[SOLVED] How to switch a Form from Show-mode to ShowModal() without
flicker?
lazarus / 31-LCL

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Hartmut · 2022-01-07 11:01

My program uses a 2nd Form to display some infos. Some of these infos
need a couple of seconds to gather them. I want to display the 1st
part immediately and add the 2nd part after it is ready to display.
Therefore I use Show() to make the 1st part visible. After the 2nd
part is displayed, I can switch the Form to modal-mode.

The problem is, that before the ShowModal() a Hide() is required and
between those 2 commands a flicker occurs. I want to avoid that
flicker. I have this flicker only on Linux Ubuntu 18.04 but not on
Windows 7. I use Lazarus 2.0.6 (but have the same results with
2.0.10).

I attached a small compilable project. Thanks in advance.

Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i // Extract
from Unit1:procedure TForm1.Button1Click(Sender: TObject);   begin 
 Form2.display_Infos;   end; // Extract from Unit2:procedure
TForm2.display_Infos;   begin   Memo1.Clear;   Memo1.Lines.Add('This
are Infos part 1');   Show; {display Infos part 1} 
 Application.ProcessMessages; {neccessary to display Infos part 1}  
 sleep(2000); {simulates gathering Infos part 2}  
 Memo1.Lines.Add('This are Infos part 2');   Hide; {is neccessary
before ShowModal()}   ShowModal; {a flicker occurs between Hide() and
ShowModal()}   end; procedure TForm2.Button_ActionClick(Sender:
TObject);   begin   // do some actions...   Close;   end; procedure
TForm2.Button_CloseClick(Sender: TObject);   begin   Close;   end;

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balazsszekely · 2022-01-07 11:16

@Hartmut

Show the form with FormX.ShowModal, then on FormShow event start a
timer, like this:
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i procedure
TFormX.FormShow(Sender: TObject);begin  Timer1.Interval := 300; 
Timer1.Enabled := True;end; procedure TFormX.Timer1Timer(Sender:
TObject);begin  Timer1.Enabled := False;  Memo1.Clear; 
Memo1.Lines.Add('This are Infos part 1');   Self.Enabled := False;
//prevent user to close form  try    //do lengthy process here   
Memo1.Lines.Add('This are Infos part 2');  finally    Self.Enabled :=
True;  end;end; 
PS: On design time set Timer1.Enabled to false.

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Hartmut · 2022-01-07 12:14

Thanks a lot GetMem for your quick reply. Before I try to implement
it, may I ask a question? Why do you recommend a timer for this? Is it
not possible to do everything directly in FormShow() - or in
FormActivate(), where I already had experimented with?
Because I'm still a beginner to GUI programming I want to understand
and to learn. Thanks.

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balazsszekely · 2022-01-07 13:14

@Hartmut
QuoteThanks a lot GetMem for your quick reply.You are welcome!

QuoteBefore I try to implement it, may I ask a question? Why do you
recommend a timer for this? Because is the least painful solution. If
you put a lengthy code in the FormShow event, the form won't be
visible until the lengthy process is over. With a timer you
immediately exit the OnShow event and the user will have a visual
feedback. FormActivate is also OK, but you have to be careful because
FormActivate is always called when the from receive focus.
There are other ways to achieve the same thing, like running the
lengthy code on FormCreate and showing a busy cursor in the caller
form. In my opinion a timer is the most straightforward solution.

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Hartmut · 2022-01-07 13:52

Hello GetMem, thank you very much for your explanations.

Meanwhile I implemented your suggestion. The good news is: no more
flicker :-))

But the 2nd Form is not longer "really" modal: as long as it is open,
you still can press Button1 from Form1 and get a Runtime-Error,
because Form2 ist already displayed. I have this both on Lazarus 2.0.6
and 2.0.10. In my "original" demo version Button1 is blocked (in 2.0.6
and 2.0.10), as it should be for modal Forms. Here is my new code:

Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i procedure
TForm2.display_Infos;   begin   ShowModal; {calls FormShow()} 
 end; procedure TForm2.FormShow(Sender: TObject);   {Event "OnShow"} 
 begin   writeln('OnShow - start Timer');   Timer1.Interval := 300;
{in ms}   Timer1.Enabled := True;   end; procedure
TForm2.Timer1Timer(Sender: TObject);   {Event "OnTimer" des Timers} 
 begin   Timer1.Enabled := False;   writeln('Timer1Timer');  
 Memo1.Clear;   Memo1.Lines.Add('This are Infos part 1'); 
 Application.ProcessMessages; {neccessary to display Infos part 1}  
 Self.Enabled := False; // prevent user to close form   try //do
lengthy process here:       sleep(2000); {simulates gathering Infos
part 2}       Memo1.Lines.Add('This are Infos part 2');   finally    
 Self.Enabled := True;   end; {try}   end;
Did I made something wrong?

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balazsszekely · 2022-01-07 14:04

QuoteDid I made something wrong?No. At least noting obvious. I would
suggest to remove method displayinfos and move the ShowModal part to
your main form:
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i procedure
TForm1.Button1Click(Sender: TObject);begin  Form2.ShowModal;end; If
your second form(Form2) does not behave like a modal form, please
attach your project or try something like this:
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i procedure
TForm1.Button1Click(Sender: TObject);begin  Form1.Enabled := False; 
try    Form2.ShowModal;  finally    Form1.Enabled := True;  end;end;
The last method really shouldn't be necessary.  ShowModal always works
fine for me.

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Hartmut · 2022-01-07 15:06

I tried both suggestions, thank you. Suggestion 1 did not help, but
suggestion 2 did.
Meanwhile I found out, that this "modal" problem is only on Linux
Ubuntu 18.04, but not on Windows 7.
I attached my little project and a screenshot of the Runtime-Error
when pressing Button1.
Are you on Windows or Linux?

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balazsszekely · 2022-01-07 15:36

@Hartmut
QuoteI tried both suggestions, thank you. Suggestion 1 did not help,
but suggestion 2 did.
Meanwhile I found out, that this "modal" problem is only on Linux
Ubuntu 18.04, but not on Windows 7. Windows and it works fine at my
side, but should work with Linux(gtk2) too. In the past, there were
some problem with modal forms(gtk2), but only when you show a modal
form, from a modal form. Sort of a double modal form.
As for the error message, you should set form2 visible property to
false.

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Hartmut · 2022-01-07 18:01

OK, I think we've got it. I have a solution. Your posts and examples
were easy to understand for me. Thanks a lot again for your help.