iOS日历（单选、区间选择，组头悬停，分组月份阴影效果）

日历效果图

做一个自己的日历
思路：整体用collectionView
难点：组织日期数据，分组背景效果，组头悬停效果

1.组织日期数据

1.两层for循环，创建一个二维数组，第一层为月份，第二层为每个月下的天数。
2.计算起始日期之间一共有几个月，用于第一层数组。
3.计算各个月份一共有几天。并计算每月第一天是星期几，用来添加对应数量的空数据模型。对应天的model对象添加到每月的数组，构成第二层数组。

#pragma mark - 数据源方法
- (NSMutableArray *)setUpDataStartDate:(NSDate *)startDate endDate:(NSDate *)endDate {
    
    NSTimeInterval timeInterval = [endDate timeIntervalSinceDate:startDate];
    if (timeInterval < 0) {
        return nil;
    }
    
    NSTimeInterval todayTime = [[NSDate new]timeIntervalSince1970];
    
//    计算开始结束日期之间相差几个月
    NSInteger monthInterval;
    
    NSDateComponents *startComponents = [startDate getDateComponents];
    NSDateComponents *endComponents = [endDate getDateComponents];
    
    NSInteger startYear = startComponents.year;
    NSInteger endYear = endComponents.year;
    
    NSInteger startMonth = startComponents.month;
    NSInteger endMonth = endComponents.month;
    
    monthInterval = (endYear - startYear) * 12 + (endMonth - startMonth) + 1;
    
    
    NSMutableArray *dataArray = [NSMutableArray new];
    
    
    for (int i = 0; i < monthInterval; i++) {
        
        NSMutableArray *monthArray = [NSMutableArray array];
        
        NSDateComponents *newComponent = [NSDateComponents new];
        NSInteger nowYear = startYear + (startMonth + i)/12;
        newComponent.year = nowYear;
        
        NSInteger monthYu = (startMonth + i) % 12;
        NSInteger nowMonth = monthYu ? monthYu : 12;
        newComponent.month = nowMonth;
        newComponent.day = 1;
        
        NSDate *newDate = [[NSCalendar currentCalendar]dateFromComponents:newComponent];
        
//        本月第一天是周几
        NSInteger weekFirstDay = [newDate getFirstWeekInMonth];
        
//        再本月第一天之前添加空的数据模型
        if (weekFirstDay == 7) {
            weekFirstDay = 0;
        }
        for (int j = 0; j < weekFirstDay; j++) {
            CalendarModel *model = [CalendarModel new];
            model.dateStatus = Calendar_EmptyDate;
            [monthArray addObject:model];
        }
        
//        添加有数据的数据模型
        NSInteger daysMonth = [newDate getDayNumOfMouth];
        
        for (int k = 1; k <= daysMonth ; k++) {
            CalendarModel *model = [CalendarModel new];
            
            model.year = nowYear;
            model.month = nowMonth;
            model.day = k;
            
//            计算周几
            newComponent.day = k;
            NSDate *weekDate = [[NSCalendar currentCalendar]dateFromComponents:newComponent];
            model.week = [weekDate getWeek];
            
            NSTimeInterval modelTimeStamp = weekDate.timeIntervalSince1970 + 60*60*24 - 1;
            if (modelTimeStamp < todayTime) {
                model.dateStatus = Calendar_PassedDate;
            }else{
                model.dateStatus = Calendar_NormalDate;
            }
            
            [monthArray addObject:model];
        }
        
        [dataArray addObject:monthArray];
    }
    
    return dataArray;
}

2.区间选择日期

逻辑：选择开始日期和结束日期区间，开始日期标记为开始变红，结束日期标记为结束变红，两个日期之间的日期变紫。特殊情况：已经选择开始日期，选择结束日期比开始日期早，取消原来的开始日期，将这次点击变为选择开始日期，每次点击选择日期，item的model状态改变重新刷新collectionView

3.添加分组月份阴影效果

1.需要自定义装饰视图，继承UICollectionReusableView实现自定义视图
2.在自定义FlowLayout中注册自定义的装饰视图

- (void)prepareLayout {
    [super prepareLayout];

    [self registerClass:[DSCollectionReusableView class] forDecorationViewOfKind:kDecorationReuseIdentifier];
}

3.因为每个collectionView的section背景装饰视图的月份是不同的，所以需要动态改变。不能直接传值，只能通过layoutAttributesForElementsInRect方法，添加装饰视图

- (NSArray *)layoutAttributesForElementsInRect:(CGRect)rect {
    NSArray *superAttrs = [super layoutAttributesForElementsInRect:rect];
    
//    避免多次调用设置section背景图片
    NSInteger sectionNum = 0;
    BOOL isSet = true;
    for (UICollectionViewLayoutAttributes *attr in superAttrs) {

        if (isSet) {
            if (attr.indexPath.section == 0) {
                sectionNum = attr.indexPath.section;
                
            }else {
                sectionNum = attr.indexPath.section + 1;//只计算当前显示的分组
                
            }
            isSet = false;
        }
        
        if (sectionNum == attr.indexPath.section) {

            [attrs addObject:[self layoutAttributesForDecorationViewOfKind:kDecorationReuseIdentifier atIndexPath:attr.indexPath top:attr.frame.origin.y]];
            
            sectionNum ++;
        }
        
    }

    return attrs;
    
}

- (UICollectionViewLayoutAttributes *)layoutAttributesForDecorationViewOfKind:(NSString *)decorationViewKind atIndexPath:(NSIndexPath *)indexPath top:(CGFloat)top {
    
//    UICollectionViewLayoutAttributes *layoutAttribute = [UICollectionViewLayoutAttributes layoutAttributesForDecorationViewOfKind:decorationViewKind withIndexPath:indexPath];
    
    DSLayoutAttributes *layoutAttribute = [DSLayoutAttributes layoutAttributesForDecorationViewOfKind:decorationViewKind withIndexPath:indexPath];

    layoutAttribute.mouthStr = [NSString stringWithFormat:@"%@",self.monthArray[indexPath.section]];
    
    layoutAttribute.frame = CGRectMake(0, top, [UIScreen mainScreen].bounds.size.width, 375);
    layoutAttribute.zIndex = -1;
    
    
    return layoutAttribute;
}

4.组头悬停效果

iOS 9.0之后直接设置flowLayout.sectionHeadersPinToVisibleBounds = true;就可以。但是9.0之前需要自己手动在自定义FlowLayout中计算，组头悬停。可以参考下这老哥的https://www.jianshu.com/p/f0a2a7a81c6f

我的github地址demo：https://github.com/D-james/Calendar