Store NSDateFormatters in thread-attached storage to avoid wasteful + ex...

[rspeyer]

...pensive recreation of these objects.

In my experience, it can be very costly to instantiate NSDateFormatter objects and there is no need since they are almost-always (like here) created using constant input parameters. We use thread-attached storage here because the formatter objects are documented as being non thread-safe, thus the best we can do is once instance per thread.

[bpoplauschi]

@rspeyer indeed, creating NSDateFormatter or other NSFormatter subclass is expensive and those objects are not thread safe. We are using the formatter just for log file names (that is not very often), but from a performance POV, that would be the correct way to write that reusable formatter. I just have a comment. Could you please compose logFileDateFormatter and the log format (in this case yyyy'-'MM'-'dd' 'HH'-'mm') for the thread dictionary key? This way, if we decide to add another formatter with a different format, it won't collide with this one.

[rspeyer]

Thanks @bpoplauschi. Fixed.

[bpoplauschi]

I meant

NSMutableDictionary *dictionary = [[NSThread currentThread] threadDictionary];
NSString *dateFormat = @"yyyy'-'MM'-'dd' 'HH'-'mm'";
NSString *threadDictKey = [@"logFileDateFormatter." stringByAppendingString:dateFormat];
NSDateFormatter *dateFormatter = [dictionary objectForKey:threadDictKey];
if (dateFormatter == nil) {
    NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
    [dateFormatter setDateFormat:dateFormat];
    [dateFormatter setTimeZone:[NSTimeZone timeZoneForSecondsFromGMT:0]];
    [dictionary setObject:dateFormatter forKey:key];
}

no need to copy paste the dateFormat.

[rspeyer]

Done

[bpoplauschi]

Great. Merging.