NSDate / NSDateComponent 问题答案

【问题标题】：NSDate / NSDateComponent problemNSDate / NSDateComponent 问题
【发布时间】：2010-07-15 19:12:35
【问题描述】：

假设我们是 2010 年 7 月 15 日，我想获得前一周的第一天，即 7 月 5 日。我有以下方法，但它不起作用。看来我的weekday属性有问题……

+ (NSDate *)getFirstDayOfPreviousWeek
{
// Get current date
NSCalendar *cal = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];
NSDate *now = [NSDate date];

// Get today date at midnight
NSDateComponents *components = [cal components:( NSYearCalendarUnit | NSMonthCalendarUnit | NSDayCalendarUnit | NSHourCalendarUnit | NSMinuteCalendarUnit | NSSecondCalendarUnit | NSTimeZoneCalendarUnit) fromDate:now];
[components setHour:0];
[components setMinute:0];
[components setSecond:0];
NSInteger *dayNbr = [components weekday];
NSLog(@"week day:%@",dayNbr); // CRASH
NSDate *todayMidnight = [cal dateFromComponents:components];

// Get first day of previous week midnight
components = [[[NSDateComponents alloc] init] autorelease];
NSInteger *wd = [dayNbr intValue] * -1 + 1;
[components setWeekDay:wd];
[components setWeek:-1];
NSDate *newDate = [cal dateByAddingComponents:components toDate:todayMidnight options:0];
[cal release];

NSLog(@"new date:%@", newDate);

return newDate;
}

你能帮忙吗？

非常感谢，

卢克

【问题讨论】：

首先弄清你的类型，NSLog(@"..%@...",xxx);将 NSString 作为参数，而您似乎可以在其中放入任何内容。
其实你可以在里面放任何东西来监听description消息，一般可以是任何对象，但是NSInteger不是对象：把NSInteger *dayNbr改成NSInteger dayNbr因为你不需要指针，所以将%@ 更改为%d，因为它是一个整数。
谢谢，我没明白 NSInteger 不是对象。

标签： iphone nsdate nsdatecomponents

【解决方案1】：

我觉得应该会容易很多

- (NSDate *)getFirstDayOfPreviousWeek
{
    NSCalendar *cal = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];
    NSDate *now = [NSDate date];
    NSLog(@"Current date: %@", now);

    // required components for today
    NSDateComponents *components = [cal components:( NSYearCalendarUnit | NSMonthCalendarUnit | NSWeekCalendarUnit | NSWeekdayCalendarUnit ) fromDate:now];

    // adjust them for first day of previous week (Monday)
    [components setWeekday:2];
    [components setWeek:([components week] - 1)];

    // construct new date and return
    NSDate *newDate = [cal dateFromComponents:components];
    NSLog(@"New date: %@", newDate);

    return newDate;
}

请注意，此解决方案假定一周的第一天是星期一。

【讨论】：

【解决方案2】：

以下 Swift 3 Playground 代码显示了如何从任意日期获取前一周的第一天：

import Foundation

let calendar = Calendar(identifier: .gregorian)
let intialDate = Date()

// Prepare the new date components
var newDateComponents = calendar.dateComponents([.year, .month, .weekOfMonth], from: intialDate)
newDateComponents.weekOfMonth = newDateComponents.weekOfMonth! - 1 // get previous week
newDateComponents.weekday = 2 // sunday is 1, monday is 2

// Get the new date
let newDate = calendar.date(from: newDateComponents)
print(newDate ?? "Could not retrieve a date")

如果需要重复此操作，可以将代码重构为函数或方法：

import Foundation

func firstDayOfPreviousWeek(from date: Date) -> Date? {
    let calendar = Calendar(identifier: .gregorian)
    var newDateComponents = calendar.dateComponents([.year, .month, .weekOfMonth], from: date)
    newDateComponents.weekOfMonth = newDateComponents.weekOfMonth! - 1
    newDateComponents.weekday = 2
    return calendar.date(from: newDateComponents)
}

因此，以下 Playground 代码显示了如何获取从 2010 年 7 月 15 日开始的前一周的第一天：

import Foundation

func firstDayOfPreviousWeek(from date: Date) -> Date? {
    let calendar = Calendar(identifier: .gregorian)
    var newDateComponents = calendar.dateComponents([.year, .month, .weekOfMonth], from: date)
    newDateComponents.weekOfMonth = newDateComponents.weekOfMonth! - 1
    newDateComponents.weekday = 2
    newDateComponents.timeZone = TimeZone(abbreviation: "GMT")
    return calendar.date(from: newDateComponents)
}

let calendar = Calendar(identifier: .gregorian)
var july15Components = DateComponents()
july15Components.day = 15
july15Components.month = 7
july15Components.year = 2010
july15Components.timeZone = TimeZone(abbreviation: "GMT")
let july15 = calendar.date(from: july15Components)!
print(july15) // prints 2010-07-15 00:00:00 +0000

let newDate = firstDayOfPreviousWeek(from: july15)
print(newDate ?? "Could not retrieve a date") // prints 2010-07-05 00:00:00 +0000

【讨论】：