各種google沒找到完整的判斷星期幾的demo，參考部分前輩的代碼，自己琢磨寫了個方法，大神看了莫拍。

- (NSString*)weekdayStringFromDate:(NSString*)inputDate {
    //截取年月日
    NSString *year = [inputDate substringToIndex:4];
    int yearNum = [year intValue];
    NSString *mon0 = [inputDate substringFromIndex:4];
    NSString *mon = [mon0 substringToIndex:2];
    int monNum = [mon intValue];
    NSString *day = [inputDate substringFromIndex:6];
    int dayNum = [day intValue];
    //定義一個NSDateComponents對象，設(shè)置為導(dǎo)入的時間點
    NSDateComponents *_comps = [[NSDateComponents alloc] init];
    [_comps setDay:dayNum];
    [_comps setMonth:monNum];
    [_comps setYear:yearNum];
    //一周的數(shù)組
    NSArray *weekdays = [NSArray arrayWithObjects: [NSNull null],@"星期天",  @"星期一", @"星期二", @"星期三", @"星期四", @"星期五", @"星期六", nil];
    //定義一個遵循某個歷法的日歷對象
    NSCalendar *gregorian = [[NSCalendar alloc]
                             initWithCalendarIdentifier:NSCalendarIdentifierGregorian];
    NSDate *_date = [gregorian dateFromComponents:_comps];
    NSDateComponents *weekdayComponents =
    [gregorian components:NSCalendarUnitWeekday fromDate:_date];
    int _weekday = [weekdayComponents weekday];
    //返回星期幾
    return [weekdays objectAtIndex:_weekday];
    
}