Consider the following simple demonstration project:
Project1.dpr
program Project1;
uses
Vcl.Forms,
Unit1 in 'Unit1.pas' {Form1};
{$R *.res}
begin
Application.Initialize;
Application.MainFormOnTaskbar := True;
Application.CreateForm(TForm1, Form1);
Application.Run;
end.
Unit1.pas
unit Unit1;
interface
uses
Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes,
Vcl.Graphics, Vcl.Controls, Vcl.Forms, Vcl.Dialogs, Vcl.ComCtrls, Vcl.StdCtrls;
type
TButton = class(Vcl.StdCtrls.TButton)
protected
procedure CreateWnd; override;
end;
TForm1 = class(TForm)
PageControl1: TPageControl;
TabSheet1: TTabSheet;
TabSheet2: TTabSheet;
TabSheet3: TTabSheet;
Button1: TButton;
Button2: TButton;
Button3: TButton;
procedure FormCreate(Sender: TObject);
end;
var
Form1: TForm1;
implementation
{$R *.dfm}
procedure TButton.CreateWnd;
begin
inherited;
Writeln('Window created: ' + Name);
end;
procedure TForm1.FormCreate(Sender: TObject);
var
i: Integer;
begin
AllocConsole;
end;
end.
Unit1.dfm
object Form1: TForm1
Caption = 'Form1'
ClientHeight = 299
ClientWidth = 635
OnCreate = FormCreate
PixelsPerInch = 96
TextHeight = 13
object PageControl1: TPageControl
Left = 40
Top = 40
Width = 537
Height = 233
ActivePage = TabSheet1
object TabSheet1: TTabSheet
Caption = 'TabSheet1'
object Button1: TButton
Caption = 'Button1'
end
end
object TabSheet2: TTabSheet
Caption = 'TabSheet2'
object Button2: TButton
Caption = 'Button2'
end
end
object TabSheet3: TTabSheet
Caption = 'TabSheet3'
object Button3: TButton
Caption = 'Button3'
end
end
end
end
When you run this, the console window says:
Window created: Button1
As you select each page in turn, the other buttons are created, as shown in the console window:
Window created: Button1
Window created: Button2
Window created: Button3
Now change the OnCreate event handler to force each page to be visible when the form is created:
procedure TForm1.FormCreate(Sender: TObject);
var
i: Integer;
begin
AllocConsole;
for i := 0 to PageControl1.PageCount-1 do begin
PageControl1.Pages[i].Visible := True;
end;
end;
Now when the form is first shown, the console window reads:
Window created: Button1
Window created: Button2
Window created: Button3
This is because, as Danny says, the windows are not created until they are shown.
Now, the nuance with regards page controls is the handling of visibility of the pages. The constructor of TTabSheet contains this:
Visible := False;
Further, the Visible property of TTabSheet is published like this:
property Visible stored False;
That means that when a page control starts its life, its pages are hidden, in the VCL sense of having Visible equal to False. As Danny said, window controls are first created when a control is shown. That happens inside TWinControl.UpdateShowing which begins like this:
procedure TWinControl.UpdateShowing;
var
ShowControl: Boolean;
I: Integer;
begin
ShowControl := (FVisible and (not (csDesigning in ComponentState) or not (csDesignerHide in ControlState)) or
((csDesigning in ComponentState) and not (csDesignerHide in ControlState)) and
not (csNoDesignVisible in ControlStyle)) and
not (csReadingState in ControlState) and not (csDestroying in ComponentState);
if ShowControl then
begin
if WindowHandle = 0 then CreateHandle; // <-- this is the key
if FWinControls <> nil then
for I := 0 to FWinControls.Count - 1 do
TWinControl(FWinControls[I]).UpdateShowing;
end;
....
end;
The pages start out not showing, and then when they become active in TPageControl.ChangeActivePage the following is executed for the newly active page:
Page.BringToFront;
Page.Visible := True;
Setting Visible to True results in TWinControl.UpdateShowing executing, and the window handle being created.
And that's why the trick above of making all the pages visible at form creation time has the effect you desire.
Now, all of the above is very page control centric. For many other controls, the window is first created when the form is created, if the control is visible. If you a specific problem with a specific form then it would be best to share the specific details.
