Objective-c format time left until certain date

I'm creating a countdown timer and I need to printout the time left (hour:minute:seconds) until a specific date. I've found how to 开发者_运维百科get the time interval between Now and the target date but I don't know how to format the time interval as a string. Does NSDateFormater work on NSTimeInterval?

NSTimeInterval is in seconds, use divide and remainder to break it up and format (code untested):

NSString *timeIntervalToString(NSTimeInterval interval)
{
   long work = (long)interval; // convert to long, NSTimeInterval is *some* numeric type

   long seconds = work % 60;   // remainder is seconds
   work /= 60;                 // total number of mins
   long minutes = work % 60;   // remainder is minutes
   long hours = work / 60      // number of hours

   // now format and return - %ld is long decimal, %02ld is zero-padded two digit long decimal 
   return [NSString stringWithFormat:@"%ld:%02ld:%02ld", hours, minutes, seconds];
}

You would first compare two NSDate objects to retrieve the difference in seconds between the two, the NSDate method you should use is

- (NSTimeInterval)timeIntervalSinceDate:(NSDate *)anotherDate

Then you could simply write a function to parse the seconds into hours/minutes/seconds, for example you could use this (untested):

-(NSDictionary*)createTimemapForSeconds:(int)seconds{
   int hours = floor(seconds /  (60 * 60) );

   float minute_divisor = seconds % (60 * 60);
   int minutes = floor(minute_divisor / 60);

   float seconds_divisor = seconds % 60;
   seconds = ceil(seconds_divisor);

   NSDictionary * timeMap = [NSDictionary dictionaryWithObjects:[NSArray arrayWithObjects:[NSNumber numberWithInt:hours], [NSNumber numberWithInt:minutes], [NSNumber numberWithInt:seconds], nil] forKeys:[NSArray arrayWithObjects:@"h", @"m", @"s", nil]];

   return timeMap;
}

This is code from my project:

-(NSString*)timeLeftString
{
    long seconds = [self msLeft]/1000;
    if( seconds == 0 )
        return @"";
    if( seconds < 60 )
        return [NSString stringWithFormat:
            pluralString(seconds,
            NSLocalizedString(@"en|%ld second left|%ld seconds left", @"")), seconds];
    long minutes = seconds / 60;
    seconds -= minutes*60;
    if( minutes < 60 ) 
        return [NSString stringWithFormat: 
            NSLocalizedString(@"%ld:%02ld left",@""),
            minutes, seconds];    
    long hours = minutes/60;
    minutes -= hours*60;
    return [NSString stringWithFormat:
        NSLocalizedString(@"%ld:%02ld:%02ld left",@""),
        hours, minutes, seconds];    
}

msLeft --- my function that returns time in milliseconds pluralString --- my function that provides different parts of format string depending on the value (http://translate.sourceforge.net/wiki/l10n/pluralforms)

Function returns different format for different timer values (1 second left, 5 seconds left, 2:34 left, 1:15:14 left).

In any case, progress bad should be visible during long operation

One more thought: In case that time left is "small" (less then a minute?), probably time left should not be shown --- just progress bar left to reduce interface "visual noise".