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Posted on 2001-08-22
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Last Modified: 2010-04-06
Hello

I want to pervent creation of the form if the form has already created.
I have put this in my application:

  if Not Assigned(PaymentForm) then
    PaymentForm := TPaymentForm.Create(Self);
  PaymentForm.Show;

and:

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

and everything is OK.

BUT! How can I close the form if Error occured when the form is creating during TPaymentForm.FormCreate precedure executiing?

Example:
procedure TPaymentForm.FormCreate(Sender: TObject);
begin
try
 ....
 ....
 ERROR
 ....
except
 there I want to close the form! HOW?
end;
 
Thaks
0
Comment
Question by:pmarkov
8 Comments
 
LVL 27

Expert Comment

by:kretzschmar
ID: 6412429
use the release-method

procedure TPaymentForm.FormCreate(Sender: TObject);
begin
try
....
....
ERROR
....
except
  release;
end;

0
 
LVL 2

Accepted Solution

by:
FrodoBeggins earned 100 total points
ID: 6413133
If you override the destructor you'll be able to do what you need.

type
  TForm1 = class(TForm)
   ...
  destructor Destroy; override;
   ...  
implementation

destructor TForm1.Destroy;
begin
  Form1 := Nil;
  inherited Destroy;
end;

It is safe to make the pointer Nil before calling the inherited destructor. The Destructor works with "Self"

Note that the destructor is always called, according the docs:
"When an exception is raised during creation of an object, Destroy is automatically called to dispose of the unfinished object. This means that Destroy must be prepared to dispose of partially constructed objects."

Rgds,
Frodo
0
 
LVL 27

Expert Comment

by:kretzschmar
ID: 6413336
hmm, frodo,

looks hazardous,
what is if i use a second instance of the tform-object,
which is not form1 and will freed before?

and if the onCreate-Event is fired,
the object is already properly created,
a exception in this event-procedure would
not cause an automatically destroying

meikl ;-)
0
 
LVL 2

Expert Comment

by:FrodoBeggins
ID: 6413428
Yes, the second form is a good question. But, as I sow form PMarkov's question he's not going to create the same form again if it is created:
"I want to pervent creation of the form if the form has already created".

If that was not the case, then his code is blowing up too:
" Action := caFree;
 PaymentForm := nil;"
I'm just using Form1 instead of PamentForm :)

And about if the form is created properly or not... I don't care. The main thing here is that ALWAYS after the destruction the pointer becoms "nil".

And about is it hazardous. Theoreticaly - yes. If you have more than one such form. But PMarkov wants to check if PaymentForm = nil. And I make PaymentForm := nil :)

Rgds,
Frodo
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LVL 2

Expert Comment

by:FrodoBeggins
ID: 6413475
As a basic rule it is not good to play with the instance in a class method. The disadvantages: you are blowing if (1) you have more than one instance, (2) inherit the class, (3) change the instance, (4) other cases. The solution is to use "self" as an interface to the instance.

But our case is special one.
(1) don't have more than one instance
(2) the class is not likely to be inherited
(3) the instance name is used elsewhere, so we're blowing anyway (on change of the name!)
(4) no comment :)
And using "self" doesen't help. None cares of it. Even more, "self := nil" is a great joke, even if we needed it ;-)

Rgds,
Frodo
0
 

Author Comment

by:pmarkov
ID: 6416363
What about if I put CLOSE in Except statement and PaymentForm := nil in TPaymentForm.FormDestroy?

Thanks
0
 
LVL 2

Expert Comment

by:ivobauer
ID: 6435656
Hi all!

I prefer the following way: When I need to perform some initialization/finalization during form creation/destruction I always override the constructor & destructor whereas leaving OnCreate/OnDestroy events untouched. So when an exception is raised in the constructor, form is automatically destroyed and you don't have to care about some OnCreate event handler.

Best regards, Ivo.
0
 
LVL 1

Expert Comment

by:Moondancer
ID: 6900915
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