【iOS开发系列】collectionView头部悬浮

//
//  WNCalendarCollectionLayout.h
//  Calendar
//
//  Created by 付国良 on 16/3/22.
//  Copyright © 2016年 日历. All rights reserved.
//

#import <UIKit/UIKit.h>

@interface WNCalendarCollectionLayout : UICollectionViewFlowLayout

@property (nonatomic, assign) BOOL isSuspend;

@end

//  Calendar
//
//  Created by 付国良 on 16/3/22.
//  Copyright © 2016年 日历. All rights reserved.
//

#import "WNCalendarCollectionLayout.h"

@implementation WNCalendarCollectionLayout

- (instancetype)init
{
self = [super init];
if (self){

}
return self;
}

- (NSArray *)layoutAttributesForElementsInRect:(CGRect)rect
{
NSMutableArray *answer = [[super layoutAttributesForElementsInRect:rect] mutableCopy];
UICollectionView * const cv = self.collectionView;
CGPoint const contentOffset = cv.contentOffset;

NSMutableIndexSet *missingSections = [NSMutableIndexSet indexSet];
for (UICollectionViewLayoutAttributes *layoutAttributes in answer) {
if (layoutAttributes.representedElementCategory == UICollectionElementCategoryCell) {
[missingSections addIndex:layoutAttributes.indexPath.section];
}
}
for (UICollectionViewLayoutAttributes *layoutAttributes in answer) {
if ([layoutAttributes.representedElementKind isEqualToString:UICollectionElementKindSectionHeader]) {
[missingSections removeIndex:layoutAttributes.indexPath.section];
}
}

[missingSections enumerateIndexesUsingBlock:^(NSUInteger idx, BOOL *stop) {

NSIndexPath *indexPath = [NSIndexPath indexPathForItem:0 inSection:idx];

UICollectionViewLayoutAttributes *layoutAttributes = [self layoutAttributesForSupplementaryViewOfKind:UICollectionElementKindSectionHeader atIndexPath:indexPath];

[answer addObject:layoutAttributes];

}];

for (UICollectionViewLayoutAttributes *layoutAttributes in answer) {

if ([layoutAttributes.representedElementKind isEqualToString:UICollectionElementKindSectionHeader]) {

NSInteger section = layoutAttributes.indexPath.section;
NSInteger numberOfItemsInSection = [cv numberOfItemsInSection:section];

NSIndexPath *firstObjectIndexPath = [NSIndexPath indexPathForItem:0 inSection:section];
NSIndexPath *lastObjectIndexPath = [NSIndexPath indexPathForItem:MAX(0, (numberOfItemsInSection - 1)) inSection:section];

UICollectionViewLayoutAttributes *firstObjectAttrs;
UICollectionViewLayoutAttributes *lastObjectAttrs;

if (numberOfItemsInSection > 0) {
firstObjectAttrs = [self layoutAttributesForItemAtIndexPath:firstObjectIndexPath];
lastObjectAttrs = [self layoutAttributesForItemAtIndexPath:lastObjectIndexPath];
} else {
firstObjectAttrs = [self layoutAttributesForSupplementaryViewOfKind:UICollectionElementKindSectionHeader
atIndexPath:firstObjectIndexPath];
lastObjectAttrs = [self layoutAttributesForSupplementaryViewOfKind:UICollectionElementKindSectionFooter
atIndexPath:lastObjectIndexPath];
}

CGFloat headerHeight = CGRectGetHeight(layoutAttributes.frame);
CGPoint origin = layoutAttributes.frame.origin;
origin.y = MIN(
MAX(
contentOffset.y + cv.contentInset.top,
(CGRectGetMinY(firstObjectAttrs.frame) - headerHeight)
),
(CGRectGetMaxY(lastObjectAttrs.frame) - headerHeight)
);

layoutAttributes.zIndex = 1024;
layoutAttributes.frame = (CGRect){
.origin = origin,
.size = layoutAttributes.frame.size
};

}
}
return answer;
}

- (BOOL)shouldInvalidateLayoutForBoundsChange:(CGRect)newBound
{
return _isSuspend;
}

- (BOOL)isSuspend{
return _isSuspend;
}
@end