iphone——日期處理

Dates

        NSDate類提供了建立date，比較date以及計算兩個date之間間隔的功能。Date對象是不可改變的。

        如果你要建立date對象並表示當前日期，你可以alloc一個NSDate對象並調用init初始化：

NSDate *now = [[NSDate alloc] init];

        或者使用NSDate的date類方法來建立一個日期對象。如果你需要與當前日期不同的日期，你可以使用NSDate的initWithTimeInterval...或dateWithTimeInterval...方法，你也可以使用更複雜的calendar或date components對象。

        建立一定時間間隔的NSDate對象：

NSTimeInterval secondsPerDay = 24 * 60 * 60;NSDate *tomorrow = [[NSDate alloc] initWithTimeIntervalSinceNow:secondsPerDay];NSDate *yesterday = [[NSDate alloc] initWithTimeIntervalSinceNow:-secondsPerDay];[tomorrow release];[yesterday release];

        使用增加時間間隔的方式來產生NSDate對象：

NSTimeInterval secondsPerDay = 24 * 60 * 60;NSDate *today = [[NSDate alloc] init];NSDate *tomorrow, *yesterday;tomorrow = [today dateByAddingTimeInterval: secondsPerDay];yesterday = [today dateByAddingTimeInterval: -secondsPerDay];[today release];

        如果要對NSDate對象進行比較，可以使用isEqualToDate:, compare:, laterDate:和 earlierDate:方法。這些方法都進行精確比較，也就是說這些方法會一直精確比較到NSDate對象中秒一級。例如，你可能比較兩個日期，如果他們之間的間隔在一分鐘之內則認為這兩個日期是相等的。在這種情況下使用，timeIntervalSinceDate:方法來對兩個日期進行比較。下面的代碼進行了樣本：

if (fabs([date2 timeIntervalSinceDate:date1]) < 60) ...

NSCalendar & NSDateComponents

        日曆對象封裝了對系統日期的計算，包括這一年開始，總天數以及劃分。你將使用日曆對象對絕對日期與date components（包括年，月，日，時，分，秒）進行轉換。

        NSCalendar定義了不同的日曆，包括佛教曆，格里高利曆等（這些都與系統提供的本地化設定相關）。NSCalendar與NSDateComponents對象緊密相關。

        你可以通過NSCalendar對象的currentCalendar方法來獲得當前系統使用者佈建的日曆。

NSCalendar *currentCalendar = [NSCalendar currentCalendar];NSCalendar *japaneseCalendar = [[NSCalendar alloc] initWithCalendarIdentifier:NSJapaneseCalendar];NSCalendar *usersCalendar = [[NSLocale currentLocale] objectForKey:NSLocaleCalendar];

        usersCalendar和currentCalendar對象是相等的，儘管他們是不同的對象。

        你可以使用NSDateComponents對象來表示一個日期對象的組件——例如年，月，日和小時。如果要使一個NSDateComponents對象有意義，你必須將其與一個日曆對象相關聯。下面的程式碼範例了如何建立一個NSDateComponents對象：

NSDateComponents *components = [[NSDateComponents alloc] init];[components setDay:6];[components setMonth:5];[components setYear:2004];NSInteger weekday = [components weekday]; // Undefined (== NSUndefinedDateComponent)

        要將一個日期對象解析到相應的date components，你可以使用NSCalendar的components:fromDate:方法。此外日期本身，你需要指定NSDateComponents對象返回組件。

NSDate *today = [NSDate date];NSCalendar *gregorian = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];NSDateComponents *weekdayComponents = [gregorian components:(NSDayCalendarUnit | NSWeekdayCalendarUnit) fromDate:today];NSInteger day = [weekdayComponents day];NSInteger weekday = [weekdayComponents weekday];同樣你也可以從NSDateComponents對象來建立NSDate對象：NSDateComponents *components = [[NSDateComponents alloc] init];[components setWeekday:2]; // Monday[components setWeekdayOrdinal:1]; // The first Monday in the month[components setMonth:5]; // May[components setYear:2008];NSCalendar *gregorian = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];NSDate *date = [gregorian dateFromComponents:components];

        為了保證正確的行為，您必須確保使用的組件在日曆上是有意義的。指定“出界”行事曆群組件，如一個-6或2月30日在西曆中的日期值產生未定義的行為。

        你也可以建立一個不帶年份的NSDate對象，這樣的作業系統會自動產生一個年份，但在後面的代碼中不會使用其自動產生的年份。

NSDateComponents *components = [[NSDateComponents alloc] init];[components setMonth:11];[components setDay:7];NSCalendar *gregorian = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];NSDate *birthday = [gregorian dateFromComponents:components];

        下面的樣本顯示了如何從一個日曆置換到另一個日曆：

NSDateComponents *comps = [[NSDateComponents alloc] init];[comps setDay:6];[comps setMonth:5];[comps setYear:2004];NSCalendar *gregorian = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];NSDate *date = [gregorian dateFromComponents:comps];[comps release];[gregorian release];NSCalendar *hebrew = [[NSCalendar alloc] initWithCalendarIdentifier:NSHebrewCalendar];NSUInteger unitFlags = NSDayCalendarUnit | NSMonthCalendarUnit | NSYearCalendarUnit;NSDateComponents *components = [hebrew components:unitFlags fromDate:date];NSInteger day = [components day]; // 15NSInteger month = [components month]; // 9NSInteger year = [components year]; // 5764

曆法計算

        在目前時間加上一個半小時：

NSDate *today = [[NSDate alloc] init];NSCalendar *gregorian = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];NSDateComponents *offsetComponents = [[NSDateComponents alloc] init];[offsetComponents setHour:1];[offsetComponents setMinute:30];// Calculate when, according to Tom Lehrer, World War III will endNSDate *endOfWorldWar3 = [gregorian dateByAddingComponents:offsetComponents toDate:today options:0];

        獲得當前星期中的星期天（使用格里高利曆）：

NSDate *today = [[NSDate alloc] init];NSCalendar *gregorian = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];// Get the weekday component of the current dateNSDateComponents *weekdayComponents = [gregorian components:NSWeekdayCalendarUnit fromDate:today];/*Create a date components to represent the number of days to subtract from the current date.The weekday value for Sunday in the Gregorian calendar is 1, so subtract 1 from the number of days to subtract from the date in question.  (If today is Sunday, subtract 0 days.)*/NSDateComponents *componentsToSubtract = [[NSDateComponents alloc] init];[componentsToSubtract setDay: 0 - ([weekdayComponents weekday] - 1)];NSDate *beginningOfWeek = [gregorian dateByAddingComponents:componentsToSubtract toDate:today options:0];/*Optional step:beginningOfWeek now has the same hour, minute, and second as the original date (today).To normalize to midnight, extract the year, month, and day components and create a new date from those components.*/NSDateComponents *components = [gregorian components:(NSYearCalendarUnit | NSMonthCalendarUnit | NSDayCalendarUnit) fromDate: beginningOfWeek];beginningOfWeek = [gregorian dateFromComponents:components];

        如何可以計算出一周的第一天（根據系統的日曆設定）：

NSDate *today = [[NSDate alloc] init];NSDate *beginningOfWeek = nil;BOOL ok = [gregorian rangeOfUnit:NSWeekCalendarUnit startDate:&beginningOfWeek interval:NULL forDate: today];

        獲得兩個日期之間的間隔:

NSDate *startDate = ...;NSDate *endDate = ...;NSCalendar *gregorian = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];NSUInteger unitFlags = NSMonthCalendarUnit | NSDayCalendarUnit;NSDateComponents *components = [gregorian components:unitFlags fromDate:startDate toDate:endDate options:0];NSInteger months = [components month];NSInteger days = [components day];

        使用Category來計算同一時代（AD|BC）兩個日期午夜之間的天數：

@implementation NSCalendar (MySpecialCalculations)-(NSInteger)daysWithinEraFromDate:(NSDate *) startDate toDate:(NSDate *) endDate {     NSInteger startDay=[self ordinalityOfUnit:NSDayCalendarUnit inUnit: NSEraCalendarUnit forDate:startDate];     NSInteger endDay=[self ordinalityOfUnit:NSDayCalendarUnit inUnit: NSEraCalendarUnit forDate:endDate];     return endDay-startDay;}@end

        使用Category來計算不同時代（AD|BC）兩個日期的天數：

@implementation NSCalendar (MyOtherMethod)-(NSInteger) daysFromDate:(NSDate *) startDate toDate:(NSDate *) endDate {     NSCalendarUnit units=NSEraCalendarUnit | NSYearCalendarUnit | NSMonthCalendarUnit | NSDayCalendarUnit;     NSDateComponents *comp1=[self components:units fromDate:startDate];     NSDateComponents *comp2=[self components:units fromDate endDate];     [comp1 setHour:12];     [comp2 setHour:12];     NSDate *date1=[self dateFromComponents: comp1];     NSDate *date2=[self dateFromComponents: comp2];     return [[self components:NSDayCalendarUnit fromDate:date1 toDate:date2 options:0] day];}@end

        判斷一個日期是否在當前一周內（使用格里高利曆）：

-(BOOL)isDateThisWeek:(NSDate *)date {     NSDate *start;     NSTimeInterval extends;     NSCalendar *cal=[NSCalendar autoupdatingCurrentCalendar];     NSDate *today=[NSDate date];     BOOL success= [cal rangeOfUnit:NSWeekCalendarUnit startDate:&start interval: &extends forDate:today];     if(!success)return NO;     NSTimeInterval dateInSecs = [date timeIntervalSinceReferenceDate];     NSTimeInterval dayStartInSecs= [start timeIntervalSinceReferenceDate];     if(dateInSecs > dayStartInSecs && dateInSecs < (dayStartInSecs+extends)){          return YES;     }     else {          return NO;     }}