[SOLVED] Placing Opening Code in EXE for Opening Associate Files

[lucamar]

FYI, when I upload this updated version tomorrow, It will have a messagebox (see screen below) that gives you credit.

It's not really necessary, just helping is a pleasure for me. But if you do, note that my name is "Luis Caballero", not "Luis Martínez"

Although just "lucamar" is good enough

[pixelink]

Okay.... I''ll change the credit

[Ally]

Hello lucamar,

you have written:

The problem with that code is that the OnShow event is trigered on many conditions, not just on starting the program, so you keep (re-)loading the parameters at odd moments during the application run-time.

For my programs, FormShow will only fire once when the program starts. Under what conditions can FormShow be executed while the program is running?

Greeting Roland

[howardpc]

For the main form of a simple app, FormShow is normally executed only once. For all other forms which can be opened and closed at will without terminating the app, every time a closed form is opened a FormShow event is fired for the form that is now showing.

[Edited to take account of lucamar's later contribution: the main form of an app can clearly have OnShow event called more than once. This is probably the exception rather than the rule, however.]

[lucamar]

For the main form of the app, FormShow is executed only once.

That depends on whether that form is always showing or not. Try this (rather contrived) example: Create a new app (call it "peek-a-boo", for example ) and add a button, a timer (Interval: 5000) an a memo to the form. Then add these handlers to it:

Code: Pascal  [Select][+][-]

1. procedure TMainForm.btDoTestClick(Sender: TObject);
2. begin
3.   Timer.Enabled := True;
4.   Hide;
5. end;
6.  
7. procedure TMainForm.TimerTimer(Sender: TObject);
8. begin
9.   Timer.Enabled := False;
10.   Show;
11. end;
12.  
13. procedure TMainForm.FormShow(Sender: TObject);
14. begin
15.   Memo.Lines.Add('OnShow event triggered');
16. end;

Run and enjoy

I don't remember exactly all the conditions in which OnShow is triggered but I did some tests some time ago and it did get called more than once. Not constantly, of course, but in enough circumstances as to merit the precaution.

[Ally]

Hello howardpc,
that's how I see it, and since it's about the program start and MainForm, there should not be any problems.

Hello lucamar,
In your example, the running program calls its own FormShow. I do not think that usually does.
Are there any situations in which TMainForm.FormShow is executed without explicitly calling it?

Greeting Roland

[lucamar]

Hello lucamar,
In your example, the running program calls its own FormShow. I do not think that usually does.
Are there any situations in which TMainForm.FormShow is executed without explicitly calling it?

As I said, I don't remember all the conditions in which it's called. There were not many but it was a long time ago; it should be easy to find them: grep the sources of (at least) the forms units for constructs of the form "if Assigned(FOnShow) then".

My example above is contrived as h**l; it's intention is to demo one way in which OnShow can be triggered more than once for the main form--but note that I don't explicitely call the event. It gets called as a result of calling Hide/Show.

Note also that there are actually quite a lot of applications in which the function of the main form is to act as a kind of "junction box" for all the forms. Such applications may show/hide the main form (and all the others) quite a lot of times. Granted that it's not what we could term "normal" but the possibility is there; why then would not one take the two seconds it takes to implement the guard even if it'll never be needed?

[Ally]

Hello lucamar,

I found something else. Also Visible: = False; > Visible: = True; calls FormShow.
Your tip, to install a guard here, can be quite helpful.
Thank you for your answers.

Greeting Roland

[lucamar]

I found something else. Also Visible: = False; > Visible: = True; calls FormShow.

That's quite logical. I haven't looked but I bet the implementation of the setter is something similar to

Code: Pascal  [Select][+][-]

1. procedure TCustomForm.SetVisible(Value: Boolean);
2. begin
3.   if Value = FVisible then Exit;
4.   FVisible := Value;
5.   if  FVisible then
6.     Show
7.   else
8.     Hide;
9. end;