CODEWITHSUNDEEP
delphi
user1175743
user1175743

Reputation:

Dynamic Form Creation

I create my forms at runtime something like this:

AboutForm := TAboutForm.Create(AboutForm);
try
  AboutForm.ShowModal;
finally
  AboutForm.Free;
end;

But what is the difference though in using any of these:

AboutForm := TAboutForm.Create(Self);
AboutForm := TAboutForm.Create(nil);
AboutForm := TAboutForm.Create(Application);

They all seem to work the same from what I can see but which is correct, or are they all correct, which is generally the best one to use?

Appreciate your comments thanks :)

Upvotes: 9

Views: 7441

Answers (3)

mjStallinger
mjStallinger

Reputation: 192

The first one

AboutForm := TAboutForm.Create(AboutForm);

tells the TAboutForm, that it's Owner is an object, which has been assigned to the pointer "AboutForm" before (could also be NIL -> no Owner).

The "Owner" parameter of 

TForm.Create(Owner: TComponent)

tells the form, who is it's owner. TComponents register themselves at their owner, if some is provided. So, there is some kind of communication between the owner and it's childs.

Example: If the owner gets freed, he can free it's owned components also.

Therefore, 

TAboutForm.Create(Self);

means, that "Self" is the Owner of the new TAboutForm, if (Self is TComponent).

TAboutForm.Create(NIL);

is also an acceptable solution, but here, you have to free the component yourself (except for, you create TAboutForm within a MDI-Window).

Upvotes: 2

PresleyDias
PresleyDias

Reputation: 3711

when you create the Form dynamically, that is the form is available form. You have to Pass the owner of the form.

enter image description here

.. So in your case

AboutForm := TAboutForm.Create(Self);

AboutForm := TAboutForm.Create(nil);

AboutForm := TAboutForm.Create(Application);

  1. Self
  2. Nil
  3. Application

is the owner of the for AboutForm (as you already may know it)

But what is the difference though in using any of these:

here is something from about.com-Form Owner

nil - specifies that no object owns the form - and therefore a developer (you) is responsible for freeing the created form (by calling myForm.Free when you no longer need the form)

Self - specifies the object in which the method is called. If, for example, you are creating a new instance of a TMyForm form from inside a Button's OnClick handler (where this button is placed on a MainForm) - self refers to "MainForm". Thus, when the MainForm is freed - it will also free "MyForm".

Application - specifies a global TApplication type variable created when you run your application. "Application" encapsulates your application as well as providing many functions that occur in the background of the program.

and this also Form Owner

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.

This is from

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

For freeing you can do this ACtion := caFree onclose of the form.

Upvotes: 5

Jørn E. Angeltveit
Jørn E. Angeltveit

Reputation: 3079

The TForm.Create takes an Owner as parameter.

In your first example, AboutForm is the owner. Which obviously is a bad idea, since it's not created yet.

When Self is the parameter, the instance that makes the call is the owner.
When Application is the parameter, the Application is the owner.
When nil is the parameter, the AboutForm doen not have a owner. That is all fine, but in those cases you must remember to free the form yourself.

When you do pass in a owner, you actually don't need to free i explicitly. The owner will free it when the owner is freed.

This is how your code should look like:

AboutForm := TAboutForm.Create(nil);
try
  AboutForm.ShowModal;
finally
  AboutForm.Free;
end;

Upvotes: 10

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