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Home > Forums > Programmers > Development Tools > Embarcadero: Delphi Forum

Better method: Auto-create forms or on demand?

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doctorjellybean (Programmer)

21 Apr 08 5:49

When one adds forms to the main one, Delphi auto-creates it at runtime. Now I've seen quite a few examples where the form is removed from the Available Forms list and created when needed

Quote:
Form2 := TForm2.Create(nil);

Is there a difference between the two, and what if yes?

Thank you.

whosrdaddy (Vendor)

21 Apr 08 7:15

this has already been discussed here:

http://www.tek-tips.com/viewthread.cfm?qid=1438941

lookup my response near the bottom of that thread.

Come back here if it is still not clear.

/Daddy

-----------------------------------------------------
What You See Is What You Get
Never underestimate tha powah of tha google!

whosrdaddy (Vendor)

21 Apr 08 7:21

to answer your question:

when autocreated, your form eats/memory resources, even if it is not visible/used at that moment.

for an app with 2 forms, this will not make a big difference (depending what's on the form offcourse)
but things could go ugly when your app has a LOT of forms.

I tend to create my objects the moment I need them.
(that's the method you showed)

/Daddy

-----------------------------------------------------
What You See Is What You Get
Never underestimate tha powah of tha google!

doctorjellybean (Programmer)

21 Apr 08 8:17

Quote:
whosrdaddy (Vendor)

a)
Formx.Create(Application) -> Form resources will be freed when application is terminated

b)
Formx.Create(Self) -> Form resources are freed when the owner object is destroyed (if Self is a descendand of TComponent)

c)
Formx.Create(nil) -> you are responsible for freeing the form

a is used by delphi when a form is autocreated
b is handy for a main form that has several child windows that need to close when the main form is closed
c is handy for showing a dialog window

I have a normal main menu, and each menu item uses its own form. If I select item 2, form 2 shows, item 3 shows form 3, and so on. When showing e.g. form 3, then form 2 should be closed.

Do I use option (c) Formx.Create(nil) for each form?

whosrdaddy (Vendor)

21 Apr 08 9:01

yes, that would be the best option.

to prevent the hassle checking which for is in memory at the moment, just create a variable. make sure all forms have an onclose event with the code Action := caFree;

CurrentForm : TForm;

initialize it to nil
CurrentForm := nil;

when clicking on the menu item create your form and associate it with the var

CODE

...

procedure Menu1Click(Sender : TObject);
var Form : TForm1;

begin
// check if we have a form open, if so, close it
// the onclose event will free the form
if Assigned(CurrentForm) then
CurrentForm.Close;
// create your new form
Form1 := TForm1.Create(nil);
CurrentForm := Form1;
end;

clearer now?

/Daddy

-----------------------------------------------------
What You See Is What You Get
Never underestimate tha powah of tha google!

sggaunt (Programmer)

21 Apr 08 9:57

Perhaps you shouldn't be doing this, but if you have data stored on/in a form that's not always visible, then creating it at run time isn't an easy option.

To give an example one of my first apps started out with all pre-created forms, later when attempting to convert it to 'create at run time', I found that major re-write would be required to change the way the data was structured.
I could only convert about 2/3 of the forms.

Steve: N.M.N.F.
Playing the blues isn't about feeling better. It's about making other people feel worse.

doctorjellybean (Programmer)

21 Apr 08 11:33

Quote:
whosrdaddy (Vendor)

clearer now?

Yes, I think so. Thank you.

This is the code on my main form.

Quote:
type
  TForm1 = class(TForm)
    MainMenu1: TMainMenu;
    Menu1: TMenuItem;
    Menu2: TMenuItem;
    Menu3: TMenuItem;
    Menu4: TMenuItem;
    Panel1: TPanel;
    procedure Menu1Click(Sender: TObject);
    procedure Menu2Click(Sender: TObject);
    procedure Menu3Click(Sender: TObject);
    procedure Menu4Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;
  CurrentForm : TForm;

implementation

uses Unit2, Unit3, Unit4;

{$R *.dfm}

procedure TForm1.Menu1Click(Sender: TObject);
var Form : TForm1;
begin
if Assigned(CurrentForm) then
  CurrentForm.Close;
Form1 := TForm1.Create(nil);
CurrentForm := Form1;
end;

procedure TForm1.Menu2Click(Sender: TObject);
var Form : TForm1;
begin
if Assigned(CurrentForm) then
  CurrentForm.Close;
Form2 := TForm2.Create(nil);
CurrentForm := Form2;
Form2.Show;
end;

procedure TForm1.Menu3Click(Sender: TObject);
var Form : TForm1;
begin
if Assigned(CurrentForm) then
  CurrentForm.Close;
Form3 := TForm3.Create(nil);
CurrentForm := Form3;
Form3.Show;
end;

procedure TForm1.Menu4Click(Sender: TObject);
var Form : TForm1;
begin
if Assigned(CurrentForm) then
  CurrentForm.Close;
Form4 := TForm4.Create(nil);
CurrentForm := Form4;
Form4.Show;
end;

I'm sure I'm doing something wrong, as it gives me a message when compiling:

Quote:
[DCC Warning] Unit1.pas(38): H2164 Variable 'Form' is declared but never used in 'TForm1.Menu1Click'
[DCC Warning] Unit1.pas(47): H2164 Variable 'Form' is declared but never used in 'TForm1.Menu2Click'
[DCC Warning] Unit1.pas(57): H2164 Variable 'Form' is declared but never used in 'TForm1.Menu3Click'
[DCC Warning] Unit1.pas(67): H2164 Variable 'Form' is declared but never used in 'TForm1.Menu4Click'

However, it does appear to work. I think.

I assume I don't need the create routine in the menu1 click procedure, as menu1 is the main form?

whosrdaddy (Vendor)

21 Apr 08 17:01

2 observations:

you don't need the form var in each unit as the form's unit already contains a global variable.

Quote:

I assume I don't need the create routine in the menu1 click procedure, as menu1 is the main form?

yeah, your main form should act as a placeholder for the other forms. so Form1 is autocreated by delphi and the others are controlled by you...

/Daddy

P.S: please use the code tags when posting code

-----------------------------------------------------
What You See Is What You Get
Never underestimate tha powah of tha google!

doctorjellybean (Programmer)

21 Apr 08 17:18

Thanks for help and advice!

Oops, I'm so used to using quote tags. Will remember to use code tags smile

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