Does VCL components have onCreate event? - delphi

Question

Need to access some components right after program start but I have found that doing so from form's onCreate event is bad because at the moment they may still be unavailable (access violation occurs). Can not find onCreate event in any component. Am I missing something?
Here is code. Empty form with ValueListEditor.
unit Unit1;
interface
uses
Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,
Vcl.Controls, Vcl.Forms, Vcl.Dialogs, Vcl.Grids, Vcl.ValEdit;
type
TForm1 = class(TForm)
ValueListEditor1: TValueListEditor;
procedure FormCreate(Sender: TObject);
private
{ Private declarations }
public
{ Public declarations }
end;
var
Form1: TForm1;
procedure: Load;
implementation
{$R *.dfm}
procedure TForm1.FormCreate(Sender: TObject);
begin
Load;
end;
procedure Load;
begin
(Application.MainForm.FindComponent('ValueListEditor1') as TValueListEditor)
.Strings.LoadFromFile('c:\li');
end;
end.

Answer 1

The problem is not because your component hasn't been created yet, because it has been. The real problem is because the Application.MainForm property hasn't been assigned yet when your main Form's OnCreate event is fired, so you are calling FindComponent() on a nil Form pointer.
Since Load() is merely accessing a member of TForm1 then Load() should also be a member of TForm1 as well, and then you can call it, and thus access your component, via the implicit Self pointer, which is valid during the Form's OnCreate event, eg:
unit Unit1;
interface
uses
Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,
Vcl.Controls, Vcl.Forms, Vcl.Dialogs, Vcl.Grids, Vcl.ValEdit;
type
TForm1 = class(TForm)
ValueListEditor1: TValueListEditor;
procedure FormCreate(Sender: TObject);
private
{ Private declarations }
procedure LoadValues;
public
{ Public declarations }
end;
var
Form1: TForm1;
implementation
{$R *.dfm}
procedure TForm1.FormCreate(Sender: TObject);
begin
LoadValues;
end;
procedure TForm1.LoadValues;
begin
ValueListEditor1.Strings.LoadFromFile('c:\li');
end;
end.
If, for whatever reason, Load() must be a standalone procedure, then at least have it use your global Form1 variable, which the call to Application.CreateForm() will assign before the main Form's OnCreate event is fired, eg:
unit Unit1;
interface
uses
Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,
Vcl.Controls, Vcl.Forms, Vcl.Dialogs, Vcl.Grids, Vcl.ValEdit;
type
TForm1 = class(TForm)
ValueListEditor1: TValueListEditor;
procedure FormCreate(Sender: TObject);
private
{ Private declarations }
procedure LoadValues;
public
{ Public declarations }
end;
var
Form1: TForm1;
procedure Load;
implementation
{$R *.dfm}
procedure TForm1.FormCreate(Sender: TObject);
begin
LoadValues;
end;
procedure TForm1.LoadValues;
begin
ValueListEditor1.Strings.LoadFromFile('c:\li');
end;
procedure Load;
begin
if Form1 <> nil then
Form1.LoadValues;
end;
end.
Alternatively, you can fallback to looking for the Form1 object in the Screen.Forms[] array, eg:
unit Unit1;
interface
uses
Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,
Vcl.Controls, Vcl.Forms, Vcl.Dialogs, Vcl.Grids, Vcl.ValEdit;
type
TForm1 = class(TForm)
ValueListEditor1: TValueListEditor;
procedure FormCreate(Sender: TObject);
private
{ Private declarations }
procedure LoadValues;
public
{ Public declarations }
end;
var
Form1: TForm1;
procedure Load;
implementation
{$R *.dfm}
procedure TForm1.FormCreate(Sender: TObject);
begin
LoadValues;
end;
procedure TForm1.LoadValues;
begin
ValueListEditor1.Strings.LoadFromFile('c:\li');
end;
procedure Load;
var
I: Integer;
Frm: TForm;
begin
for I := 0 to Screen.FormCount-1 do
begin
Frm := Screen.Forms[I];
if Frm is TForm1 then
begin
TForm1(Frm).LoadValues;
Exit;
end;
end;
end;
end.

Answer 2

Use the form's onShow() -event.
But be aware, that the onShow() -event is called every time the form is shown, not only the first time.

Answer 3

In your form, you should override the DoShow method and insert your code there. Since DoShow is called everytime the form changes from invisible to visible, you should also add a boolean variable in your form and check in the DoShow override, if it is false. In that case, it is the first time DoShow is called. Then set the variable to true and do whatever you need to do the first time.
Please note that within DoShow the form is not visible yet. If you need to do something once the form is made visible, you can post a custom message from DoShow and put your code in the corresponding message handler. At the time it is executed, the form just became visible.
const
WM_APP_STARTUP = WM_USER + 1;
type
TForm1 = class(TForm)
protected
FInitialized : Boolean;
procedure DoShow; override;
procedure WMAppStartup(var Msg: TMessage); message WM_APP_STARTUP;
end;
implementation
procedure TForm1.DoShow;
begin
// Form is NOT visible but will soon be
if not FInitialized then begin
FInitialized := TRUE;
// Insert your code here
PostMessage(Handle, WM_APP_STARTUP, 0, 0);
end;
inherited DoShow;
end;
procedure TForm1.WMAppStartup(var Msg: TMessage);
begin
// The form is now visible
// Insert your code here
end;