Question

我用于在活动A中填充Recycleview Adapter表单firebase数据库。我正在寻找解决方案，以在其父级为空或子级为空时返回Mainactivity。

这是我的代码

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();
               }
            }

我最初已经声明

int child_count=0;

但是，当它没有任何子值时，.still progressDialog会无限加载，除非按下任何键。

感谢您的帮助。

Answer 1

onChildAdded()仅在添加了新的孩子或孩子已经存在时被调用。

在您的情况下，您没有添加任何子级，因此永远不会调用onChildAdded()。这就是为什么 progressDialog保持无限加载

的原因

要检测是否存在孩子，请使用ValueEventListener而不是ChildEventListener。这是链接how to use ValueEventListener

Answer 2

您可以使用exist检查引用数据库引用的路径是否存在

 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();

            }
}

存在

exists（）返回布尔值

如果此DataSnapshot包含任何数据，则返回true。稍微比使用snapshot.val（）！== null更有效。

这将检查dbReference所指出的路径内是否有数据，但不会检查其中的子级是否大于0。

为此，您可以在else语句之后检查该节点内是否有多个子节点

   ...
            }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();

                        }
}

如果Firebase数据库没有子值，则开始下一个活动

2 个答案:

存在