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iOS8 Day-by-Day :: Day 15 :: New NSFormatters

Posted on 7 Aug 2014 Written by Sam Davies

Updated 9 Apr 2015: This post has been updated to Swift 1.2

This post is part of a daily series of posts introducing the most exciting new parts of iOS8 for developers – #iOS8DayByDay. To see the posts you’ve missed check out the introduction page, but have a read through the rest of this post first!

To enjoy each of the 39 posts all in one place, you can now also download the iOS8 Day-by-Day eBook free from our website! 


Introduction

Many apps that we built represent data flow in some form or another. However, since apps are primarily visual it’s often important to display this data to the user. This requires converting numbers, dates etc to strings which the user will understand. This is more complicated than it might sound – since different locales will have different formatting conventions, different names and even different calendars.

Cocoa has traditionally been very strong in this area – with the NSFormatter abstract class – and more specifically the NSNumberFormatter and NSDateFormatter concrete implementations. You’ll almost certainly have come across them in your app development – e.g. converting an NSDate object to a string for display in an UILabel.

iOS8 adds to this family of NSFormatter concrete implementations with some new date/time formatters and some physical quantity formatters. This article will take a quick tour of the new formatters, and how to use them.

The sample code which accompanies this post is in the form of a Xcode 6 playground, and is available in the iOS8 Day-by-Day git repo on the ShinobiControls github at github.com/ShinobiControls/iOS8-day-by-day.

Temporal Formatters

Joining the hugely popular NSDateFormatter are two new formatters – NSDateComponentsFormatter and NSDateIntervalFormatter.

NSDateComponentsFormatter

NSDateFormatter is useful for creating a string which represents a specific point in time (e.g. “12th June 2010″), but it can’t be used for specifying some kind of temporal duration (e.g. “12 days 13 hours and 12 minutes”). These durations are still very much in need of a formatter, since they need to be carefully localized. Enter NSDateComponentsFormatter, which can create a suitably localized string for an instance of NSDateComponents.

let dateComponentsFormatter = NSDateComponentsFormatter()
let components = NSDateComponents()
components.hour   = 2
components.minute = 45

NSDateComponentsFormatter has a unitsStyle property which can be any one of .Positional, .Abbreviated, .Short, .Full, or .SpellOut:

The following snippet demos the different unit styles:

let dcfStyles: [NSDateComponentsFormatterUnitsStyle] =
  [.Positional, .Abbreviated, .Short, .Full, .SpellOut]
for style in dcfStyles {
  dateComponentsFormatter.unitsStyle = style
  dateComponentsFormatter.stringFromDateComponents(components)
}
Units Style Result
.Positional 2:45
.Abbreviated 2h 45m
.Short 2 hrs, 45 mins
.Full 2 hours, 45 minutes
.SpellOut two hours, forty-five minutes

By default these string are localized to the default locale, but you can override this by providing a specific NSCalendar instance:

let calendar = NSCalendar.currentCalendar()
calendar.locale = NSLocale(localeIdentifier: "th-TH")
dateComponentsFormatter.calendar = calendar
dateComponentsFormatter.stringFromDateComponents(components)
// => สอง ชั่วโมง และ สี่​สิบ​ห้า นาที

You can also specify that “approximation” and “remaining” phrases be added to the output string:

dateComponentsFormatter.includesApproximationPhrase = true
dateComponentsFormatter.stringFromDateComponents(components)
// => About 2 hrs, 45 mins
dateComponentsFormatter.includesTimeRemainingPhrase = true
dateComponentsFormatter.stringFromDateComponents(components)
// => About 2 hrs, 45 mins remaining

NSDateIntervalFormatter

NSDateComponentsFormatter is really useful for creating duration strings, but if you want to specify a specific time interval, then NSDateIntervalFormatter is you friend. It takes two NSDate objects, and again has a selection of different time and date styles, which can be set independently for the time and the date:

let dateIntervalFormatter = NSDateIntervalFormatter()
let now = NSDate()
let longTimeAgo = NSDate(timeIntervalSince1970: 0.0)
let difStyles: [NSDateIntervalFormatterStyle] =
  [.NoStyle, .ShortStyle, .MediumStyle, .LongStyle, .FullStyle]
for style in difStyles {
  dateIntervalFormatter.dateStyle = style
  dateIntervalFormatter.timeStyle = style
  dateIntervalFormatter.stringFromDate(longTimeAgo, toDate: now)
}
Style Result
.NoStyle  
.ShortStyle 1/1/70, 1:00 AM – 7/30/14, 9:32 AM
.MediumStyle Jan 1, 1970, 1:00:00 AM – Jul 30, 2014, 9:32:35 AM
.LongStyle January 1, 1970, 1:00:00 AM GMT+1 – July 30, 2014, 9:32:35 AM GMT+1
.FullStye Thursday, January 1, 1970, 1:00:00 AM GMT+01:00 – Wednesday, July 30, 2014, 9:32:35 AM British Summer Time

Physical Quantity Formatters

A completely new addition to the NSFormatter family in iOS are the physical quantity formatters, which have primarily been added in support of HealthKit. They all contain the ability to format numbers with a selection of relevant units.

NSLengthFormatter

NSLengthFormatter is used to format numbers as lengths, and includes localizable units such as metres, inches, yards and miles. The stringFromMeters() method will convert from a value in meters to the unit appropriate for the current locale, whereas the stringFromValue() method takes a units value, allowing you to specify what the number represents, and should be output:

let lengthFormatter = NSLengthFormatter()
lengthFormatter.stringFromMeters(1.65)
// => 1.804 yd
let lfUnits: [NSLengthFormatterUnit] =
  [.Millimeter, .Centimeter, .Meter, .Kilometer, .Inch, .Foot, .Yard, .Mile]
for unit in lfUnits {
  lengthFormatter.stringFromValue(15.2, unit: unit)
  lengthFormatter.unitStringFromValue(10.3, unit: unit)
}

NSLengthFormatterUnit allows you to specify which unit type should be used:

Unit Result
.Millimeter 15.2 mm
.Centimeter 15.2 cm
.Meter 15.2 m
.Kilometer 15.2 km
.Inch 15.2 in
.Foot 15.2 ft
.Yard 15.2 yd
.Mile 15.2 mi

You can also specify unit styles as well, which allow you to control the length of the units themselves:

let unitStyles: [NSFormattingUnitStyle] = [.Short, .Medium, .Long]
for style in unitStyles {
  lengthFormatter.unitStyle = style
  lengthFormatter.stringFromValue(1.65, unit: .Meter)
}
Style Result
.Short 1.65m
.Medium 1.65 m
.Long 1.65 meters

In some locales, the units used for measuring a person’s height are different from those used to measure other lengths – e.g. human height is rarely measured in yards. NSLengthFormatter has a boolean forPesonHeightUse property which controls this behavior.

If you only require the units, as opposed to the number with the units then the unitsStringFromValue(,unit:) method allows will do that for you. It respects the current unitStyle, and uses the provided value to determine whether or not the unit should be plural.

NSMassFormatter

In many respects, the NSMassFormatter is very similar to the NSLengthFormatter, but representing the physical quantity of mass instead of distance. It too can convert from the standard unit (kilograms) to the unit used in the appropriate locale, and also has a selection of units available for manual use:

let massFormatter = NSMassFormatter()
massFormatter.stringFromKilograms(56.4)
// => 124.08 lb
let mfUnits: [NSMassFormatterUnit] =
  [.Gram, .Kilogram, .Ounce, .Pound, .Stone]
for unit in mfUnits {
  massFormatter.stringFromValue(165.2, unit: unit)
}
Unit Result
.Gram 165.2 g
.Kilogram 165.2 kg
.Ounce 165.2 oz
.Pound 165.2 lb
.Stone 165.2 st

And again, you can specify the style of the units:

massFormatter.stringFromKilograms(76.2)
for style in unitStyles {
  massFormatter.unitStyle = style
  massFormatter.stringFromValue(34.2, unit: .Kilogram)
}
Style Result
.Short 34.2kg
.Medium 34.2 kg
.Long 34.2 kilograms

Again, some locales tend to use different (some may say strange) units to measure the mass of a human, and therefore there is a forPersonMassUse property which will switch the formatter into ‘human’ mode.

NSEnergyFormatter

The final new formatter is for formatting energy, and its existence is driven by the requirement to measure the energy content of food in HealthKit. It shares many of the same features of the previous two physical quantity formatters.

The stringFromJoules() method will convert to the unit it thinks is most appropriate for the locale, but once again you can specify units with stringFromValue(,unit:):

let energyFormatter = NSEnergyFormatter()
energyFormatter.stringFromJoules(42.5)
// => 10.158 cal
let efUnits: [NSEnergyFormatterUnit] =
  [.Joule, .Kilojoule, .Calorie, .Kilocalorie]
for unit in efUnits {
  energyFormatter.stringFromValue(54.2, unit: unit)
}
Unit Result
.Joule 54.2 J
.Kilojoule 54.2 kJ
.Calorie 54.2 cal
.Kilocalorie 54.2 kcal

In the same way as before, you can specify a selection of unit styles:

for style in unitStyles {
  energyFormatter.unitStyle = style
  energyFormatter.stringFromValue(5.6, unit: .Kilojoule)
}
Style Result
.Short 5.6kJ
.Medium 5.6 kJ
.Long 5.6 kilojoules

For some completely unimaginable reason, when referring to the energy content of food, some locales tend to invent a new unit, helpfully called “calories”, which is identical to a traditional “kilocalorie”. Luckily, before I go off on a rant, NSEnergyFormatter has your back – with the forFoodEnergyUse property:

energyFormatter.forFoodEnergyUse = true
energyFormatter.stringFromJoules(4200)
// => 1.004 Cal

Conclusion

Formatting values is hard enough in the locale you use every day, but attempting to get it right across all locales is near-enough impossible. In the past numbers and dates have been possible through the existing NSFormatter subclasses, but iOS8 adds some great new formatters to ease this pain.

This new functionality isn’t something that you’re likely to be able to rush out and use immediately to implement some cool new feature, but it’s one of those things that is incredibly useful when you need it. The important part is to remember that these formatters exist, so that next time you are creating an app which needs to represent a distance, or a duration, you know that you can have localization for free.

The code for this post is all available in a playground which you can get from the ShinobiControls github at [github.com/ShinobiControls/ios8-day-by-day]. If you have any questions, or fancy a new person to follow (I’d very much like that – it makes me feel loved) then I’m @iwantmyrealname on Twitter.

sam

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