Robust Form Initialization

Initialization Best Pratices

Written by Poul Halgaard, Blibler Software 26. Feb. 2008. Copyright.

Search keys:

form-creation initialization robust form-show start-up

When you need to use a form in you Delphi program, it is important how you control the creation and initialization, if you want the code to be completely failsafe. This article describes how this can be accomplished with simple means and describes the problems you can get into, if you do not consider this.

RAD Approach

In Delphi a form has a FormCreate and a FormShow event, these two is where you normally initialize the form or othervice you initialize it in the code wrapping the displaying of the form, e.g. like this:

var

f: TForm;

FormNotCancelled: Boolean;

begin

f := TForm.Create(nil);

try

f.Initialize;

FormNotCancelled := f.ShowModal = mrOk;

if FormNotCancelled then

ShowMessage(‘Hello’);

finally

f.Free;

end;

This is a nice way to do it and errors are handled correctly.

If however circumstances is that you do not want an external call to a Initialize method, but you want to use FormCreate or FormShow instead, you need to be careful.

If code in the FormCreate event fails, an exception is thrown, but the expected result is not what then actually happens. This has to do with the Windows message system.

The remaining part of the FormCreate is skipped, but then the FormShow is invoked!

So if you want to use FormCreate for some initialization, you actually need to check that it completed in the FormShow.

E.g. use a flag like FormCreatedCompletedSuccessfully.

You actually need to check it in all code until the form is successfully shown on the screen. This is errorprone, so the approach with an initialize method is recommended.

Sometimes you want the form to display and then make some timeconsuming initialization.

You can use the Initialize method for this. If you want 2 steps you would ofcause need both an Initialize and RemainingInitialize methods for example. The RemainingInitialize you could call in the FormShow event.

If something goes wrong there, there is a way to handle it. Close the form and raise an exception.

Events Beginning to Fire to Early

When a control has been created there is a good chance it can start generating event.

But that might lead to other errors, if the form failed during its initialization!

It is not safe to begin to execute code in the events before the form has been completely initialized and displayed. Except if you really know what is going on.

To prevent unwanted events this you can use a flag like:

FormShowCompletedSuccessfully

You set this to True right before you exit FormShow and in all dangereous events you start with this code:

If not FormShowCompletedSuccessfully then

Exit;

For this and other reasons you should be very alerted when using events. As the application grows you will get more and more unexpected situations if you use a lot of events.

Some controls can fire after the form has been closed.

You can then set FormShowCompletedSuccessfully to False in the beginning of FormClose to prevent this.