What am I doing wrong with allowsFractionalUnits on NSDateComponentsFormatter?

问题

Basically what I want is to get the value of a time interval represented in hours only, without rounding it to full hours (using NSDateComponentsFormatter to get it properly formatted and localized). I don't know if I misunderstand the use of NSDateComponentsFormatter.allowsFractionalUnits, but I can't get the formatter to give me a decimal value. Can anyone help me spot my error or tell me in what way I misunderstand this?

From Apple docs about allowsFractionalUnits property:

Fractional units may be used when a value cannot be exactly represented using the available units. For example, if minutes are not allowed, the value “1h 30m” could be formatted as “1.5h”.

Swift example code:

let formatter = NSDateComponentsFormatter()
formatter.unitsStyle = .Abbreviated
formatter.allowedUnits = .Hour
formatter.allowsFractionalUnits = true

let onePointFiveHoursInSeconds = NSTimeInterval(1.5 * 60.0 * 60.0)
print(formatter.stringFromTimeInterval(onePointFiveHoursInSeconds)!)
//"1h" instead of expected "1.5h"

Same example in Objective-C code:

NSDateComponentsFormatter *formatter = [[NSDateComponentsFormatter alloc] init];
formatter.unitsStyle = NSDateComponentsFormatterUnitsStyleAbbreviated;
formatter.allowedUnits = NSCalendarUnitHour;
formatter.allowsFractionalUnits = YES;

NSTimeInterval onePointFiveHoursInSeconds = 1.5 * 60.0 * 60.0;
NSLog(@"%@", [formatter stringFromTimeInterval:onePointFiveHoursInSeconds]);
//"1h" instead of expected "1.5h"

Update: I have reported a bug to Apple about this problem (rdar://22660145).

回答1:

According to Open Radar #32024200:

After doing some digging (disassembling Foundation), it looks like every call to -[_unitFormatter stringFromNumber:] in -[NSDateComponentsFormatter _stringFromDateComponents:] is passed an +[NSNumber numberWithInteger:] which drops floating point data.

You're not doing anything wrong. The flag is simply broken.

回答2:

Looking at the documentation, it’s using a lot of interesting language (emphasis mine):

Fractional units may be used when a value cannot be exactly represented using the available units. For example, if minutes are not allowed, the value “1h 30m” could be formatted as “1.5h”.

While to me it seems that it would only make sense for the values in the documentation to be values that actually work, it’s certainly possible that there is some combination of time value, formatter options, and calendar/locale settings that would make this work. Definitely worth filing a Radar on both the functionality and the documentation.

回答3:

enter code hereAs far as i understood , You want to display time in 12-Hour formate Right ? Below is the code : Swift->

let dateAsString = "20:30"
let dateFormatter = NSDateFormatter()
dateFormatter.dateFormat = "HH:mm"
let date = dateFormatter.dateFromString(dateAsString)

dateFormatter.dateFormat = "h:mm a"
let date12 = dateFormatter.stringFromDate(date!)
txtText.text = date12 

来源：https://stackoverflow.com/questions/32522965/what-am-i-doing-wrong-with-allowsfractionalunits-on-nsdatecomponentsformatter