Gridview项目不显示

Question

我想将GridView与ArrayList绑定。这是我的适配器：

public class GridAdapter extends BaseAdapter {
    ArrayList<Upload>uploads;
    ImageView preview;

    public GridAdapter (ArrayList<Upload>uploads) {
        this.uploads=uploads;
    }
    @Override
    public int getCount() {
        return uploads.size();
    }

    @Override
    public Object getItem(int position) {
        return uploads.get(position);
    }

    @Override
    public long getItemId(int position) {
        return position;
    }

    @Override
    public View getView(int position, View convertView, ViewGroup parent) {
        LayoutInflater layoutInflater=(LayoutInflater) getSystemService(Context.LAYOUT_INFLATER_SERVICE);

        convertView=layoutInflater.inflate(R.layout.row_data,null);

        preview=(ImageView) convertView.findViewById(R.id.gridImageView);
        Picasso.with(getApplicationContext()).load(uploads.get(position).getImageUrl()).into(preview);

        return null;
    }} 

这就是我设置适配器的方式以及向ArrayList添加项目的方式

        pb = (ProgressBar) findViewById(R.id.pb);
        uploadList=new ArrayList<>();
        adapter = new GridAdapter(uploadList);
        intent=getIntent();
        directory=intent.getStringExtra("directory");

        gridView= findViewById(R.id.gridView);
        mDatabaseRef=FirebaseDatabase.getInstance().getReference("uploads/"+directory);

        mDatabaseRef.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
               for(DataSnapshot postSnapshot:dataSnapshot.getChildren()) {
                   Upload upload=postSnapshot.getValue(Upload.class);
                   uploadList.add(upload);

                }
                gridView.setAdapter(adapter);
               pb.setVisibility(View.INVISIBLE);

            } 

在活动中，没有显示任何内容，没有可单击的内容，因此没有创建GridView项