obj-c编程10:Foundation库中类的使用(5)[时间对象]

    隔了好久才有了这新的一篇，还是无奈的时间啊！so这次我们就着重谈谈它喽。

    F库中有很多时间相关的类，比如NSDate，NSTimeInterval，NSTimeZone，NSDateComponents，NSCalendar，最后还有一个NSDateFormatter。新手很快就被弄的云里雾里了，我也在雾里呆了好长段时间了。简单地说NSDate只是一个单纯的时间类；NSTimeInterval用来表示2个时间之间的间隔；NSTimeZone和NSCalendar顾名思义自然是表示时区和日历啦；NSDateComponents可以从多个单个元素构建任意一个时间，而NSDateFormatter可以将时间转换为一个人类可读的字符串或者反向转换。下面我们用代码来说明问题喽：

#import 

//static int g_idx = 0;

#define msg(...) printf("[[%d]]:\n",__LINE__);NSLog(__VA_ARGS__)

int main(int argc,char *argv[])
{
	@autoreleasepool{
		//创建现在时间的NSDate对象
		NSDate *now = [NSDate date];
		NSDate *now_too = [[NSDate alloc] init];
		msg(@"now is %@ and %@",now,now_too);

		//使用时间间隔创建NSDate对象
		NSDate *now_sub_hour = [now dateByAddingTimeInterval:-3600];
		NSDate *now_add_hour = [now dateByAddingTimeInterval:3600];
		msg(@"now +- hour is %@ and %@",now_add_hour,now_sub_hour);

		//计算2个时间的间隔(以秒计)
		NSTimeInterval interval1 = [now timeIntervalSinceDate:now_sub_hour];
		NSTimeInterval interval2 = [now timeIntervalSinceDate:now_add_hour];
		msg(@"intervals is %f and %f",interval1,interval2);

		//比较2个时间的“大小”
		assert([now laterDate:now_add_hour] == now_add_hour);	//true
		assert([now earlierDate:now_sub_hour] == now_sub_hour);	//true
		assert([now compare:now_sub_hour] == NSOrderedDescending);	//true

		NSTimeZone *zone = [NSTimeZone timeZoneWithName:@"Asia/Shanghai"];

		//使用时间组件对象和日历对象构造时间
		NSDateComponents *cps = [[NSDateComponents alloc] init];
		[cps setTimeZone:zone];

		[cps setMonth:11];
		[cps setDay:23];
		[cps setYear:2018];
		[cps setHour:23];
		[cps setMinute:59];
		[cps setSecond:31];
		NSCalendar *calar = [NSCalendar currentCalendar];
		NSDate *date = [calar dateFromComponents:cps];
		msg(@"date is %@",date);
		[cps setDay:([cps day] - 1)];
		[cps setMinute:([cps minute] + 1)];
		date = [calar dateFromComponents:cps];
		msg(@"date is %@",date);

		//设置日历类型为中国日历类型
		NSCalendar *china_calar = [[NSCalendar alloc] initWithCalendarIdentifier:\
			NSRepublicOfChinaCalendar];
		date = [china_calar dateFromComponents:cps];
		msg(@"china_calar style is %@",date);

		//列出所有已知时区
		//msg(@"all zone :\n%@",[NSTimeZone knownTimeZoneNames]);
		//在时间对象中考虑时区因素
		msg(@"时区缩写:%@",[zone abbreviation]);
		NSData *data = [zone data];
		msg(@"time data is %@",data);
		NSCalendar *calar_zone = [NSCalendar currentCalendar];
		[calar_zone setTimeZone:zone];
		cps = [calar_zone components:(NSYearCalendarUnit|NSMonthCalendarUnit|\
			NSDayCalendarUnit|NSHourCalendarUnit|NSMinuteCalendarUnit|\
			NSSecondCalendarUnit) fromDate:now];
		[cps setTimeZone:zone];
		date = [calar_zone dateFromComponents:cps];
		msg(@"time with zone : %@",date);

		NSDateFormatter *f = [[NSDateFormatter alloc]init];
		[f setDateFormat:@"yyyy-MM-dd hh:mm:ss"];

		msg(@"init time_zone is %@",[f timeZone]);
		date = [f dateFromString:@"2019-10-11 03:14:59"];
		msg(@"date is %@",date);
		[f setTimeZone:[NSTimeZone timeZoneWithName:@"GMT"]];
		date = [f dateFromString:@"2019-10-11 03:14:59"];
		msg(@"date is %@",date);
		[f setTimeZone:[NSTimeZone timeZoneWithName:@"GMT+8"]];
		NSString *fix_date_str = [f stringFromDate:date];
		msg(@"fix_str is %@",fix_date_str);
		[f setTimeZone:[NSTimeZone timeZoneWithName:@"GMT"]];
		date = [f dateFromString:fix_date_str];
		msg(@"date is %@",date);
	}	
	return 0;
}

编译执行结果如下：

apple@kissAir: objc_src$./8

[[13]]:

2014-07-20 17:42:03.593 8[2468:507] now is 2014-07-20 09:42:03 +0000 and 2014-07-20 09:42:03 +0000

[[18]]:

2014-07-20 17:42:03.594 8[2468:507] now +- hour is 2014-07-20 10:42:03 +0000 and 2014-07-20 08:42:03 +0000

[[23]]:

2014-07-20 17:42:03.594 8[2468:507] intervals is 3600.000000 and -3600.000000

[[44]]:

2014-07-20 17:42:03.595 8[2468:507] date is 2018-11-23 15:59:31 +0000

[[48]]:

2014-07-20 17:42:03.596 8[2468:507] date is 2018-11-22 16:00:31 +0000

[[54]]:

2014-07-20 17:42:03.597 8[2468:507] china_calar style is 3929-11-22 16:00:31 +0000

[[59]]:

2014-07-20 17:42:03.599 8[2468:507] 时区缩写:GMT+8

[[61]]:

2014-07-20 17:42:03.601 8[2468:507] time data is

[[69]]:

2014-07-20 17:42:03.602 8[2468:507] time with zone : 2014-07-20 09:42:03 +0000

[[74]]:

2014-07-20 17:42:03.603 8[2468:507] init time_zone is Asia/Harbin (GMT+8) offset 28800

[[76]]:

2014-07-20 17:42:03.604 8[2468:507] date is 2019-10-10 19:14:59 +0000

[[79]]:

2014-07-20 17:42:03.604 8[2468:507] date is 2019-10-11 03:14:59 +0000

[[82]]:

2014-07-20 17:42:03.605 8[2468:507] fix_str is 2019-10-11 11:14:59

[[85]]:

2014-07-20 17:42:03.606 8[2468:507] date is 2019-10-11 11:14:59 +0000

要注意的是，我开始在用NSDateFormatter转换时间时，会发现设置时区后，转换的时间时区总是不对，后来发现NSDateFormatter总会把任意时区时间转换为GMT时间，如果你是GMT+8时区，即中国标准时间，则会将时间减去8小时。就像代码中最后一个例子，如果你不设置zone的时区，默认会使用本机时区，我这里就是GMT+8时区,则date对象实际上时间为2019-10-11 03:14:59减去8小时哦。当我们把f对象的时区变为GMT后，则时间上就不会有任何差别。所以你想显示当前时间是不可以的：

NSDate *now2 = [NSDate date];
msg(@"now2 is %@",now2);

当前时间是 2014-07-20 17:49:10,可是你显示出来会减去8小时的哦，你必须如下做转换：

NSDate *now2 = [NSDate date];
		msg(@"now2 is %@",now2);
		NSDateFormatter *f2 = [[NSDateFormatter alloc] init];
		[f2 setDateFormat:@"yyyy-MM-dd hh:mm:ss"];
		[f2 setTimeZone:[NSTimeZone timeZoneWithName:@"GMT+8"]];
		NSString *now2_str = [f2 stringFromDate:now2];
		msg(@"now2 is %@",now2_str);

了解了吧？运行结果如下：

2014-07-20 17:52:59.513 8[2502:507] now2 is 2014-07-20 09:52:59 +0000

[[93]]:

2014-07-20 17:52:59.513 8[2502:507] now2 is 2014-07-20 17:52:59