如何获取当前日期在Cocoa [英] How do I get the current date in Cocoa

问题描述

我开始为iPhone开发，因此我在网上查看不同的教程，以及尝试一些不同的东西。目前，我想创建一个倒计时直到午夜。要获取小时，分钟和秒数，我执行以下操作（我在某处找到）：

  NSDate * now = [NSDate date]; 
 
 int hour = 23  -  [[now dateWithCalendarFormat：nil timeZone：nil] hourOfDay]; 
 int min = 59  -  [[now dateWithCalendarFormat：nil timeZone：nil] minuteOfHour]; 
 int sec = 59  -  [[now dateWithCalendarFormat：nil timeZone：nil] secondOfMinute]; 
 countdownLabel.text = [NSString stringWithFormat：@％02d：％02d：％02d，hour，min，sec];但是，我使用的每个地方 -dateWithCalendarFormat：timeZone： 
 $ c>我得到以下错误：
 警告：'NSDate'可能不响应'-dateWithCalendarFormat：timeZone：'
（没有匹配方法签名的消息将被假定为返回'id'并接受'...'作为参数。）
警告：没有找到'-hourOfDay'方法
错误：无效的操作数到二进制 - （有'int'和'id'）
  
这看起来很简单。 
 
 
 
此外，我注意到在不同的地方，在不同的时间星号（*）位于时间 NSDate * now 或紧靠变量 NSDate * now 之前。两者有什么区别，为什么你会使用一个对另一个？ 
解决方案
您必须使用以下方式：
 code> NSCalendar * gregorian = [[NSCalendar alloc] initWithCalendarIdentifier：NSGregorianCalendar]; 
 NSDateComponents * dateComponents = [gregorian components：（NSHourCalendarUnit | NSMinuteCalendarUnit | NSSecondCalendarUnit）fromDate：yourDateHere]; 
 NSInteger hour = [dateComponents hour]; 
 NSInteger minute = [dateComponents minute]; 
 NSInteger second = [dateComponents second]; 
 [gregorian release]; 
  
现在NSDate * now和NSDate *之间没有区别，这只是一个偏好。从编译器的角度来看，没有什么改变。
 
I'm getting started developing for the iPhone and as such I am looking at different tutorials online as well as trying some different things out myself. Currently, I'm trying to create a countdown until midnight. To get the number of hour, minutes, and seconds, I do the following (which I found somewhere):
NSDate* now = [NSDate date];

int hour = 23 - [[now dateWithCalendarFormat:nil timeZone:nil] hourOfDay];
int min = 59 - [[now dateWithCalendarFormat:nil timeZone:nil] minuteOfHour];
int sec = 59 - [[now dateWithCalendarFormat:nil timeZone:nil] secondOfMinute];
countdownLabel.text = [NSString stringWithFormat:@"%02d:%02d:%02d", hour, min,sec];
However, each place I use -dateWithCalendarFormat:timeZone: I get the following error:
warning: 'NSDate' may not respond to '-dateWithCalendarFormat:timeZone:'
(Messages without a matching method signature will be assumed to return 'id' and accept '...' as arguments.)
warning: no '-hourOfDay' method found
error: invalid operands to binary - (have 'int' and 'id')
This seems like something very simple. What am I missing?

Also, I've noticed at different places and at different times the asterisk (*) is located either right after the time NSDate* now or right before the variable NSDate *now. What is the difference in the two and why would you use one versus the other? 
解决方案
You must use the following:
NSCalendar *gregorian = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];
NSDateComponents *dateComponents = [gregorian components:(NSHourCalendarUnit  | NSMinuteCalendarUnit | NSSecondCalendarUnit) fromDate:yourDateHere];
NSInteger hour = [dateComponents hour];
NSInteger minute = [dateComponents minute];
NSInteger second = [dateComponents second];
[gregorian release];
There is no difference between NSDate* now and NSDate *now, it's just a matter of preference. From the compiler perspective, nothing changes.

                        
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