.How我可以声明数组列表来获取全局元素吗？我的代码片段是，

public class BookingDetails extends Fragment {
private RecyclerView recyclerView;
private DatabaseReference dataRef;
private FirebaseAuth mAuth;
ArrayList<String> uids=new ArrayList<String>();
private ArrayList<String> dates=new ArrayList<>();
private int hour;
@Nullable
@Override
public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
    InputMethodManager in=(InputMethodManager) getActivity().getSystemService(Context.INPUT_METHOD_SERVICE);
    in.hideSoftInputFromWindow(getActivity().getCurrentFocus().getWindowToken(),InputMethodManager.HIDE_NOT_ALWAYS);
    final View v=inflater.inflate(R.layout.activity_booking_details,container,false);
    recyclerView = (RecyclerView) v.findViewById(R.id.book_detail_view);
    recyclerView.setHasFixedSize(true);
    LinearLayoutManager llm = new LinearLayoutManager(getActivity());
    llm.setOrientation(LinearLayoutManager.VERTICAL);
    recyclerView.setLayoutManager(llm);
    recyclerView.addItemDecoration(new SimpleDividerItemDecoration(getActivity()));
    mAuth=FirebaseAuth.getInstance();
    final String user=mAuth.getCurrentUser().getUid();

    dataRef= FirebaseDatabase.getInstance().getReference();
    dataRef.child("Users").child(user).child("booked docters").addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            for (DataSnapshot data:dataSnapshot.getChildren()) {
                uids.add(data.getKey());
            }
       }
        @Override
        public void onCancelled(DatabaseError databaseError) {

        }
    });
    Toast.makeText(getActivity(),uids.get(0), Toast.LENGTH_SHORT).show();
    return v;
    }

}

当我在方法值上使用这个toast时，事件侦听器给出了正确的elements.How来解决这个问题，这就给了我empty.But。

Answer 1

Firebase侦听器是异步的，因此请在Firebase侦听器中按如下方式填充数据后在其中使用arraylist：

ArrayList<String> itemList;

private void someFunction() {
    itemList = new ArrayList<>();
    someDatabaseRef.addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            for (DataSnapshot child : snapshot.getChildren()) {
                String item = child.getKey();
                itemList.add(item);
            }
            // Now your itemList is filled and ready to be used...
        }

        @Override
        public void onCancelled(DatabaseError databaseError) {

        }
    });
    // If you try to use itemList here then you may get error since it is empty...
}

Answer 2

Mehmed关于哪里出了问题的理由是正确的。您需要将需要访问uids 的代码移到的` `onDataChange方法中。这样做的一种方法是：

dataRef= FirebaseDatabase.getInstance().getReference();
dataRef.child("Users").child(user).child("booked docters").addValueEventListener(new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
        for (DataSnapshot data:dataSnapshot.getChildren()) {
            uids.add(data.getKey());
        }
        Toast.makeText(getActivity(),uids.get(0), Toast.LENGTH_SHORT).show();
   }
    @Override
    public void onCancelled(DatabaseError databaseError) {
        throw databaseError.toException();
    }
});