overrideing TScrollBox OnResize event in a custom component

Question

I am making a custom component derived from a TScrollBox, that displays thumbnail images of images files stored in a Strings Property. It works good, until the Scrollbox is resized at runtime. I tried overriding the Risize method, but then when i add images to the component at runtime, after adding two to three images, it errors out (Invalid Pointer Operation). See my code below

type
  TViewer = class(TScrollBox)
  private
    { Private declarations }
    fColumns: Integer;
    fImages : TStrings;
    fThumbWidth: Integer;
    fThumbHeight: Integer;
    procedure SetColumns(Value: Integer);
    procedure SetImages(Value: TStrings);
    procedure SetThumbWidth(Value: Integer);
    procedure SetThumbHeight(Value: Integer);
    function GetColumns: Integer;
    function GetThumbWidth: Integer;
    function GetThumbHeight: Integer;
  protected
    { Protected declarations }
    procedure LinesChanged(Sender: TObject);
    procedure UpdateViewer;
    procedure ClearViewer;
    **//procedure Resize;override;**
  public
    { Public declarations }
    constructor Create(AOwner: TComponent);override;
    destructor Destroy;override;
  published
    { Published declarations }
     property Columns : Integer read GetColumns write SetColumns default 5;
     property Images  : TStrings read fImages write SetImages;
     property ThumbHeight: Integer read GetThumbHeight write SetThumbHeight default 100;
     property ThumbWidth: Integer read GetThumbWidth write SetThumbWidth default 100;
     property OnImageClick: TNotifyEvent read  fOnImageClick write  fOnImageClick;
  end;

procedure Register;

implementation

procedure TViewer.ClearViewer;
var
 I: Integer;
begin
 for I := ControlCount-1 downto 0 do
   TPanel(Components[I]).Free;
end;

procedure TViewer.UpdateViewer;
var
 I,X,Y,C, D: Integer;
 Thumb, btnPnl: TPanel;
 img: TImage;
 btn1, btn2, btn3, btn4: TSpeedButton;
begin
 self.
 ClearViewer;
  X:= 0;
  Y:= 0;
  C:= 1;
  D:= 5;
  Thumb:= nil;
  for I := 0 to fimages.Count - 1 do
  begin
   if fileExists(fimages[I]) then
   begin
     Thumb:= TPanel.Create(self);
     Thumb.Parent:= self;
     thumb.Tag:= I;

     Thumb.Caption:= '';
     Thumb.Left:= X + D;
     Thumb.Top:= Y + D;
     Thumb.Width:= fThumbWidth;
     Thumb.Height:= fThumbHeight; 
     img:= TImage.Create(Thumb);
     img.Parent:= Thumb;
     img.Tag:= I;
     img.Align:= alClient;
     img.Stretch:= true;
     img.OnClick:= fOnImageClick;
     img.Picture.LoadFromFile(fImages[I]);
   end;
   X:= X + Thumb.Width + D;
   if C = fColumns then
   begin
     X:= 0;
     Y:= Y + Thumb.Height + D;
     C:= 0;
   end
   else
   Inc(C);
  end;
end;


procedure TViewer.SetImages(Value: TStrings);
begin
  fImages.Assign(Value);
  UpdateViewer;
end;

procedure TViewer.SetColumns(Value: Integer);
begin
 if fColumns <> value then
  fColumns:= Value;
end;

procedure TViewer.SetThumbWidth(Value: Integer);
begin
 if fThumbWidth <> Value  then
  fThumbWidth:= Value;
end;
procedure TViewer.SetThumbHeight(Value: Integer);
begin
 if fThumbHeight <> Value  then
  fThumbHeight:= Value;
end;

function TViewer.GetColumns: Integer;
begin
 result:= fColumns;
end;

function TViewer.GetThumbWidth: Integer;
begin
  result:= fThumbWidth;
end;

function TViewer.GetThumbHeight: Integer;
begin
  result:= fThumbHeight;
end;

procedure TViewer.LinesChanged(Sender: TObject);
begin
  UpdateViewer;
end;

**{procedure TViewer.Resize;
begin
  inherited;
  UpdateViewer;
end;}**

constructor TViewer.Create(AOwner: TComponent);
begin
  inherited Create(aOwner);
  DoubleBuffered:= True;
  fImages := TStringList.Create;
  TStringList(fImages).OnChange := LinesChanged;
  fThumbHeight:= 100;
  fThumbWidth:= 100;
  fColumns:= 5;
end;

destructor TViewer.Destroy;
begin
  fImages.Free;
  inherited;
end;

Answer 1

The code

for I := ControlCount-1 downto 0 do
  TPanel(Components[I]).Free;

is problematic. Indeed, ControlCount is to Controls as ComponentCount is to Components. You cannot mix them!

You should do like this:

for I := ControlCount-1 downto 0 do
  Controls[I].Free;

However, I don't think that is the issue.