Start next activity if firebase database has no child value

问题

I am using to populate the Recycleview Adapter form firebase database in Activity A. I am looking for solution to return to Mainactivity when it has empty parent or no child.

Here is my code

public class SubjectBooks extends AppCompatActivity {
    FirebaseAuth fauth;
    String user;
    DatabaseReference dbreference,dbref;
    RecyclerView rv;
    String subject;
    int child_count=0;
    ArrayList<Books> list;
    SubjectBooksAdapter staggeredBooksAdapter;
    ProgressDialog progressDialog;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_subject_books);
       


        fauth = FirebaseAuth.getInstance();
        subject = getIntent().getStringExtra("subject").trim();

        dbreference = FirebaseDatabase.getInstance().getReference("books").child(subject);
        dbreference.keepSynced(true);
        user = fauth.getCurrentUser().getEmail();

      

        final ProgressDialog  progressDialog = ProgressDialog.show(this, "Loading...", "Please wait...", true);
        progressDialog.setMessage("Please wait ...");
        progressDialog.setCanceledOnTouchOutside(true);
        progressDialog.show();

       
        dbreference.addChildEventListener(new ChildEventListener() {
            @Override
            public void onChildAdded(DataSnapshot dataSnapshot, String s) {

                final Books b1 = dataSnapshot.getValue(Books.class);
              //  Log.e("Value is ",dataSnapshot.getKey()+" "+b1.getBauthor());
                //Log.e("Book"," received");
                if(!user.equals(b1.getSelleremail()) || (user.equals(b1.getSelleremail())) ) {

                    child_count++;
                    list.add(b1);
                    staggeredBooksAdapter.notifyDataSetChanged();

                    progressDialog.dismiss();
                }
                


             
              if(child_count==0){
                   progressDialog.dismiss();
                    Toast.makeText(SubjectBooks.this, "No books found!", Toast.LENGTH_SHORT).show();
                   Intent in = new Intent(SubjectBooks.this,MainActivity.class);
                   startActivity(in);
                    finish();
               }
            }

I have initially declared

int child_count=0;

But also when it has no any child value..still progressDialog keeps loading infinitely unless any key is pressed.

Any help is appreciated.

回答1:

onChildAdded() will only be called when a new child has been added or a child already exist.

In your case, you don't have any child added, So onChildAdded() is never been called. that's why progressDialog keeps loading infinitely

To detect if a child exists, use ValueEventListener instead of ChildEventListener. Here is link how to use ValueEventListener

回答2:

You can use exists to check if the path that refers your database reference exists

 dbreference.addChildEventListener(new ChildEventListener() {
            @Override
            public void onChildAdded(DataSnapshot dataSnapshot, String s) {
                if(!dataSnapshot.exists()){

                final Books b1 = dataSnapshot.getValue(Books.class);
              //  Log.e("Value is ",dataSnapshot.getKey()+" "+b1.getBauthor());
                //Log.e("Book"," received");
                if(!user.equals(b1.getSelleremail()) || (user.equals(b1.getSelleremail())) ) {

                    child_count++;
                    list.add(b1);
                    staggeredBooksAdapter.notifyDataSetChanged();

                    progressDialog.dismiss();
                }




           }else{
                    progressDialog.dismiss();
                    Toast.makeText(SubjectBooks.this, "No books found!", Toast.LENGTH_SHORT).show();
                    Intent in = new Intent(SubjectBooks.this,MainActivity.class);
                    startActivity(in);
                    finish();

            }
}

exists

exists() returns boolean

Returns true if this DataSnapshot contains any data. It is slightly more efficient than using snapshot.val() !== null.

That will check if there is data inside the path your dbReference points out, but it will not check if the childs inside are > 0.

To do so you can check after the else statment if inside that node you have more than 1 child node

   ...
            }else{

                 long childrenQty = dataSnapshot.getChildrenCount();;
                  if(childrenQty > 0 ) {
                   //We have >= than 1 children to show in our recyclerView  :-)
                  }else{
                   //Theres nothing inside to show in our recyclerView  :-(
                               progressDialog.dismiss();
                               Toast.makeText(SubjectBooks.this, "No books found!", Toast.LENGTH_SHORT).show();
                               Intent in = new Intent(SubjectBooks.this,MainActivity.class);
                               startActivity(in);
                               finish();

                        }
}

来源：https://stackoverflow.com/questions/54165807/start-next-activity-if-firebase-database-has-no-child-value