ios - NSCalendar的dateBySettingUnit方法返回的结果不明显

我的问题已由NSDateComponents解决，但我想了解为何此方法如此有效

NSDate *today = [NSDate date];
NSLog(@"today is: %@", today);

NSDate *dayChanged = [calendar dateBySettingUnit:(NSCalendarUnitDay)
                                           value:10
                                          ofDate:today
                                         options:0];
NSLog(@"dayChanged is: %@", dayChanged);

NSDate *monthChanged = [calendar dateBySettingUnit:(NSCalendarUnitMonth)
                                             value:5
                                            ofDate:today
                                           options:0];
NSLog(@"monthChanged is: %@", monthChanged);

结果是:

today is: 2016-09-30 06:40:36 +0000
dayChanged is: 2016-10-10 00:00:00 +0000
monthChanged is: 2017-05-01 00:00:00 +0000

为什么dateBySettingUnit下一个更大的单位递增？

最佳答案

根据观察，dateBySettingUnit does not return past date

因此，使用dateBySettingUnit进行 future 日期的快速测试设置可以按预期工作，请参见以下代码:

NSCalendar *calendar = [[NSCalendar alloc] initWithCalendarIdentifier:NSCalendarIdentifierGregorian];
NSDate *today = [NSDate date];
NSLog(@"today is: %@", today);

NSDate *dayChanged = [calendar dateBySettingUnit:(NSCalendarUnitDay)
                                           value:31
                                          ofDate:today
                                         options:NSCalendarWrapComponents];
NSLog(@"dayChanged is: %@", dayChanged);

NSDate *monthChanged = [calendar dateBySettingUnit:(NSCalendarUnitMonth)
                                             value:12
                                            ofDate:dayChanged
                                           options:NSCalendarWrapComponents];
NSLog(@"monthChanged is: %@", monthChanged);

日志:

2016-09-30 12:58:30.506 TestRegex[3785:72957] today is: 2016-09-30 07:58:30 +0000
2016-09-30 12:58:30.508 TestRegex[3785:72957] dayChanged is: 2016-09-30 19:00:00 +0000
2016-09-30 12:58:30.509 TestRegex[3785:72957] monthChanged is: 2016-11-30 19:00:00 +0000

您可以尝试在链接的答案中提到的NSCalendar扩展名，该扩展名实际上可以计算偏移量，也可以使用NSDateComponents解:

 NSDateComponents *comps = [[NSCalendar currentCalendar] components:NSCalendarUnitDay | NSCalendarUnitMonth | NSCalendarUnitYear fromDate:today];
[comps setCalendar:calendar];
[comps setDay:5];

希望有帮助!