Detect when Delphi FMX ListBox is scrolled to the bottom?

Question

I need to detect when the user has scrolled down to the bottom in a ListBox , so that I can fetch the next 25 items to show in the listBox, Any Tip?

Answer 1

Okay so let us break this down, first we go to ScrollToItem in FMX.ListBox unit

procedure TCustomListBox.ScrollToItem(const Item: TListBoxItem);
begin
  if (Item <> nil) and (Content <> nil) and (ContentLayout <> nil) then
  begin
    if VScrollBar <> nil then
    begin
      if Content.Position.Y + Item.Position.Y + Item.Margins.Top + Item.Margins.Bottom + Item.Height >
        ContentLayout.Position.Y + ContentLayout.Height then
        VScrollBar.Value := VScrollBar.Value + (Content.Position.Y + Item.Position.Y + Item.Margins.Top +
          Item.Margins.Bottom + Item.Height - ContentLayout.Position.Y - ContentLayout.Height);
      if Content.Position.Y + Item.Position.Y < ContentLayout.Position.Y then
        VScrollBar.Value := VScrollBar.Value + Content.Position.Y + Item.Position.Y - ContentLayout.Position.Y;
    end;
    if HScrollBar <> nil then
    begin
      if Content.Position.X + Item.Position.X + Item.Margins.Left + Item.Margins.Right + Item.Width >
        ContentLayout.Position.X + ContentLayout.Width then
        HScrollBar.Value := HScrollBar.Value + (Content.Position.X + Item.Position.X + Item.Margins.Left +
          Item.Margins.Right + Item.Width - ContentLayout.Position.X - ContentLayout.Width);
      if Content.Position.X + Item.Position.X < 0 then
        HScrollBar.Value := HScrollBar.Value + Content.Position.X + Item.Position.X - ContentLayout.Position.X;
    end;
  end;
end;

Now as you can see. the procedure checks for many values (margins, paddings, top, ....) and then moves VScrollBar by setting VScrollBar.Value to the appropriate position.

You want to know when the Vertical scroll bar has reached the bottom.

so we use the same idea as my other answer for the list view.

We first add this hack to expose the private and protected parts of TListBox class

TListBox = class(FMX.ListBox.TListBox)
  end;

add that to the form where the listbox is and then use the VScrollChange(Sender: TObject); event and reverse engineer the if conditions.

Something like this will work for you

procedure TForm1.ListBox1VScrollChange(Sender: TObject);
var
  S:single;
begin
  S:= ListBox1.ContentRect.Height;

  if ListBox1.VScrollBar.ValueRange.Max = S + ListBox1.VScrollBar.Value then
    Caption := 'hit'
  else
    Caption := 'no hit';
end;

when trying to solve these types of problems always look for a ScrollToControl function and get the inspiration from there. The code above is working with simple items added to the scroll box. if you have any problems with margins or paddings just evolve the formula to cope with that.