Am 03.02.2012 um 01:04 schrieb "Scott Aron Bloom" <[email protected]>:
> I have found with windows at least, its instantaneous. Essentially it
> sets up a blocking win32 call, that gets release and emits the signal...
>
> But it does take the event loop of the connected slots thread to be
> running...
... and the later is exactly my point: the underlying implementation can be
threaded ("asynchronous") or polling - but in the end it goes through the Qt
event queue, and if *your* code is not multi-threaded then there is no point in
blocking the signals of QFileWatcher, since the signal would get delivered only
*after* your method returns to the Qt event queue anyway (and at that point the
mentioned instance of the "signal blocker" would have gone, not blocking
anything anymore).
But even if the application code itself was multi-threaded there is no
*guarantee* when the OS would actually inform your application.
"Instantaneously" might not be fast enough: depending on how fast your method
is terminating you might indeed successfully block the signal in 99.9% of all
cases - making you wonder why in the other 0.1% your code still seems to think
that another process had modified the file, when in fact it was *your* process.
And that off course still does not take into account that on some other
platforms the underlying implementation might be polling, and that the polling
intervall X might just be slightly larger than the time Y it takes your method
to execute (and hence the signal blocker would "live"). And again that would
leave you wondering "But my code works on Windows (by chance), but not on Foo
OS!"
So if your requirement would be: "For the lifetime of that *method* I don't
want to get informed when *I* change that file" then the signal blocker
approach would work (taking any possible race conditions out of the equation
for now).
For all other scenarios one has to apply application-specific measures, such as
some MD5 checksum which has been mentioned several times by now (possibly
together with some "file-locking mechanisms" which make the story even more
interesting when we talk about files located on network-mapped drives - did I
hear "cloud services" just now? ;))
Cheers, Oliver
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