Creating my own ListControl, some problems in Delphi - delphi

Question

Some time ago I have decided to create my own ListControl. What is mean under ListControl - is a control that similar to standard TListBox in Delphi.
I know, it is 'reinventing a wheel', but I want to finish my control.
So, I implemented not so much features in that control like TListBox has, but my control allows:
Add items;
Select item;
Navigate through items via keyboard (arrow keys Up an Down).
I plan to implement my ScrollBar, but this is another topic.
But I have a problem: when summary height of items is more than control's height and last item selected and I try to increase control's height I got a 'blank space', but I want to 'scroll' items down to fill blank space.
At the picture above you can see that control has lack of items to draw them onto 'blank space'.
May be I explain my problem not so clear, but do next:
Put standard TListBox on form and set its height equal 100 px;
Put standard TrackBar on form, set Max value to 100 and in event OnChange write this:
ListBox1.Height := ListBox1.Height + TrackBar1.Position;
Add 12 items at this Listbox;
Compile project and select last item in Listbox, then begin to change its height via TrackBar. You will see, that 'invisible top items' are come from top to down one by one.
That effect I want to add in my control, but I have no idea why.
Control's code
unit aListBox;
interface
uses
Windows,
Messages,
SysUtils,
Classes,
Graphics,
Controls,
Forms,
StdCtrls,
ExtCtrls,
StrUtils,
Dialogs,
Math;
type
{ main class }
TaListBox = class;
{>>>>>>>>>>>>>>>>>>>>>>>>>}
TaListBox = class(TCustomControl)
private
{ Private declarations }
protected
{ Protected declarations }
FItemBmp: TBitmap;
FEnabled: Boolean;
FSelected: Boolean;
FItems: TStringList;
FItemHeight: Integer;
FCurrentItemIndex: Integer;
FMode: Integer;
FGlobalY: Integer;
FScrollOffset: Integer;
FDownScroll: Integer;
procedure SetItems(value: TStringList);
procedure WMSIZE(var Message: TWMSize); message WM_SIZE;
procedure WMGETDLGCODE(var Message: TWMGETDLGCODE); message WM_GETDLGCODE;
procedure MouseDown(Button: TMouseButton; Shift: TShiftState; X, Y: Integer); override;
procedure KeyDown(var Key: Word; Shift: TShiftState); override;
function GetItemIndex: Integer;
function GetVisibleItemsCount: Integer;
function GetScrollItemIndex: Integer;
procedure PaintItemStandard(BmpInOut: TBitmap; AMode, AIndex: Integer);
procedure PaintControlStandard(ACanvas: TCanvas; AMode: Integer);
procedure Paint; override;
public
{ Public declarations }
constructor Create(AOwner: TComponent); override;
destructor Destroy; override;
procedure Click; override;
property ItemIndex : Integer read FCurrentItemIndex;
published
{ Published declarations }
property Items : TStringList read FItems write FItems;
property OnClick;
end;
{<<<<<<<<<<<<<<<<<<<<<<<<<}
implementation
{ TaListBox }
procedure Register;
begin
RegisterComponents('MyControl', [TaListBox]);
end;
constructor TaListBox.Create(AOwner: TComponent);
begin
Inherited Create(AOwner);
{ standard declarations }
ControlStyle := ControlStyle + [csOpaque, csCaptureMouse, csDoubleClicks];
Width := 100;
Height := 120;
DoubleBuffered := true;
{ control's declarations }
FItemBmp := TBitmap.Create;
FEnabled := true;
FSelected := false;
FItems := TStringList.Create;
FItemHeight := 20;
FCurrentItemIndex := -1;
FScrollOffset := 0;
FDownScroll := 0;
FMode := 1;
end;
destructor TaListBox.Destroy;
begin
FreeAndNil(FItemBmp);
FreeAndNil(FItems);
Inherited Destroy;
end;
procedure TaListBox.Click;
begin
if FEnabled then
Inherited Click
else
Exit;
end;
procedure TaListBox.SetItems(value: TStringList);
begin
Invalidate;
end;
procedure TaListBox.WMSize(var Message: TWMSize);
var
LScrollIndex, LVisibleCount: Integer;
begin
inherited;
LScrollIndex := FScrollOffset div FItemHeight;
LVisibleCount := GetVisibleItemsCount;
if (FItems.Count - LScrollIndex) < LVisibleCount then
FScrollOffset := FItemHeight * max(0, FItems.Count - GetVisibleItemsCount);
end;
procedure TaListBox.WMGETDLGCODE(var Message: TWMGETDLGCODE);
begin
Inherited;
Message.Result := DLGC_WANTARROWS;
end;
procedure TaListBox.MouseDown(Button: TMouseButton; Shift: TShiftState; X, Y: Integer);
begin
if Button = mbLeft then
begin
Windows.SetFocus(Handle);
if PtInRect(Rect(1, 1, Width - 1, Height - 1), Point(X, Y)) then
FGlobalY := Y - 2;
if GetItemIndex > FItems.Count - 1 then
Exit
else
begin
FSelected := true;
FCurrentItemIndex := GetItemIndex;
// prevent selecting next item if height too low
if Height >= FItemHeight then
if PtInRect(Rect(1, Height - FDownScroll - 1, Width - 1, Height - 1), Point(X, Y)) then
FScrollOffset := FScrollOffset + FItemHeight;
Invalidate;
end;
end;
Inherited MouseDown(Button, Shift, X, Y);
end;
procedure TaListBox.KeyDown(var Key: Word; Shift: TShiftState);
var
ScrollIndex: Integer;
begin
Inherited KeyDown(Key, Shift);
if FEnabled then
begin
case Key of
VK_UP:
begin
if FCurrentItemIndex = 0 then
Exit
else
begin
if (FCurrentItemIndex + 1) > 0 then
begin
Dec(FCurrentItemIndex);
ScrollIndex := FScrollOffset div FItemHeight;
if FCurrentItemIndex < ScrollIndex then
FScrollOffset := FScrollOffset - FItemHeight;
end;
end;
end;
VK_DOWN:
begin
if FCurrentItemIndex = FItems.Count - 1 then
Exit
else
begin
if (FCurrentItemIndex + 1) < FItems.Count then
begin
Inc(FCurrentItemIndex);
ScrollIndex := FScrollOffset div FItemHeight;
if (FCurrentItemIndex - GetVisibleItemsCount + 1) > ScrollIndex then
FScrollOffset := FScrollOffset + FItemHeight;
end;
end;
end;
end;
Invalidate;
end
else
Exit;
end;
function TaListBox.GetItemIndex: Integer;
begin
Result := (FGlobalY + FScrollOffset) div FItemHeight;
end;
function TaListBox.GetVisibleItemsCount: Integer;
begin
Result := Height div FItemHeight;
end;
function TaListBox.GetScrollItemIndex: Integer;
begin
Result := FScrollOffset div FItemHeight;
end;
procedure TaListBox.PaintItemStandard(BmpInOut: TBitmap; AMode, AIndex: Integer);
var
Text: String;
R: TRect;
begin
BmpInOut.Width := Width - 2;
BmpInOut.Height := FItemHeight;
case AMode of
1:
begin
if FSelected then
begin
BmpInOut.Canvas.Brush.Color := clWebCrimson;
BmpInOut.Canvas.Font.Color := clWhite;
end
else
begin
BmpInOut.Canvas.Brush.Color := clWhite;
BmpInOut.Canvas.Font.Color := clBlack;
end;
BmpInOut.Canvas.Pen.Color := clGray;
end;
4:
begin
BmpInOut.Canvas.Brush.Color := clSilver;
BmpInOut.Canvas.Pen.Color := clGray;
BmpInOut.Canvas.Font.Color := clBlack;
end;
end;
BmpInOut.Canvas.FillRect(BmpInOut.Canvas.ClipRect);
// paint item's text
if AIndex = - 1 then
Exit
else
BmpInOut.Canvas.TextOut(18, 2, FItems.Strings[AIndex]);
end;
procedure TaListBox.PaintControlStandard(ACanvas: TCanvas; AMode: Integer);
var
i: Integer;
OldSelected: Boolean;
TempBmp: TBitmap;
begin
case AMode of
1:
begin
ACanvas.Brush.Color := clWhite;
ACanvas.Pen.Color := clBlack;
end;
4:
begin
ACanvas.Brush.Color := clSilver;
ACanvas.Pen.Color := clBlack;
end;
end;
ACanvas.Rectangle(Rect(0, 0, Width, Height));
// calculate DownButton size
FDownScroll := Height - GetVisibleItemsCount * FItemHeight - 1 {top border pixel} - 1 {bottom border pixel};
// create output bitmap
TempBmp := TBitmap.Create;
TempBmp.Width := Width - 2;
TempBmp.Height := Height - 2;
// turn off selected flag
OldSelected := FSelected;
FSelected := false;
for i:=0 to FItems.Count - 1 do
begin
PaintItemStandard(FItemBmp, FMode, i);
TempBmp.Canvas.Draw(0, 0 + (FItemHeight * i) - FScrollOffset, FItemBmp);
end;
// output result
ACanvas.Draw(1, 1, TempBmp);
// restore selected flag
FSelected := OldSelected;
if FSelected then
begin
// paint selected item
PaintItemStandard(FItemBmp, FMode, FCurrentItemIndex);
ACanvas.Draw(1, 1 + (FItemHeight * FCurrentItemIndex) - FScrollOffset, FItemBmp);
end;
// free resources
FreeAndNil(TempBmp);
end;
procedure TaListBox.Paint;
begin
if FEnabled then
PaintControlStandard(Canvas, 1)
else
PaintControlStandard(Canvas, 4);
end;
end.
I hope I can find some help here.
Thank you for your attention!
P.S.
In the source code was added implementation of scrolling items by changing control's size, written by Tom Brunberg.
P.S.S.
Thanks to user fantaghirocco for formatting my question ;)

Answer 1

Following your directions to create a standard TListBox I noted, as you said, that the number of visible items increased when increasing the list box (regardless of any item being selected).
But, decreasing the size did not scroll items up again, regardless of any item being selected. I understand that you ask about the same functionality, since you refer to the standard TListBox.
Add to the uses clause and to the TaListBox class declaration:
uses ... Math;
...
TaListBox = class(TCustomControl)
private
procedure WMSize(var Message: TWMSize); message WM_SIZE;
and to the implementation
procedure TaListBox.WMSize(var Message: TWMSize);
var
LScrollIndex, LVisibleCount: Integer;
begin
inherited;
LScrollIndex := FScrollOffset div FItemHeight;
LVisibleCount := GetVisibleItemsCount;
if (FItems.Count - LScrollIndex) < LVisibleCount then
FScrollOffset := FItemHeight * max(0, FItems.Count - GetVisibleItemsCount);
end;
A side note: you use the following kind of expressions in many places, e.g.
Round(FScrollOffset div FItemHeight);
The div operator means integer division. It always returns an integer, thus the call to Round is meaningless. Read about div and also mod in the documentation.

Answer 2

The idea is simple:
Always know how many items can be displayed when your control is a certain height. That means if your clientheight is 100px and an item's height is 10px then you obviously will be able to display 10 items completely whitout anyone being clipped. Save that amount in a variable. Keep as a float because sometimes an item will be clipped. (Visible Count)
Keep a variable of which direction you scrolled last. This is important as that will help you decide whether to bring items into view from the bottom or from the top or whether to hide items at the top or bottom when the control's height decreases/increases.
Keep an index of the item that is at the top or bottom the last time you scrolled. Whether to keep the top one or the bottom one will depend on which direction you last scrolled (point 2). It will obviously change as you add items, etc.
So let's say the situation is that you have more items than can be displayed and the last time you scrolled was up, so you will keep the item index of the top most visible item. If that index is 0 (zero) then obviously you just need to bring items into view from the bottom. But if that index is for example; 5, then you will keep bringing items into view also from the bottom but only until the Visible Count grows as large or larger than the Item Count in which case you wil start to bring as many items into view from the top as is needed to fill the client area.
You just have to adapt according to last scroll direction and whether the height increases or decreases