[Gambas-user] Events not firing in dynamically instantiated forms.

[Tobias Boege]

On Thu, 27 Nov 2014, T Lee Davidson wrote:
> On 11/27/2014 12:55 PM, Tobias Boege wrote:
> > On Thu, 27 Nov 2014, T Lee Davidson wrote:
> >> On 11/27/2014 08:39 AM, Beno?t Minisini wrote:
> >>> Le 27/11/2014 14:29, Stephen a ?crit :
> >>>> I'm not consciously overriding the event observer so I'm not clear how
> >>>> to not do it.
> >>>> Obviously I still have much to learn.
> >>>>
> >>>
> >>> mhForm1 = New Form1(TabStrip1) As "Form1"
> >>>                                   \________/
> >>
> >> Now *I'm* confused. The documentation for "New"
> >> (http://gambaswiki.org/wiki/lang/new) gives its syntax as:
> >> Object = NEW Class [ ( Constructor parameters... ) ] [ AS Name ]
> >>
> >>
> >> And, there is a warning box that says, "If you forget to specify the
> >> Name part, your object will never raise events!"
> >>
> >> But after stripping "As Form1" off the end of that line, the form
> >> obviously *does* raise events.
> >>
> >> Is the documentation in need of correction? If so, what should that
> >> warning actually say?
> >>
> >
> > What the documentation says is generally true. But Form is a special class:
> > it attaches to itself as event observer by default when instantiated.
> >
> > Regards,
> > Tobi
> >
> 
> Thanks Tobi. I understand that a form is by default its own event observer.
> 
> What confuses me is that the warning seems to indicate that the "As 
> Name" part is required for the object to be able to raise events.
> 
> Thinking that the warning should say, "*If you specify As* but forget to 
> specify the Name part ...", I took off just the Name part leaving "As" 
> at the end of the line. I then got, "Unexpected end of line".
> 
> So, I guess I'm just confused as to what that warning is actually saying.
> 

OK, "As name" comes as a bundle. It makes your newly created object raise
events under the given name and makes the current class its event observer.
This is called "attaching" the object to its observer. "As" without a name
is a syntax error.

This means that in a class Bob the lines

  Private h As Alice

  Public Sub _new()
    h = New Alice As "Beth"
  End

create a new Alice in the variable "h" and assigns her the event name "Beth"
(just to show that the event name is totally free to choose).

The current Bob where you create that Alice is now her event observer, that
is you can intercept the events of "h" in the code of Bob. Say, Alice raises
an event called "Send", you can write

  Public Sub Beth_Send()
    ' This is called when the h object raises the Send event.
  End

in your Bob.class. Take your time to experiment if you're still insecure
here. (Or ask for a concrete example!)

If you don't give "As name", the object will not be attached to any observer
and this will prevent it from raising events (actually it _can_ raise events
but nobody will hear it).

But Form plays a special role. It will attach itself to itself automatically,
using the event name "Form". You have very likely already seen code like

  Public Sub Form_Open()
    ' ...
  End

without worrying about it. Well, you don't need to worry about it because of
the automatic attaching logic that is built into the Form class.

However, if you create a new form and explicitely give it an event name, the
procedure I described above kicks in. That is, the form is not attached to
itself as "Form" but to the object which creates the form, with the given
name (an object can have at most have one primary observer). Consequently,
Form_Open within the form code will no longer fire. That's what the thread
was originally about.

Regards,
Tobi

-- 
"There's an old saying: Don't change anything... ever!" -- Mr. Monk