如何从Firebase获取数据并将其显示在一个RecyclerView中？[副本]

Question

我开发了一个聊天应用程序。我试图从firebase数据库中获取数据，并将其显示在一个recyclerview中，但这并不起作用。我在谷歌上搜索了它，我试了很多选项，但它什么都显示不出来。我做错了什么？这些活动是：

1.主活动（我想在哪里显示数据）

public class SearchActivity extends AppCompatActivity {
Toolbar sToolbar;
SearchView searchView;
RecyclerView recyclerView;
BottomNavigationView sBnv;
SearchAdapter searchAdapter;
LinearLayoutManager llm;
ArrayList<String> arrayList;
User user;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_search);

    sToolbar = findViewById(R.id.sToolbar);
    recyclerView = findViewById(R.id.sRecyclerView);
    sBnv = findViewById(R.id.sBnv);

    searchAdapter = new SearchAdapter(arrayList, getApplicationContext());
    recyclerView.setHasFixedSize(true);
    llm = new LinearLayoutManager(getApplicationContext());

    final DatabaseReference ref = FirebaseDatabase.getInstance().getReference().child("users");

    ref.addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            if (dataSnapshot.exists()) {
                for (DataSnapshot snapshot : dataSnapshot.getChildren()) {
                    String username = (String)
                            snapshot.child("username").getValue();
                    arrayList.add(username);

                }
                recyclerView.setLayoutManager(llm);
                recyclerView.setAdapter(searchAdapter);
            }
        }

        @Override
        public void onCancelled(DatabaseError databaseError) {
        Toast.makeText(SearchActivity.this, "Database error", Toast.LENGTH_SHORT)
                .show();
        }
    });

public class SearchAdapter extends RecyclerView.Adapter<SearchAdapter.ViewHolder> {

ArrayList<String> friendsSearchedList = new ArrayList<>();
Context mContext;

public SearchAdapter(ArrayList<String> friendsSearchedList, Context mContext){
    this.friendsSearchedList = friendsSearchedList;
    this.mContext = mContext;
}

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

@Override
public void onBindViewHolder(@NonNull ViewHolder holder, int position) {
    final String username = friendsSearchedList.get(position);
    holder.text.setText(username);

}

@Override
public int getItemCount() {
return friendsSearchedList == null ? 0:friendsSearchedList.size();
}

public class ViewHolder extends RecyclerView.ViewHolder {
TextView text, text1, text2;
Button button;
RelativeLayout relativeLayout;
public ViewHolder(View itemView){
super(itemView);

text = itemView.findViewById(R.id.sText);
button = itemView.findViewById(R.id.addFriend);
text1 = itemView.findViewById(R.id.sText1);
text2 = itemView.findViewById(R.id.sText2);
relativeLayout = itemView.findViewById(R.id.relativeLayout);
    }
}

public class User {
public String  email, id, password;
public String username;

public User(String email, String username, String password){
this.email = email;
this.username = username;
this.password = password;
}
public User(){}
public void setUsername(String username){
this.username = username;
}
public String getUsername(){
return username;
}
public void setEmail(String email){
this.email = email;
}
public String getEmail(){
return email;
}
public String getPassword(){
return password;
}
public void setPassword(String password){
this.password = password;
}

}

Answer 1

最初，您为recyclerview设置的arraylist为空。将数据添加到该列表时，必须通知回收器视图适配器。所以您的代码应该是这样的：

searchAdapter = new SearchAdapter(arrayList, getApplicationContext());
  // remove this line
    /* recyclerView.setHasFixedSize(true);*/
    llm = new LinearLayoutManager(getApplicationContext());
    recyclerView.setLayoutManager(llm);
                recyclerView.setAdapter(searchAdapter);

    final DatabaseReference ref = FirebaseDatabase.getInstance().getReference().child("users");

    ref.addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            if (dataSnapshot.exists()) {
                for (DataSnapshot snapshot : dataSnapshot.getChildren()) {
                    String username = (String)
                            snapshot.child("username").getValue();
                    arrayList.add(username);

                }
                  ***searchAdapter.notifyDataSetChanged();***

            }
        }