Error message:Control " has no parent control.

I learn component writing from the online help on Delphi. When I try to write my first component. But I get Control " has no parent window when I try to use it.  Although it is not completed, it should at least display something, not a error message. The source is:
unit ButtonPro;

interface

uses
  Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms, Dialogs,
  ExtCtrls;

type
  TButtonPro = class(TCustomControl)
  private
    { Private declarations }
    FNormalPic, FActivePic, FPressingPic, FDisabledPic, FDownPic: TBitmap;
    FBtnDown, FStretch: Boolean;
    FTranspColor, FBkColor: TColor;
    procedure UpdatePic(NewPic: TBitmap);
    procedure SetNormalPic(Value: TBitmap);
    procedure SetActivePic(Value: TBitmap);
    procedure SetPressingPic(Value: TBitmap);
    procedure SetDisabledPic(Value: TBitmap);
    procedure SetDownPic(Value: TBitmap);
    procedure SetTranspColor(Value: TColor);
    procedure SetBkColor(Value: TColor);
  protected
    { Protected declarations }
    procedure CMMouseEnter(var Message: TMessage); message CM_MOUSEENTER;
    procedure CMMouseLeave(var Message: TMessage); message CM_MOUSELEAVE;
    procedure WMLButtonDown(var Message: TMessage); message WM_LBUTTONDOWN;
    procedure WMLButtonUp(var Message: TMessage); message WM_LBUTTONUP;
  public
    { Public declarations }
    constructor Create(AOwner: TComponent); override;
    destructor Destroy; override;

  published
    { Published declarations }
    Property NormalPicture: TBitmap read FNormalPic write SetNormalPic;
    Property ActivePicture: TBitmap read FActivePic write SetActivePic;
    Property PressingPicture: TBitmap read FPressingPic write SetPressingPic;
    Property DisabledPicture: TBitmap read FDisabledPic write SetDisabledPic;
    Property DownPicture: TBitmap read FDownPic write SetDownPic;
    Property Down: Boolean read FBtnDown write FBtnDown;
    Property Stretch: Boolean read FStretch write FStretch;
    Property TransparentColor: TColor read FTranspColor write SetTranspColor default clBackground;
    Property BackgroundColor: TColor read FBkColor write SetBkColor default clBackground;
    Property Height default 50;
    Property Width default 50;
    Property Visible;
  end;

procedure Register;

implementation

procedure Register;
begin
  RegisterComponents('My Components', [TButtonPro]);
end;

procedure TButtonPro.CMMouseEnter(var Message: TMessage);
begin
  UpdatePic(FActivePic);
  inherited;
end;

procedure TButtonPro.CMMouseLeave(var Message: TMessage);
begin
  If Enabled = False then
    UpdatePic(FDisabledPic)
  else if FBtnDown = True then
         UpdatePic(FDownPic)
       else UpdatePic(FNormalPic);
  inherited;
end;

procedure TButtonPro.WMLButtonDown(var Message: TMessage);
begin
  inherited;
  UpdatePic(FDownPic);
end;

procedure TButtonPro.WMLButtonUp(var Message: TMessage);
begin
  inherited;
  If not Enabled then
    UpdatePic(FDisabledPic)
  else if FBtnDown then
         UpdatePic(FDownPic)
       else UpdatePic(FNormalPic);
end;

constructor TButtonPro.Create(AOwner: TComponent);
begin
  inherited Create(AOwner);
  Height := 50;
  Width := 50;
  FNormalPic := TBitmap.Create;
  FActivePic := TBitmap.Create;
  FPressingPic := TBitmap.Create;
  FDisabledPic := TBitmap.Create;
  FDownPic := TBitmap.Create;
  FBtnDown := False;
  FTranspColor := clBackground;
  FBkColor := clBackground;
  UpdatePic(FNormalPic);
end;

destructor TButtonPro.Destroy;
begin
  FNormalPic.Free;
  FActivePic.Free;
  FPressingPic.Free;
  FDisabledPic.Free;
  FDownPic.Free;
  inherited Destroy;
end;

procedure TButtonPro.UpdatePic(NewPic: TBitmap);
var TmpRect: TRect;
begin
  TmpRect := Rect(0,0,NewPic.Width,NewPic.Height);
  Canvas.Brush.Color := FBkColor;
  If NewPic.Empty then Canvas.FillRect(BoundsRect)
  else If FStretch then Canvas.BrushCopy(BoundsRect,NewPic,TmpRect,FTranspColor)
         else Canvas.BrushCopy(TmpRect,NewPic,TmpRect,FTranspColor);
  Invalidate;
end;

procedure TButtonPro.SetBkColor(Value: TColor);
begin
  FBkColor := Value;
  UpdatePic(FNormalPic);
end;

procedure TButtonPro.SetTranspColor(Value: TColor);
begin
  FTranspColor := Value;
  UpdatePic(FNormalPic);
end;

procedure TButtonPro.SetNormalPic(Value: TBitmap);
begin
  FNormalPic.Assign(Value);
  UpdatePic(FNormalPic);
  If not Stretch then Height := FNormalPic.Height;
end;

procedure TButtonPro.SetActivePic(Value: TBitmap);
begin
  FActivePic.Assign(Value);
end;

procedure TButtonPro.SetPressingPic(Value: TBitmap);
begin
  FPressingPic.Assign(Value);
end;

procedure TButtonPro.SetDisabledPic(Value: TBitmap);
begin
  FDisabledPic.Assign(Value);
end;

procedure TButtonPro.SetDownPic(Value: TBitmap);
begin
  FDownPic.Assign(Value);
end;

end.

What's wrong with my code? Please tell me. It is very easy for any people who know component writing.
sccheungAsked:
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sassas081597Commented:
I see you did not declare the procedure UpdatePic.
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StevenBCommented:
 I've only had a quick glance at your code but heres a few things to try:

  1) The error message you describe is often caused when you try to do things to a canvas during design mode. Try preventing some of the canvas operations with :
  If Not(csDesigning in ComponentState) then begin
    +++CODE+++
  end;

  2) From a quick glance it seems like you should probably be using the Paint method to perform the canvas operations anyway. Put all canvas code within an overridden paint method and call 'refresh' to update the apearance of your component. This will also have the advantage of having your component automatically repaint itself if obscured by another window.

  If any of this helps then I'll see what more I can do.

  Steven.
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icampbe1Commented:
First of all, it looks like you should have descended from TGraphicControl not TCustomControl (if I understand OK).

Then, you don't say exactly when the error occurs.  Is it when you deposit the control on the form?  When you call one of its methods?   Do you create it manually?  


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arhCommented:
 This error usually happens when you try to do something
with your component, but it has its Parent property not
assigned yet. VCL will complain about it and raise an exception.
  In your code UpdatePic is called from the constructor before
Parent is initialized, and UpdatePic refers to the Canvas property, which most probably causes an exception.
  Try to remove call to UpdatePic from TButtonPro.Create.
This means you should update your button image somewhere else. The best way to do it is to override the Paint procedure,
which will be called by Delphi when it will need your button
to be displayed or re-displayed.
 Also if you are creating component at run time, you should
assign its Parent property by yourself. eg:
 myButton:=TButtomPro.Create( myForm );
 myButon.Parent:=myForm;
 { myButton.Top, Left, Bottom, Right should be set as well }

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sccheungAuthor Commented:
icampbel,
I should have descended from TGraphicControl not TCustomControl? What's the difference between them? What will happen if I use the  wrong one?
0
icampbe1Commented:
TCustomControl is derived from TWinControl.  If you are not going to require windows input focus, and it looks like you don't, then you can save resources by using TGraphicControl (like TImage does).

Also, arh... has a good suggestion about removing UpdatePic from the constructor.

Cheers,
Ian C.
0
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