Filtering and loading data, compared value in firebase android

问题

I want to display ListTesting data sorted by projectName from the current user. I try this but it does not work (data not showing):

    public View onCreateView(@NonNull LayoutInflater inflater,
                         ViewGroup container, Bundle savedInstanceState) {
    View view = inflater.inflate(R.layout.fragment_list_testing, container, false);
if (getActivity() != null)
        Objects.requireNonNull(((AppCompatActivity) getActivity()).getSupportActionBar()).hide();

    firebaseAuth = FirebaseAuth.getInstance();

    DatabaseReference databaseReference = FirebaseDatabase.getInstance().getReference("Users");
    Query query = databaseReference.orderByChild("id").equalTo(myId);
    query.addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
            for (DataSnapshot dataSnapshot1:dataSnapshot.getChildren()){
                projectName = "" +dataSnapshot1.child("projectName").getValue();
            }
        }

        @Override
        public void onCancelled(@NonNull DatabaseError databaseError) {

        }
    });

    //rv and properties
    rvListTest = view.findViewById(R.id.rv_postsListTest);
    FloatingActionButton fabAddListTest = view.findViewById(R.id.fab_addListTest);

    fabAddListTest.setOnClickListener(view1 -> startActivity(new Intent(getActivity(), AddListTestingActivity.class)));
    LinearLayoutManager linearLayoutManager = new LinearLayoutManager(getActivity());
    //show newest post first
    linearLayoutManager.setStackFromEnd(true);
    linearLayoutManager.setReverseLayout(true);

    //set layout to rv
    rvListTest.setLayoutManager(linearLayoutManager);

    listTestings = new ArrayList<>();

    checkUserStat();
    loadPosts();
    EditText search_listTest = view.findViewById(R.id.search_listTest);
    search_listTest.addTextChangedListener(new TextWatcher() {
        @Override
        public void beforeTextChanged(CharSequence charSequence, int i, int i1, int i2) {
        }

        @Override
        public void onTextChanged(CharSequence charSequence, int i, int i1, int i2) {
            String searchUsers = search_listTest.getText().toString().trim();
            if (!searchUsers.isEmpty()){
                rvListTest.setVisibility(View.VISIBLE);
                searchListTest(charSequence.toString().toLowerCase());
            }
        }

        @Override
        public void afterTextChanged(Editable editable) {

        }
    });
    return view;
}

private void checkUserStat() {
    FirebaseUser user = firebaseAuth.getCurrentUser();
    if (user != null) {
        myId = user.getUid();
    } else {
        startActivity(new Intent(getActivity(), ErrorActivity.class));
        requireActivity().finish();
    }
}

private void loadPosts() { //path of all posts
    DatabaseReference reference = FirebaseDatabase.getInstance().getReference("ListTesting");
    //query to load posts
    Query query = reference.orderByChild("projectName").equalTo(projectName);
    //get all data from node Posts
    query.addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
            listTestings.clear();
            for (DataSnapshot ds : dataSnapshot.getChildren()) {
                ListTesting listTesting = ds.getValue(ListTesting.class);

                listTestings.add(listTesting);

                //adapter
                adapter = new ListTestingAdapter(getActivity(), listTestings);
                //set adapter to rv
                rvListTest.setAdapter(adapter);
            }
        }

        @Override
        public void onCancelled(@NonNull DatabaseError databaseError) {
            //if error
            Toast.makeText(getActivity(), "" + databaseError.getMessage(), Toast.LENGTH_SHORT).show();
        }
    });
}`

回答1:

You can't make a global variable for value in the firebase database, because the database is asynchronous which means there is some time to get the value from the database.

There are two options to solve this:

1- Put the code inside the firebase listener, in your case like this

DatabaseReference databaseReference = FirebaseDatabase.getInstance().getReference("Users");
Query query = databaseReference.orderByChild("id").equalTo(myId);
query.addValueEventListener(new ValueEventListener() {
    @Override
    public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
        for (DataSnapshot dataSnapshot1:dataSnapshot.getChildren()){
            projectName = "" +dataSnapshot1.child("projectName").getValue();
        }

     //then here put your code that exists in loadPosts function

     DatabaseReference reference = 
     FirebaseDatabase.getInstance().getReference("ListTesting");
     //query to load posts
     Query query = reference.orderByChild("projectName").equalTo(projectName);

     //get all data from node Posts
     query.addValueEventListener(new ValueEventListener() {
     @Override
     public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
        listTestings.clear();
        for (DataSnapshot ds : dataSnapshot.getChildren()) {
            ListTesting listTesting = ds.getValue(ListTesting.class);

            listTestings.add(listTesting);

            //adapter
            adapter = new ListTestingAdapter(getActivity(), listTestings);
            //set adapter to rv
            rvListTest.setAdapter(adapter);
        }
    }

    @Override
    public void onCancelled(@NonNull DatabaseError databaseError) {
        //if error
        Toast.makeText(getActivity(), "" + databaseError.getMessage(), 
        Toast.LENGTH_SHORT).show();
    }
});

    }

    @Override
    public void onCancelled(@NonNull DatabaseError databaseError) {

    }
});

2- Create your own callback to wait for Firebase to return your data like this.

Hope this helps you :)

来源：https://stackoverflow.com/questions/62187299/filtering-and-loading-data-compared-value-in-firebase-android