How to prevent multiple childs to be created in MDI

Question

How to prevent a child from being creating multiple times, ex: a login form. Is there a solution without looping over the childs before the login form is getting opened and check if another instance of it is created.

Answer 1

By default, each Form has a global pointer declared. Even if you don't auto-create the Forms, you can still utilize that pointer. For any give Form, initialize its global pointer to nil at startup, check the pointer for nil before creating a new instance of that Form, destroy the Form when it is closed, and reset its global pointer back to nil when the Form is destroyed. For example:

unit LoginForm;

interface

...

type
  TLoginForm = class(TForm)
    ...
    procedure FormClose(Sender: TObject; var Action: TCloseAction);
    procedure FormDestroy(Sender: TObject);
    ...
  end;

var
  LoginForm: TLoginForm = nil; // <-- here

implementation

...

procedure TLoginForm.FormClose(Sender: TObject; var Action: TCloseAction);
begin
  Action := caFree;
end;

procedure TLoginForm.FormDestroy(Sender: TObject);
begin
  LoginForm := nil;
end;

...

end.

procedure TMainForm.SomeProcedure;
begin
  ...
  if not Assigned(LoginForm) then
    LoginForm := TLoginForm.Create(Self);
  LoginForm.Show;
  ...
end;