Destroying form while in event handler

I've got a main form and a subform. My main form is responsible for creating and destroying the subform. In an event handler (a ButtonClick) of the subform there's a loop like

for i := 0 to whatever do
  DoStuff;
  Application.ProcessMessages;   // To refresh some controls
  DoMoreStuff;   //  (*)
end;

I want the user to be able to destroy the subfrom via main form while the subform is in that loop. After the destructor has finished the program continues at (*) which leads to a crash (because the resources accessed in DoMoreStuff are gone).
I thought about not to close the subform in the main form but start another thread that monitors the loop and closes the subform from this thread but that's breaking a fly on the wheel...
Any hints? Thanks in advance!
LVL 2
__alexAsked:
Who is Participating?
 
Ferruccio AccalaiConnect With a Mentor Senior developer, analyst and customer assistance Commented:
why start another thread (and so run other memory) just to do this?

i'd simply add a check on the subform. An example to better explain:

unit Unit1; {main}

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls;

type
  TForm1 = class(TForm)
    Button1: TButton;
    Button2: TButton;
    procedure Button1Click(Sender: TObject);
    procedure Button2Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

uses Unit2;

{$R *.dfm}

procedure TForm1.Button1Click(Sender: TObject);
begin
form2.toclose:= True;
end;

procedure TForm1.Button2Click(Sender: TObject);
begin
application.CreateForm(Tform2,Form2);
Form2.ToClose := False;
form2.Show;
end;

end.

unit Unit2; {subform}

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls;

type
  TForm2 = class(TForm)
    Button1: TButton;
    procedure Button1Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
    ToClose: Boolean; //a public check
  end;

var
  Form2: TForm2;

implementation

{$R *.dfm}

procedure TForm2.Button1Click(Sender: TObject);
var
i,x: Integer;

begin
for i := 1 to 10000 do begin //a sort of loop
caption := inttostr(i);
Application.ProcessMessages;
end;
showmessage(caption); // a domorestuff check
If ToClose then Close;
end;

end.
0
 
DragonSlayerConnect With a Mentor Commented:
a bit improved over F68's, so that you don't have to have a public ToClose variable in every such form that you want to send the close to (of course, you could always make a Form with ToClose and make all child forms inherit from it, but that's another story, heh):

unit Unit1;

const
  WM_CloseChild = WM_USER + 4096;

...

procedure TForm1.CloseForm2Button(Sender: TObject);
begin
  Form2.Perform(WM_CloseChild, 0, 0);
end;

and at form2:

unit Unit2;

const
  WM_CloseChild = WM_USER + 4096; // perhaps put this declaration in a global unit?

type
  TForm2 = class(TForm)
  private
    isClosing: Boolean;
  protected
    procedure WMCloseChild(var Msg: TMessage); message WM_CloseChild;
  end;

...

procedure TForm2.Form2Create(Sender: TObject);
begin
  isClosing := False;
end;

procedure TForm2.Button1Click(Sender: TObject);
var
  i, x: Integer;
begin
  for i := 1 to 10000 do begin //a sort of loop
    caption := inttostr(i);
    Application.ProcessMessages;
    if isClosing then Break;
  end;
  showmessage(caption); // a domorestuff check
  If isClosing then Close;
end;

0
 
__alexAuthor Commented:
Aaargh... That simple. Thanks a lot.
0
[Webinar] Improve your customer journey

A positive customer journey is important in attracting and retaining business. To improve this experience, you can use Google Maps APIs to increase checkout conversions, boost user engagement, and optimize order fulfillment. Learn how in this webinar presented by Dito.

 
__alexAuthor Commented:
BTW: Closing a form is not destroying it ;-)
0
 
DragonSlayerCommented:
Closing a form will lead to it being destroyed, unless you set Action to caNone at the OnClose =p

you can also optionally, call Release, when you receive the message
0
 
__alexAuthor Commented:
Default Action is caHide (for non MDI childs)
0
All Courses

From novice to tech pro — start learning today.