[Lazarus] Very strange errors when editing programs using Frames

[Tony Whyman]

Don,

Welcome to the wonderful world of Lazarus frames!

I use them a lot including subclassed frames as you are doing. They are 
very useful for structuring complex forms as well as common elements, 
but the IDE will bite you back if you are not careful. I don't think I 
have seen exactly what you have, but I have seen many strange behaviours.

The main advice I would give is to make sure that when you are editing a 
frame, all forms using that frame are closed. Otherwise, the IDE 
typically decides that any properties you update in the frame do not 
apply to the form's instance of the frame and overrides them. The worst 
case is when you add an event to a component on the original frame. If 
any forms using that frame are open, that event usually gets set to 
"none" on the form and you have to edit the .lfm to restore sanity. 
Getting used to editing lfm files is one of the delights of using frames 
as it is the only way of recovering from the IDE overriding property 
values in using forms - Lazarus unfortunately does not have the Delphi 
Object Inspector's "revert to inherited" option.

In many cases I have it found it more reliable to add a frame 
programmatically in the form's "loaded" method. That way you ensure that 
the IDE does not helpfully modify properties for you. If you do do that, 
beware, for some reason "loaded" can get called more than once for each 
form.

Tony Whyman
MWA

On 08/04/15 14:58, Donald Ziesig wrote:
> Hi All!
>
> I'm not sure if the following is a new bug or whether it only occurs 
> in Lazarus for Windows
> 1.4RC2 or RC3.
>
> I have been testing Lazarus 1.4RC2 and now Lazarus 1.4RC3 on Windows 7 
> and have encountered
> strange behavior with frames.  I have simplified the code down to an 
> app that displays
> a very simple frame after a button push.  There is only one *unusual* 
> factor in this app
> (but it is common to many programs I wrote in Delphi and Lazarus for 
> Linux).  That is,
> I subclass TFrame to add functionality to all descendent frames, for 
> example:
>
>     TFrameX = class(TFrame)
>
>       public Print; virtual;   // Now the main program can call Print 
> on a framex and
>                                         // the frame-specific printing 
> occurs.
>
>       constructor Create; override;
>     end;
>
> the remainder of the frames in the apps are subclassed from TFrameX.
>
>     TFrame1 = class(TFrameX) ...
>
> The issue is that when editing the MAIN program's appearance using the 
> Object Inspector,
> (specifically changing *Position*),  the .lfm file for the FRAME 
> /occasionally/ gets an entry
> for*Position*!  (sometimes *TabOrder*, too.).  I can't pinpoint 
> exactly the time at which
> this extraneous data appears, but it doing a series of normal editing 
> operations such as
> moving the windows, changing the properties of the MAIN or FRAME for a 
> few minutes
> (but not changing the code) will definitely cause it to happen. Once 
> it does, the object
> inspector for Frame1 has an entry for *Position* (and sometimes 
> *TabOrder*) which is not
> present when a frame is first created.  After this appears, reloading 
> the Frame form
> will /sometimes/ raise an exception in the IDE complaining about the 
> unknown property
> for the Frame.
>
> The intermittent (apparently) nature of this is puzzling.
>
> Subsequently, when I attempt to run the program, one of two possible 
> situations occur:
>
> 1.    Clicking on the button that displays the frame calls the code 
> but does not display
>         the frame, or,
> 2.    Clicking on the button raises an exception describing the 
> extraneous entry in the
>         .lfm file.
>
> I haven't been able to figure out why one or the other happens, but 
> for any given
> compilation the error remains the same.
>
> I have a simple app that demonstrates this problem if anyone would 
> like to try to
> replicate my troubles ;-) .
>
> Thanks,
>
> Don Ziesig
>
>
> --
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