将NSDate的默认值设置为Today

NSCalendar *calendar = [NSCalendar currentCalendar];
NSDateComponents *components = [calendar components:NSCalendarUnitYear | NSCalendarUnitMonth | NSCalendarUnitDay fromDate:[NSDate date]];
NSDate *today = [calendar dateFromComponents:components];

let calendar = Calendar.current
let components = calendar.dateComponents([.year, .month, .day], from: Date())
let today = calendar.date(from: components)

#import <objc/runtime.h>

@implementation NSDate (DefaultToday)

+ (void)load {
    Method originalMethod = class_getInstanceMethod([self class], @selector(init));
    Method swizzledMethod = class_getInstanceMethod([self class], @selector(initDefaultToday));
    method_exchangeImplementations(originalMethod, swizzledMethod);
}

- (instancetype)initDefaultToday {
    NSCalendar *calendar = [NSCalendar currentCalendar];
    NSDateComponents *components = [calendar components:NSCalendarUnitYear | NSCalendarUnitMonth | NSCalendarUnitDay fromDate:[NSDate date]];
    NSDate *today = [calendar dateFromComponents:components];
    
    return [self initDefaultToday];
}

@end

extension NSDate {
    @objc convenience init(defaultToday: Void) {
        let calendar = Calendar.current
        let components = calendar.dateComponents([.year, .month, .day], from: Date())
        let today = calendar.date(from: components)
        
        self.init(timeInterval: 0, since: today!)
    }
}