[fpc-pascal] FP/GTK-GLib : Sending/Accessing Callback GPointer data ?

[Marc Santhoff]

Am Mittwoch, den 06.08.2008, 12:15 +0200 schrieb T. Guilleminot:
> Hi,
> 
> I'd like to be able to pass/access variable(s ?) to/from the "data" part
> of the "g_signal_connect" function, eg :
> 
>     ...
>     Procedure MyActionProcedure (widget : pGtkWidget ; data : gpointer)
> cdecl;
>     begin
>       ... // Using data content here...
>     end;
>     ...
>     g_signal_connect (G_OBJECT(pMyButton), 'button-release-event',
> G_CALLBACK(@MyActionProcedure), MyData);
>     // Passing information here
> -------------------------------------------------------------------^^^^^^
>     ...
> 
> However I'm unable to handle it once MyActionProcedure called.
> 
>   - Does anyone have a *simple* example to do this ? I can only find C
> sample on the web.
>   - Is "data" able to store single or multiple (and different) variable ?
>   - If multiple variables possible, can they be of different types
> (integer, string...) ?

You can pass inside the data-pointer what you like to, from my memory:

type
  TMyDataRec: record
    a: integer;
    b: string;
  end;

  TMyDataObj: class
    procedure DoSomething;
  end;

Procedure MyActionProcedure (widget : pGtkWidget ; data : gpointer)
cdecl;
begin
  { you have to know or check the type of data's content
    and cast it! }
  with data as TMyDataObj do
  begin
    DoSomething;

or

  if (TMyDataRec(data).a = 123) then
  begin
    ...

At the spot where you are passing the pointer the data has to be set up
*before* connecting the callback.

var
  MyData: TMyDataRec;
  MyData2: TMyDataObj;

begin

MyData.a := 124;
g_signal_connect (G_OBJECT(pMyButton), 'button-release-event',
	 G_CALLBACK(@MyActionProcedure), MyData);

MyData2 := TMyDataObj.create;

g_signal_connect (G_OBJECT(pMyButton2), { <-- another button }
	 'button-release-event', G_CALLBACK(@MyActionProcedure),
	MyData2); { <-- another data item }

The only way to change the data item after assigning would be to hold a
reference to it and set it on the fly (e.g. for remembering a state or
the like).

HTH,
Marc