为什么recyclerview数据只有在第二次点击按钮时才加载

Question

I am developing an app based on placement. I have used firebase realtime database for this. I am matching company name from the "Job Post" db and "Applied Candidate" db, so that only applied candidates detail for that particular company will be displayed. Everything is working fine but the issue is the recyclerview's data loads only when the button is clicked the second time.

public class AppliedCandidateActivity extends AppCompatActivity {
    Toolbar toolbarAppliedcandidate;
    RecyclerView rvAppliedCandidate;
    List<appliedData> ls;
    String compNameposted;

    AppCandidateAdapter adapter;

    DatabaseReference dbJobPost,dbAppliedCandidate;

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

        toolbarAppliedcandidate = findViewById(R.id.toolbarAppliedcandidate);
        rvAppliedCandidate = findViewById(R.id.rvAppliedCandidate);

        setSupportActionBar(toolbarAppliedcandidate);
        getSupportActionBar().setTitle("Applied Candidate");
        getSupportActionBar().setDisplayHomeAsUpEnabled(true);

        rvAppliedCandidate.setHasFixedSize(true);
        rvAppliedCandidate.setLayoutManager(new LinearLayoutManager(this));

        ls = new ArrayList<>();
        adapter = new AppCandidateAdapter(getApplicationContext(),ls);
        rvAppliedCandidate.setAdapter(adapter);

        getCompany();
        matchCompanyName();
}

    void getCompany()
    {
        //To retrieve company name
        dbJobPost = FirebaseDatabase.getInstance().getReference("Job Post").child(FirebaseAuth.getInstance().getCurrentUser().getUid());

        dbJobPost.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot snapshot) {
                for (DataSnapshot ds : snapshot.getChildren())
                {
                    PostJobData postJobData = ds.getValue(PostJobData.class);
                    compNameposted = postJobData.getCompName().toString();

                    //Toast.makeText(getApplicationContext(),postJobData.getCompName(),Toast.LENGTH_LONG).show();

                }

            }

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

            }
        });

    }

    void matchCompanyName()
    {
        //To retrieve data of applied candidate for particular company
        dbAppliedCandidate = FirebaseDatabase.getInstance().getReference("Applied Candidate");

        dbAppliedCandidate.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot snapshot) {
                for (DataSnapshot ds: snapshot.getChildren())
                {
                    for (DataSnapshot ds1 : ds.getChildren())
                    {
                        appliedData data = ds1.getValue(appliedData.class);
                        String compName = data.getCompName().toString();
                        //Toast.makeText(getApplicationContext(),compName,Toast.LENGTH_LONG).show();

                        if(compName.equals(compNameposted))
                        {

                            ls.add(data);
                        }
                        else if(ls.isEmpty()== true){
                            Toasty.info(AppliedCandidateActivity.this,"No One Applied Yet!!",Toast.LENGTH_LONG,true).show();
                        }

                    }

                }
            }

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

            }
        });
    }

    public class AppCandidateAdapter extends RecyclerView.Adapter<AppCandidateAdapter.AppCandidateViewHolder>{
        List<appliedData> appliedDataList;
        Context context;

        public  AppCandidateAdapter(Context mcontext,List list){
            this.context = mcontext;
            this.appliedDataList = list;

        }
        @NonNull
        @Override
        public AppCandidateViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
            View view = LayoutInflater.from(parent.getContext())
                    .inflate(R.layout.applied_candidate_compside,parent,false);
            return new AppCandidateViewHolder(view);
        }

        @Override
        public void onBindViewHolder(@NonNull AppCandidateViewHolder holder, int position) {
            appliedData data = appliedDataList.get(position);
            holder.tvCandidateName.setText(data.getName());
            holder.tvCandidateAppliedPost.setText(data.getPosition());
            holder.tvCandidateQual.setText(data.getQualification());
            holder.tvCandidateSkills.setText(data.getSkills());
        }

        @Override
        public int getItemCount() {
            return appliedDataList.size();
        }

        class AppCandidateViewHolder extends RecyclerView.ViewHolder{
            TextView tvCandidateName,tvCandidateAppliedPost,tvCandidateQual,tvCandidateSkills;
            Button btnDeleteCandidate,btnSendMail;

            public AppCandidateViewHolder(@NonNull View itemView) {
                super(itemView);

                tvCandidateName = itemView.findViewById(R.id.tvCandidateName);
                tvCandidateAppliedPost = itemView.findViewById(R.id.tvCandidateAppliedPost);
                tvCandidateQual = itemView.findViewById(R.id.tvCandidateQual);
                tvCandidateSkills = itemView.findViewById(R.id.tvCandidateSkills);
                btnDeleteCandidate = itemView.findViewById(R.id.btnDeleteCandidate);
                btnSendMail = itemView.findViewById(R.id.btnSendMail);


            }
        }
    }


}

Answer 1

Assuming your onDataChange does get called, successfully reads the PostJobData data from the snapshot, and adds it to ls , you're not telling Android that the list has changed. Only once you notify the adapter of the change, will it rerender the view.

dbJobPost = FirebaseDatabase.getInstance().getReference("Job Post").child(FirebaseAuth.getInstance().getCurrentUser().getUid());

dbJobPost.addValueEventListener(new ValueEventListener() {
    @Override
    public void onDataChange(@NonNull DataSnapshot snapshot) {
        for (DataSnapshot ds : snapshot.getChildren()) {
            PostJobData postJobData = ds.getValue(PostJobData.class);
            compNameposted = postJobData.getCompName().toString();
        }
        adapter.notifyDataSetChanged(); // notify the adapter
    }

    @Override
    public void onCancelled(@NonNull DatabaseError error) {
        throw error.toException(); // never ignore errors
    }
});