滚动加载更多数据，按降序无法使用到RecyclerView

Question

我试图向下滚动将数据加载到Recyclerview中。按升序工作正常。但是，当我按降序尝试时，它不起作用。我的服务器端代码是PHP。请参见下面的代码。

@Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_bank_transection);

        recyclerView = findViewById(R.id.recycler_view);

        progressDialog = new ProgressDialog(BankTransection.this);
        progressDialog.setCancelable(false);

        transactionList = new ArrayList<>();
        adapter = new TransactionAdapter(BankTransection.this, transactionList);

        session = new Session(BankTransection.this);
        userid = session.getUserDetails().get(session.KEY_USERID);


        mLayoutManager= new GridLayoutManager(BankTransection.this,1);
        recyclerView.setLayoutManager(mLayoutManager);
        recyclerView.addItemDecoration(new BankTransection.GridSpacingItemDecoration(1, dpToPx(10), true));
        recyclerView.setItemAnimator(new DefaultItemAnimator());
        recyclerView.setAdapter(adapter);

        recyclerView.addOnScrollListener(new RecyclerView.OnScrollListener() {

            @Override
            public void onScrolled(RecyclerView recyclerView, int dx, int dy) {
                //super.onScrolled(recyclerView, dx, dy);

                if(mLayoutManager.findLastCompletelyVisibleItemPosition()  == transactionList.size()-1){
                    GetTransactionlist(transactionList.get(transactionList.size()-1).getId());
                }
            }
        });

        GetTransactionlist(0);
    }

    private void GetTransactionlist(int id){

        progressDialog.setMessage("Loading Please Wait...");
        showDialog();

        AsyncTask<Integer,Void,Void> task = new AsyncTask<Integer, Void, Void>() {
            @Override
            protected Void doInBackground(Integer... integers) {

                OkHttpClient client = new OkHttpClient();
                okhttp3.Request request = new okhttp3.Request.Builder()
                        .url("http://api.prokart.in/gettransaction.php?userid="+userid+"&id="+integers[0])
                        .build();
                try {
                    Response response = client.newCall(request).execute();
                    JSONArray array = new JSONArray(response.body().string());

                    for (int i=0; i<array.length(); i++){

                        JSONObject object =array.getJSONObject(i);

                        Log.e("Trandata", String.valueOf(object));

                        int id =object.has("id")?object.getInt("id"):0;
                        String userid = object.has("userid")?object.getString("userid"):"";
                        String cdate = object.has("cdate")?object.getString("cdate"):"";
                        int credit = object.has("credit")?object.getInt("credit"):0;
                        int debit = object.has("debit")?object.getInt("debit"):0;
                        int balance =object.has("balance")?object.getInt("balance"):0;
                        String tremark = object.has("tremark")?object.getString("tremark"):"";

                        Log.e("TranResponse", String.valueOf(object));

                        Transactions transactions = new Transactions(id,userid,cdate,credit,debit,balance,tremark);
                        transactionList.add(transactions);
                    }

                }catch (IOException e){
                    e.printStackTrace();
                } catch (JSONException e) {
                    System.out.println("End of Members");
                }
                return null;
            }

            @Override
            protected void onPostExecute(Void aVoid) {
                hideDialog();
                adapter.notifyDataSetChanged();
            }
        };
        task.execute(id);
    }

    public class GridSpacingItemDecoration extends RecyclerView.ItemDecoration {

        private int spanCount;
        private int spacing;
        private boolean includeEdge;

        public GridSpacingItemDecoration(int spanCount, int spacing, boolean includeEdge) {
            this.spanCount = spanCount;
            this.spacing = spacing;
            this.includeEdge = includeEdge;
        }

        @Override
        public void getItemOffsets(Rect outRect, View view, RecyclerView parent, RecyclerView.State state) {
            int position = parent.getChildAdapterPosition(view); // item position
            int column = position % spanCount; // item column

            if (includeEdge) {
                outRect.left = spacing - column * spacing / spanCount; // spacing - column * ((1f / spanCount) * spacing)
                outRect.right = (column + 1) * spacing / spanCount; // (column + 1) * ((1f / spanCount) * spacing)

                if (position < spanCount) { // top edge
                    outRect.top = spacing;
                }
                outRect.bottom = spacing; // item bottom
            } else {
                outRect.left = column * spacing / spanCount; // column * ((1f / spanCount) * spacing)
                outRect.right = spacing - (column + 1) * spacing / spanCount; // spacing - (column + 1) * ((1f /    spanCount) * spacing)
                if (position >= spanCount) {
                    outRect.top = spacing; // item top
                }
            }
        }
    }

    private void showDialog() {
        if (!progressDialog.isShowing())
            progressDialog.show();
    }

    private void hideDialog() {
        if (progressDialog.isShowing())
            progressDialog.dismiss();
    }
    /**
     * Converting dp to pixel
     */
    private int dpToPx(int dp) {
        Resources r = getResources();
        return Math.round(TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_DIP, dp, r.getDisplayMetrics()));
    }

在我的PHP代码中

$id = $_GET["id"];
$userid = $_GET["userid"];

$arraylist =array();
$sql=$dbh->prepare("SELECT * FROM commission WHERE userid=? AND id > '$id' ORDER BY id DESC LIMIT 20");
$sql->bindValue(1,$userid);
$sql->execute();

        if ($sql->rowCount() >0) {
            while($row = $sql->fetch(PDO::FETCH_ASSOC)) {

                 $catagory["userid"] = $row["userid"];
                 $catagory["id"] = $row["id"];
                 $catagory["cdate"] = $row['cdate'];
                 $catagory["credit"] = $row['credit'];
                 $catagory["debit"] = $row['debit'];
                 $catagory["balance"] = $row['balance'];
                 $catagory["tremark"] = $row['tremark'];

                     $arraylist[] = $catagory;
                 header('Content-Type:Application/json');
                 $json = json_encode($arraylist);
                 }
        } else {
 echo "0 results";
}
 echo $json;

我如何在向下滚动时加载更多数据，并且应按降序排列。 请建议我还有什么更好的方法。