objective-c - NSTime : Why [NSDateComponent date] returns nil here?

Question

我想将 NSDate 从一个时区转换为另一个时区。

这是我的代码:

NSDate* convertTime(NSDate* fromDate, NSTimeZone* fromTimeZone, NSTimeZone* toTimeZone) {
    if (fromTimeZone == toTimeZone) {
        return fromDate;
    }

    NSCalendarUnit val = NSCalendarUnitYear| NSCalendarUnitMonth | NSCalendarUnitDay | NSCalendarUnitHour| NSCalendarUnitMinute | NSCalendarUnitSecond | NSCalendarUnitTimeZone;
    NSCalendar* dupCal =  [[NSCalendar currentCalendar] copy];

    [dupCal setTimeZone:toTimeZone];
    NSDateComponents *dupComponents = [dupCal components:val fromDate:fromDate];

    return [dupComponents date]; // <- Why return nil?

}

int testTimeZone() {
    NSDate* res = convertTime([NSDate date], [NSTimeZone defaultTimeZone], [NSTimeZone timeZoneWithName:@"America/New_York"]);
    NSLog(@"convertTime: %@", res);
    return 0;

}

在输出窗口中，我可以看到这样的输出:

2014-02-15 11:15:18.040 MyApp[28742:70b] convertTime: (null)

出于某种原因，return [dupComponents date]; 始终返回 nil，即使我可以在调试器中清楚地看到它已正确初始化。它包含我期望的值。

Printing description of dupComponents:
<NSDateComponents: 0x100112260>
    TimeZone: America/New_York (EST) offset -18000
    Calendar Year: 2014
    Month: 2
    Leap month: no
    Day: 14
    Hour: 19
    Minute: 19
    Second: 29

为什么会这样？

Answer 1

我找到了解决方案。

我需要在调用 date 之前将日历对象设置为 NSDateCompoent 对象:

NSDateComponents *dupComponents = [dupCal components:val fromDate:fromDate];

[dupComponents setCalendar:dupCal]; // THIS IS THE SOLUTION

return [dupComponents date];