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Re: Call Backs

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On 2/15/2012 14:33, Edward Rutherford wrote:
> I've read plenty of discussion about callback functions as used in C (ie
> in qsort).  I'm comfortable with this: really it is a
> function pointer.  What I'm not sure about is how (or if) a
> callback differs to events like GUI's use.  One of the things I
> found about events, at least in a VB6 project I did, is that an event
> can be broadcast to all objects and they can use it if desired (code
> only and GUI objects).  Each object has to have a receive function for
> that event though.
>
> The only way I can see to do event broadcasting with a callback is to
> have a special callback that in turn calls a list of receiver functions
> that would be in each module.  For example, a timer that broadcasts a
> time tick every second would call a function (for example)
> Timer_1min(void) that in turn would call functions in all of the modules
> that need to know the time.  I've also thought about a message que, but
> that is begining to sound like too much work.  Is there a better way?

You can 'typedef' a function type:

   typedef int f_click(struct s_clickable *);

Then a function pointer to that type of function might be:

   f_click * click;

So you might have:

   struct s_clickable {
       f_click * click;
     };

   struct s_button {
       struct s_clickable clickable;
       const char * caption;
     };

Then, where you might have 'Sub Command1_Click' in Microsoft Visual 
Basic, in C, you might do:

   /* Declare using 'f_click' to ensure the type */
   static f_click cmdbtn1_click;

   /* Definition */
   static int cmdbtn1_click(struct s_clickable * clickable) {
       unsigned char * ptr;
       struct s_button * button;

       /*
        * Non-standard 'container_of' and 'CONTAINING_RECORD'
        * do something like this, but usually without the 'ptr'
        * intermediary
        */
       ptr = (void *) clickable;
       ptr -= offsetof(struct s_button, clickable);
       button = (void *) ptr;

       printf("You pressed %s!\n", button->caption);
       return 0;
     }

   struct s_button cmdbtn1 = {
       /* clickable */
       { cmdbtn1_click },
       /* caption */
       "MyCommandButton1",
     };

Then, as others have mentioned, you can have a "everything-clicked 
event" and it can have a linked-list of subscribers.  You can add 
'&cmdbtn1.clickable' to that linked-list.  The event can have an 
associated function which walks the list and invokes 
'list_item->clickable->click(&list_item->clickable)' for each item in 
the list.

I hope this helps. :)

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Thread

Call Backs Edward Rutherford <edward.p.rutherford79@REMOVETHIS.gmail.com> - 2012-02-15 19:33 +0000
  Re: Call Backs tom st denis <tom@iahu.ca> - 2012-02-15 11:36 -0800
    Re: Call Backs James Kuyper <jameskuyper@verizon.net> - 2012-02-15 15:02 -0500
    Re: Call Backs Devil with the China Blue Dress <chine.bleu@yahoo.com> - 2012-02-15 12:27 -0800
    Re: Call Backs Nick Keighley <nick_keighley_nospam@hotmail.com> - 2012-02-16 02:56 -0800
  Re: Call Backs Lew Pitcher <lpitcher@teksavvy.com> - 2012-02-15 15:20 -0500
  Re: Call Backs Devil with the China Blue Dress <chine.bleu@yahoo.com> - 2012-02-15 12:24 -0800
  Re: Call Backs Malcolm McLean <malcolm.mclean5@btinternet.com> - 2012-02-15 16:02 -0800
  Re: Call Backs Shao Miller <sha0.miller@gmail.com> - 2012-02-16 01:21 -0500
    Re: Call Backs Shao Miller <sha0.miller@gmail.com> - 2012-02-16 01:25 -0500
  Re: Call Backs Willem <willem@toad.stack.nl> - 2012-02-17 21:58 +0000

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