Protecting against callbacks queuing up?

[MRAB]

Esben von Buchwald wrote:
> Dennis Lee Bieber wrote:
>>
>>     The only other thing I could suggest is exactly what is done on:
>> http://pys60.garage.maemo.org/doc/s60/node59.html
>>
>>     Initialize a counter value to 0, then increment it in the callback,
>> only doing REAL work every n calls.
>>
>>
>>     def doCallback(self):
>>         if self.count % 35 == 0:    #doc says 35 hits/second, so this
>>             self.data_callback()    #will run one once per second
>>         self.count += 1
>>
>>     You'll still get that slew of backlogged callbacks that built up
>> while doing the real processing, but unless self.data_callback() takes
>> more time than the "35" covers, most of the callbacks will just come in
>> and exit with an increment.
> 
> Of course I can do that.
> 
> 
> But it'll only make a noticable delay EVERY time the user moves, and not 
> prevent the build up of calls if it doesn't finish within the 35 callbacks.
> 
> The point is that I don't know in advance, how long the call will take. 
> It depends on the amount of data i load and process during the call.
> I only know when the calculations have finished, and when they are 
> called, and think there might be some way to block further callbacks 
> until the first one returns?

You could record when your callback finishes (is about to return) and
then ignore any callback that happens too soon after that:

     def doCallback(self):
         now = time.time()
         if now - self.last_call >= MIN_TIME:
             self.data_callback()
             now = time.time()
         self.last_call = now