1、oc中各种nil。

1. nil
   • 用来表示空对象，数组、字典结束判断
2. Nil
   • 用于类的空指针
3. NSNull
   • 用来标识什么都没有 [NSNull null]，通常用于集合占位
4. NULL
   • 通用用于c指针变量为空
     NSLog输出nil、Nil为(null)，输出NSNull为<null>

2、判断两个日期是否在同一周

// 方法一
        NSDateFormatter *formatter = [[NSDateFormatter alloc] init];
        formatter.dateFormat = @"yyyy-MM-dd HH:mm:ss";
        
        NSString *todayString = @"2019-10-07 03:02:00";
        NSString *afterString = @"2019-10-06 00:08:33";
        
        // 方法一
        NSDate *today = [formatter dateFromString:todayString];
        NSDate *afterDay = [formatter dateFromString:afterString];
        
        formatter.dateFormat = @"yyyy-MM-dd HH:mm:ss E";
        NSLog(@"今天: %@", [formatter stringFromDate:today]);
        NSLog(@"之前: %@", [formatter stringFromDate:afterDay]);
        
        NSInteger weekday = [[[NSCalendar currentCalendar] components:NSCalendarUnitWeekday fromDate:today] weekday];
        NSInteger afterweekday = [[[NSCalendar currentCalendar] components:NSCalendarUnitWeekday fromDate:afterDay] weekday];
        weekday = weekday == 1 ? 7 : weekday - 1;
        afterweekday = afterweekday == 1 ? 7 : afterweekday - 1;
        NSLog(@"今天周 : %ld, 之前是: %ld",(long)weekday, afterweekday);

        NSInteger startDay=[NSCalendar.currentCalendar ordinalityOfUnit:NSCalendarUnitDay inUnit: NSCalendarUnitYear forDate:afterDay];
        NSInteger endDay=[NSCalendar.currentCalendar ordinalityOfUnit:NSCalendarUnitDay  inUnit: NSCalendarUnitYear forDate:today];
        NSLog(@"startDay : %ld,  endDay: %ld ", (long)startDay, (long)endDay);
         NSInteger distance = endDay - startDay;

        NSLog(@"test : today: %@,  afterDay: %@ %ld", today, afterDay, (long)distance);
        if (labs(distance) > 6) {
            NSLog(@"不在同一周");
        } else if (distance > 0) {
            if (distance < weekday) {
                NSLog(@"在同一周");
            } else {
                NSLog(@"不在同一周");
            }
        } else {
            if (labs(distance) <= 7 - weekday) {
                NSLog(@"在同一周");
            } else {
                NSLog(@"不在同一周");
            }
        }

        // 方法二(推荐使用)
        
        NSCalendar *calendar = [[NSCalendar alloc] initWithCalendarIdentifier:NSCalendarIdentifierGregorian];
        calendar.firstWeekday = 2;  // 周一是一周的开始第一天
        
        NSCalendarUnit units = NSCalendarUnitWeekday | NSCalendarUnitYear | NSCalendarUnitWeekOfYear;
        
        NSDateComponents *todayComp = [calendar components:units fromDate:today];
        NSDateComponents *afterComp = [calendar components:units fromDate:afterDay];
        
        NSLog(@"%@, \n %@ \n %@", todayComp, afterComp);
        
        if (todayComp.year == afterComp.year && todayComp.weekOfYear == afterComp.weekOfYear) {
            NSLog(@"在同一周");
        } else{
            NSLog(@"不在同一周");
        }

3、Objective-C 中 nullable、__nullable、_Nullable 的区别

• 最开始使用的是__nullable,但是由于可能与第三方库冲突，苹果引进勒_Nullable，而且苹果支持不带下划线写法nullable

- (nullable NSString *)method;
- (NSString * __nullable)method;
- (NSString * _Nullable)method;

• 对于 双指针类型对象 、Block 的返回值、Block 的参数 等，这时候就不能用 nonnull/nullable 修饰，只能用带下划线的 __nonnull/__nullable 或者 _Nonnull/_Nullable：
• 苹果建议，对于属性、方法返回值、方法参数的修饰，使用：nonnull/nullable；
  对于 C 函数的参数、Block 的参数、Block 返回值的修饰，使用：_Nonnull/_Nullable，建议弃用 __nonnull/__nullable。