Observers in C

observer_example.c

/* observed component */
typedef struct
{
  int listener_count;
  /* array of function pointers */
  void (*listener_list[10])(unsigned,unsigned);
} Button;

void button_Init( Button *pButton )
{
  pButton->listener_count = 0;
}

void button_RemoveClickListener( Button *pButton,
                                 void (*fp)(unsigned,unsigned) )
{
  unsigned i;
  void (*temp)(unsigned,unsigned);

  for( i = 0; i < pButton->listener_count; ++i )
  {
    if( pButton->listener_list[i] == fp )
    {
      pButton->listener_count--;
      pButton->listener_list[i] =
        pButton->listener_list[pButton->listener_count];
    }
  }
}

void button_AddClickListener( Button *pButton,
                              void (*fp)(unsigned,unsigned) )
{
  button_RemoveClickListener( pButton, fp );
  pButton->listener_list[ pButton->listener_count++ ] = fp;
}

void button_Click( Button *pButton, unsigned x, unsigned y )
{
  unsigned i;
  for( i = 0; i < pButton->listener_count; ++i )
  {
    pButton->listener_list[i](x,y);
  }
}