listview is not showing up

Question

我正在从firebase获取PDF并借助Listview显示，但是当我单击查看PDF button时，列表没有显示出来。

请让我知道代码中有什么问题。这真的很有帮助。上传按钮工作县仅视图button什么都没有显示。

以下是我的代码：

 public class View_life_science_pdf extends AppCompatActivity 
    {
    ListView lifesciencepdflist;
    DatabaseReference databaseReference;
    List<UploadPDF> uploadPDFS;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_view_life_science_pdf);
        lifesciencepdflist = findViewById(R.id.life_science_pdfs);
        uploadPDFS = new ArrayList<>();
        viewAllFiles();
        lifesciencepdflist.setOnItemSelectedListener(new AdapterView.OnItemSelectedListener() {
            @Override
            public void onItemSelected(AdapterView<?> parent, View view, int position, long id) {
                UploadPDF uploadPDF = uploadPDFS.get(position);
                Intent intent = new Intent();
                intent.setData(Uri.parse(uploadPDF.getUrl()));
                startActivity(intent);
            }
            @Override
            public void onNothingSelected(AdapterView<?> parent) {
            }
        });
    }
    private void viewAllFiles() {
        databaseReference = FirebaseDatabase.getInstance().getReference("uploads_Life_science_jour");
        databaseReference.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                for(DataSnapshot postSnapshot : dataSnapshot.getChildren()){
                    UploadPDF uploadPDF = postSnapshot.getValue(com.example.publications.UploadPDF.class);
                    uploadPDFS.add(uploadPDF);
                }
                String[] uploads = new String[uploadPDFS.size()];
                for (int i=0;i<uploads.length;i++)
                {
                    uploads[i]= uploadPDFS.get(i).getPdfname();
                }
                ArrayAdapter<String> adapter = new ArrayAdapter<String>(getApplicationContext(),android.R.layout.simple_list_item_1,uploads)
                {
                    @Override
                    public View getView(int position,  View convertView,  ViewGroup parent) {
                        View view = super.getView(position, convertView, parent);
                        TextView mytext = view.findViewById(android.R.id.text1);
                        mytext.setTextColor(Color.BLACK);
                        return view;
                    }
                } ;
                lifesciencepdflist.setAdapter(adapter);
            }
            @Override
            public void onCancelled(@NonNull DatabaseError databaseError) {
            }
        });
    }
}

Answer 1

当前代码没有任何点击功能。因此，仅当加载应用程序屏幕时，它才会显示。如果您只想在用户单击按钮时要获取，则需要添加onclicklistener并将其连接到按钮