Re: [gtk-list] Passing Multiple Arguments to a Callback

[Havoc Pennington <rhpennin midway uchicago edu>]

On Mon, 28 Sep 1998, Jordan Nelson wrote:
> 
> Is there a way to pass multiple arguments to a callback?
>

C won't let you have more than one signature for the same function. 
Gtk-- allows it. In C you will have to make the args into one arg; for
example, 

struct multi_arg_t {
  gchar* data1;
  gchar* data2;
}; 

Of course, use the appropriate types and think of better names according
to your application.

> strcpy( arg, "Test Value" );
>

g_strdup is easier BTW.
 
> gtk_signal_connect( GTK_OBJECT( object ), "clicked",
> GTK_SIGNAL_FUNC( callback ), structure, arg );
> 

First off you are going to need a pointer, not an entire struct, as the
callback data.

There is only one data argument; you will have to pack all your data into
one place somehow. Either create a struct, as above, or you can use
gtk_object_set_data to store additional data on either the object emitting
the signal or an object you pass in the data field. 

> void callback( char *structure, char *value )
> {
> 	strcpy( structure->data, value );
> 	g_print( "%s\n", structure->data );
> 	g_print( "%s\n", value );
> }
> 

This won't compile; you can't call -> on a char*. It will also SEGV
because your function signature is wrong.

For callbacks, you should use the signature shown in the object's class
struct. e.g., 
  void (* pressed)  (GtkButton *button);
  void (* released) (GtkButton *button);
  void (* clicked)  (GtkButton *button);
  void (* enter)    (GtkButton *button);
  void (* leave)    (GtkButton *button);

for GtkButton. There is an implicit "gpointer data" on each signature.

So the clicked signal callback should be:

void clicked_callback(GtkButton* button, gpointer data);

If your data is a struct multi_arg_t*, the callback can be:
{
 struct multi_arg_t* mat = (struct multi_arg_t*) data;
 g_print("Got data %s\n", mat->data1);
 g_print("Got data %s\n", mat->data2);
}

You could also have set_data on the button, then:

{
  gchar* str = (gchar*)gtk_object_get_data(GTK_OBJECT(button),"my_key");
  g_print("Got data %s\n", str);
}

You can pass as much data as you want by using different keys. But this is
a little less efficient than the struct approach probably.

Be careful with casts. Notice that the callback for "clicked" on a
GtkButton is going to get a GtkButton* and a gpointer. If you give the
function a different signature you're implicitly casting those two args to
something else; and you had better be right or things will break.

 Havoc