Allow customizing the naming convention for log files to use timestamps.

[michaelmelanson]

We find UUIDs to quite awkward in the field because the files cannot be
sorted in chronological order by filename. This patch adds an optional
convention to the DDLogFileManagerDefault class which uses timestamps
instead. The existing UUID-based naming convention is retained as the
default convention for backwards compatibility.

Timestamps are formatted as RFC 3339 dates in the UTC time zone.

[rivera-ernesto]

Nice!

[bpoplauschi]

we could make this NSDateFormatter reusable (i.e. static) to improve performance

[rivera-ernesto]

Right. It would suffice to add the static keyword on this line.

On the other hand this is only called when creating new files, which is far less often than when sending a new log message.

[bpoplauschi]

We could leave it as it is, but from experience, you never know when you start using that method more often. I think it's a good practice to always reuse NSFormatters.

[randarnold]

Just remember that NSDateFormatter is not thread safe https://developer.apple.com/library/mac/documentation/Cocoa/Conceptual/Multithreading/ThreadSafetySummary/ThreadSafetySummary.html

I have not looked closely to see if this method is (or can be) called from multiple threads.

[michaelmelanson]

Honestly guys, I don't think this is something worth optimizing. Unless someone is rotating their logs very very frequently (OTOO several times a second, which would be silly), there's no way creating a date formatter object each time will have any significant performance impact.