Re: [ctypes-users] judge a return callback is NULL, how?

[Lenard L. <le...@te...>]

oyster wrote:
> that is the defination in C:
> typedef struct Ihandle_ Ihandle;
> typedef int (*Icallback)(Ihandle*);
> Icallback   IupGetFunction   (const char *name);
>
> and this is in the user application in C
> int btn_pause_cb(void)
> {
>   if(!IupGetFunction(IUP_IDLE_ACTION))
>     IupSetFunction(IUP_IDLE_ACTION, (Icallback) idle_cb);
>   else
>     IupSetFunction(IUP_IDLE_ACTION, NULL);
>
>   return IUP_DEFAULT;
> }
>
>
> in my python interface, even I call IupGetFunction(IUP_IDLE_ACTION)
> for the first time, I get a value such as
> IupGetFunction(IUP_IDLE_ACTION)= <CFunctionType object at 0x00A3D378>
> obviously, it is always True
>
> so, how can I judge whether a returned value( which  is  a function
> type) is NULL or not? Thank you
>
>   
A null pointer evaluates to the boolean value False. Other pointers are 
True. So

if not IupGetFunction(IUP_IDLE_ACTION):
    .....

> BTW: I think I can modify the Python code to use a global variant to
> index wheter we have called btn_pause_cb, but I think it is not a good
> way.
>
>   
Callbacks need not be functions. Any callable Python object will do.

class SomeState(object):
    def __init__(self):
        self.was_called = False

    def btn_pause_cb(self):
        .....
        self.was_called = True
        return IUP_DEFAULT

state = SomeState()


Then create a ctypes callback for state.btn_pause_cb, keep a Python 
reference to the callback and the state object, and register the 
callback. Whenever it is called then state.was_called becomes True. Good 
Python programming practice usually involves wrapping C structures as 
classes and calling the C functions that manipulate them from the 
class's methods.

-- 
Lenard Lindstrom
<le...@te...>