NSDateFormatter毫秒bug

Question

我想創建一個NSDateFormatter，它應該解析像這樣的「2014-05-13 23：31：41.374577」日期。所以：

NSDateFormatter *formatter = [[NSDateFormatter alloc] init]; 
formatter.dateFormat = @"yyyy-MM-dd HH:mm:ss.SSSSSS"; 
NSString *dateString = @"2014-05-13 23:31:41.374577"; 
NSDate *date = [formatter dateFromString:dateString]; 
NSString *anotherDateString = [formatter stringFromDate:date]; 

但是，anotherDateString是2014-05-13 23:31:41.374000。爲什麼它截斷毫秒？

Answer 1

似乎NSDateFormatter作品僅是毫秒級，對於 原因如下：

• 通過設置在CFDateFormatterCreateDateFromString斷點，可以 看到這個功能是從dateFromString:稱爲：

  (lldb) bt 
  * thread #1: tid = 0x26d03f, 0x018f47d0 CoreFoundation`CFDateFormatterCreateDateFromString, queue = 'com.apple.main-thread', stop reason = breakpoint 3.1 
      frame #0: 0x018f47d0 CoreFoundation`CFDateFormatterCreateDateFromString 
      frame #1: 0x0116e0ea Foundation`getObjectValue + 248 
      frame #2: 0x0116dfc7 Foundation`-[NSDateFormatter getObjectValue:forString:errorDescription:] + 206 
      frame #3: 0x0116879f Foundation`-[NSDateFormatter dateFromString:] + 71 
      * frame #4: 0x00002d56 foo`main(argc=1, argv=0xbfffee54) + 182 at main.mm:25 
  

• CFDateFormatterCreateDateFromString()是從 CFDateFormatter.c 這是開源的。可以看到，所有的日曆計算都是使用 ICU Calendar Classes。

• 它在calendar.h指出，Calendar使用UDate具有毫秒分辨率：

  /** 
  * <code>Calendar</code> is an abstract base class for converting between 
  * a <code>UDate</code> object and a set of integer fields such as 
  * <code>YEAR</code>, <code>MONTH</code>, <code>DAY</code>, <code>HOUR</code>, 
  * and so on. (A <code>UDate</code> object represents a specific instant in 
  * time with millisecond precision. See UDate 
  * for information about the <code>UDate</code> class.) 
  * ...