#nsdateformatter

Also

https://nshipster.com/formatter/

A hiatus would be an understatement. Sincere apologies for my inactiveness. =S

Anyway, today's post is going to be really short, but I would say really useful. Just the kind of thing that I like. 

We have all heard about the term "expensive" in programming. Some of us don't really know what it means, but we all know it's bad! The term "expensive" appears in iOS development too. One of the most prominent example would be NSDateFormatter.  So, how do we overcome this issue? Referring to NSDateFormatter case, since it is most likely we are going to use the same date format in the entire app, wouldn't it be better to just create and set the date format just once? That sounds about right, but how do we do it? One way to do so is by using GCD, in particular dispatch_once. By using dispatch_once, we can create a singleton of NSDateFormatter to be called at appropriate places. A singleton is a design pattern in programming where we restrict the instantiation of a class to an object. By using dispatch_once, we ensure that our code is only run ONCE.  How hard is it? Not at all, take a look at the sample code below:

-(NSDateFormatter*)formatter {    static NSDateFormatter *formatter;    static dispatch_once_t onceToken;    dispatch_once(&onceToken, ^{        formatter = [NSDateFormatter new];        formatter.dateFormat = @"yyyy-MM-dd'T'HH:mm:ss.SSSZ";    });    return formatter; }

fBam! There you have it, now when you go ahead and call self.formatter in that particular class, it already takes the date format that you have set. Viola! Expensive no more! =]

How to: NSDate and NSDateFormatter – short format date and time in iphone sdk

NSDate and NSDateFormatter – short format date and time in iphone sdk

Searched for answers, but the ones i found didn’t seem to be iPhone specific.

I basically need to get current date and time separately, formatted as:

2009-04-26 11:06:54

Edit:

The code below,from another question on the same topic, generates

now: |2009-06-01 23:18:23 +0100| dateString: |Jun 01, 2009 23:18| parsed:…

NSDateFormatters are great to use especially in Mac OS X apps because they work so well with Cocoa Bindings in Interface Builder.

There are two kinds of them. One with an OS X 10.0 behavior and one with an OS X 10.4 behavior. iOS uses only the more modern one. In OS X the 10.0 behavior is deprecated and you should use the 10.4 version. But there's a problem.

Let's say you have a text field with a date formatter attached to it in Interface Builder. Now when you run your app, put a date string into the text field and hit enter. So far so good. Then delete the date string and hit enter. It seems that the 10.0 behavior supports this. But the 10.4 behavior complains that it is not a valid date format. So once your text field has a value in it, there's no way to remove that value. Weird!

If you were using an NSNumberFormatter, there is a pair of methods, -setNilSymbol and -nilSymbol, that help you deal with this. You can call [myNumberFormatter setNilSymbol:@""] and then the number formatter will know that if it sees an empty string, it should make the resulting number be nil.

But there is no -nilSymbol for NSDateFormatter. And the weird part is there is almost no mention of this on the internet. How is this possible that more people aren't having to deal with this? Here is about the only mention of it I can find, in a post about NSNumberFormatter:

http://osdir.com/ml/cocoa-dev/2009-08/msg01350.html

Note at the very bottom of the mailing list post, the respondent makes a comment about how it's too bad that NSDateFormatter doesn't have that attribute.

So what is the solution? There is a project on github that adds -setNilSymbol to NSDateFormatter. It's https://github.com/oriontransfer/SWApplicationSupport

You can go a little simpler and just create a subclass of NSDateFormatter that will allow an empty string to be converted to a nil date like this:

@implementation EPDateFormatter /** It is INSANE that NSDateFormatter doesn't allow empty values to resolve to a nil date!!!. */ - (BOOL)getObjectValue:(out id *)obj forString:(NSString *)string errorDescription:(out NSString **)error { BOOL result = [super getObjectValue:obj forString:string errorDescription:error]; if (result == NO && [string length] == 0) { *obj = nil; result = YES; } return result; } @end

Many web services choose to return dates in something other than a unix timestamp (unfortunately). [...] Luckily for us, SQLite is quite good at parsing some ISO-8601 dates and works blazingly fast. 

Many web services choose to return dates in something other than a unix timestamp (unfortunately).

Recently I set out trying to identify performance bottlenecks with a large import operation in our iOS app. Having tweaked most of the variables, I was surprised to find out that date parsing was actually one of the biggest bottlenecks. The date parsing was done using NSDateFormatter using an ISO-8601 formatted date that looked like this:

2013-09-07T23:45:00Z

That looks simple enough. We had the NSDateFormatter's format setup with a timezone of +0 GMT, and everything was great, expect parsing dates like this was consuming around 20% of the entire import operation. To provide some context, we were testing the import performance by importing roughly 250,000 objects into a SQLite database, and each object had 4 dates associated with it. That meant that we were dealing with a million dates, and parsing a million dates can get expensive.

Almost all of that time was being spent inside NSDateFormatter's dateFromString: method, so there was not much we could do to optimize things ourselves.

The main goal was to get a unix timestamp from the ISO date. Luckily for us, SQLite is quite good at parsing some ISO-8601 dates and works blazingly fast. Here's how SQLite can parse the above date:

sqlite> SELECT strftime("%s", "2013-09-07T23:45:00Z"); 1378597500

Let's convert this to use SQLite's C library.

sqlite3_stmt *statement = NULL; sqlite3_prepare_v2(db, "SELECT strftime('%s', ?);", -1, &statement, NULL); sqlite3_bind_text(statement, 1, [dateString UTF8String], -1, SQLITE_STATIC); sqlite3_step(statement); sqlite3_int64 interval = sqlite3_column_int64(statement, 0); NSDate *date = [NSDate dateWithTimeIntervalSince1970:interval];

Looks ugly, but we're trying to solve a genuine problem here. So, how did this do in a performance run against NSDateFormatter? About 1400% faster. To parse a million randomly generated dates on an iPhone 5 running iOS 7, NSDateFormatter took a whooping 106.27 seconds, while the SQLite version took just 7.02 seconds.

Here's a link to a gist containing the source code used for this comparison.

So since my last post about parsing python datetime.isoformat using NSDateFormatter I’ve discovered a lot more about the intricacies of doing it successfully, and heres what I’ve gleaned:

First: When there is no microseconds in the datetime isoformat() wont print it

Second: if the microseconds aren’t included in the output, the previous code I posted wont parse it (it’ll return a nil date).

The fixed code:

NSString * inputDate = @"2012-07-19T08:31:23Z"; NSDate * parsedDate = nil; NSDateFormatter *dateFormatter = [NSDateFormatter new]; [dateFormatter setDateFormat:@"yyyy-MM-dd'T'HH:mm:ss.SSSSSS"]; dateFormatter.timeZone = [NSTimeZone timeZoneWithAbbreviation:@"UTC"]; dateFormatter.locale = [[NSLocale alloc] initWithLocaleIdentifier:@"en_US_POSIX"]; parsedDate = [dateFormatter dateFromString:inputDate]; if (parsedDate==nil) { NSDateFormatter *iso8601Formatter = [NSDateFormatter new]; [iso8601Formatter setDateFormat:@"yyyy-MM-dd'T'HH:mm:ss'Z'"]; iso8601Formatter.timeZone = [NSTimeZone timeZoneWithAbbreviation:@"UTC"]; iso8601Formatter.locale = [[NSLocale alloc] initWithLocaleIdentifier:@"en_US_POSIX"]; parsedDate = [iso8601Formatter dateFromString:inputDate]; }

This code will “nicely” fail over and use a non-microsecond parser.

BUT WAIT THERES MORE

If you don’t actually need the microsecond accuracy then theres a “better” way to do this, from the python side. Anywhere you are outputting a datetime do the following:

output_ts = str(dt.replace(microsecond = 0).isoformat())+"Z"

(where dt is our datetime object). 

This has the advantage of making sure that all datetimes are “iso8601 compatible”. To decode it use the following code (taken from the code above)

NSDateFormatter *iso8601Formatter = [NSDateFormatter new]; [iso8601Formatter setDateFormat:@"yyyy-MM-dd'T'HH:mm:ss'Z'"]; iso8601Formatter.timeZone = [NSTimeZone timeZoneWithAbbreviation:@"UTC"]; iso8601Formatter.locale = [[NSLocale alloc] initWithLocaleIdentifier:@"en_US_POSIX"]; parsedDate = [iso8601Formatter dateFromString:inputDate];

** its bad-form to allocate a NSDateFormatter each time, it has a fairly hefty setup overhead, you should allocate it in your instance init.

And in summation

If you need to include microseconds in your output, you would be better off outputting it as a float in the format seconds.microseconds and parsing it using [NSDate dateWithTimeIntervalSince1970:] which supports floating point NSTimeInterval input.

To output a float from a datetime (dt) in Python:

In one of my projects, I encountered a bug that involves time formatting and this code has fixed it for me: