// // XLPlainFlowLayout.m // XLPlainFlowLayout // // Created by hebe on 15/7/30. // Copyright (c) 2015年 ___ZhangXiaoLiang___. All rights reserved. // // 工作邮箱E-mail: k52471@126.com #import "XLPlainFlowLayout.h" #import "XHCollectionViewLayoutAttributes.h" #import "XHCollectionReusableView.h" NSString *const XHCollectionViewSectionBackground = @"XHCollectionViewSectionBackground"; @implementation XLPlainFlowLayout -(instancetype)init { self = [super init]; if (self) { self.decorationViewAttrs = [NSMutableArray array]; _naviHeight = 64.0; [self setup]; } return self; } - (void)setup { [self registerClass:[XHCollectionReusableView class] forDecorationViewOfKind:XHCollectionViewSectionBackground]; } - (NSArray *) layoutAttributesForElementsInRect:(CGRect)rect { //UICollectionViewLayoutAttributes：我称它为collectionView中的item（包括cell和header、footer这些）的《结构信息》 //截取到父类所返回的数组（里面放的是当前屏幕所能展示的item的结构信息），并转化成不可变数组 NSMutableArray *superArray = [[super layoutAttributesForElementsInRect:rect] mutableCopy]; //创建存索引的数组，无符号（正整数），无序（不能通过下标取值），不可重复（重复的话会自动过滤） NSMutableIndexSet *noneHeaderSections = [NSMutableIndexSet indexSet]; //遍历superArray，得到一个当前屏幕中所有的section数组 for (UICollectionViewLayoutAttributes *attributes in superArray) { //如果当前的元素分类是一个cell，将cell所在的分区section加入数组，重复的话会自动过滤 if (attributes.representedElementCategory == UICollectionElementCategoryCell) { [noneHeaderSections addIndex:attributes.indexPath.section]; } } //遍历superArray，将当前屏幕中拥有的header的section从数组中移除，得到一个当前屏幕中没有header的section数组 //正常情况下，随着手指往上移，header脱离屏幕会被系统回收而cell尚在，也会触发该方法 for (UICollectionViewLayoutAttributes *attributes in superArray) { //如果当前的元素是一个header，将header所在的section从数组中移除 if ([attributes.representedElementKind isEqualToString:UICollectionElementKindSectionHeader]) { [noneHeaderSections removeIndex:attributes.indexPath.section]; } } //遍历当前屏幕中没有header的section数组 [noneHeaderSections enumerateIndexesUsingBlock:^(NSUInteger idx, BOOL *stop) { //取到当前section中第一个item的indexPath NSIndexPath *indexPath = [NSIndexPath indexPathForItem:0 inSection:idx]; //获取当前section在正常情况下已经离开屏幕的header结构信息 UICollectionViewLayoutAttributes *attributes = [self layoutAttributesForSupplementaryViewOfKind:UICollectionElementKindSectionHeader atIndexPath:indexPath]; //如果当前分区确实有因为离开屏幕而被系统回收的header if (attributes) { //将该header结构信息重新加入到superArray中去 [superArray addObject:attributes]; } }]; //遍历superArray，改变header结构信息中的参数，使它可以在当前section还没完全离开屏幕的时候一直显示 for (UICollectionViewLayoutAttributes *attributes in superArray) { //如果当前item是header if ([attributes.representedElementKind isEqualToString:UICollectionElementKindSectionHeader]) { //得到当前header所在分区的cell的数量 NSInteger numberOfItemsInSection = [self.collectionView numberOfItemsInSection:attributes.indexPath.section]; //得到第一个item的indexPath NSIndexPath *firstItemIndexPath = [NSIndexPath indexPathForItem:0 inSection:attributes.indexPath.section]; //得到最后一个item的indexPath NSIndexPath *lastItemIndexPath = [NSIndexPath indexPathForItem:MAX(0, numberOfItemsInSection-1) inSection:attributes.indexPath.section]; //得到第一个item和最后一个item的结构信息 UICollectionViewLayoutAttributes *firstItemAttributes, *lastItemAttributes; if (numberOfItemsInSection>0) { //cell有值，则获取第一个cell和最后一个cell的结构信息 firstItemAttributes = [self layoutAttributesForItemAtIndexPath:firstItemIndexPath]; lastItemAttributes = [self layoutAttributesForItemAtIndexPath:lastItemIndexPath]; }else{ //cell没值,就新建一个UICollectionViewLayoutAttributes firstItemAttributes = [UICollectionViewLayoutAttributes new]; //然后模拟出在当前分区中的唯一一个cell，cell在header的下面，高度为0，还与header隔着可能存在的sectionInset的top CGFloat y = CGRectGetMaxY(attributes.frame)+self.sectionInset.top; firstItemAttributes.frame = CGRectMake(0, y, 0, 0); //因为只有一个cell，所以最后一个cell等于第一个cell lastItemAttributes = firstItemAttributes; } //获取当前header的frame CGRect rect = attributes.frame; /** * 算法 */ //当前的滑动距离 + 因为导航栏产生的偏移量，默认为64（如果app需求不同，需自己设置） CGFloat offset = self.collectionView.contentOffset.y + _naviHeight; //第一个cell的y值 - 当前header的高度 - 可能存在的sectionInset的top CGFloat headerY = firstItemAttributes.frame.origin.y - rect.size.height - self.sectionInset.top; //哪个大取哪个，保证header悬停 //针对当前header基本上都是offset更加大，针对下一个header则会是headerY大，各自处理 CGFloat maxY = MAX(offset,headerY); //最后一个cell的y值 + 最后一个cell的高度 + 可能存在的sectionInset的bottom - 当前header的高度 //当当前section的footer或者下一个section的header接触到当前header的底部，计算出的headerMissingY即为有效值 CGFloat headerMissingY = CGRectGetMaxY(lastItemAttributes.frame) + self.sectionInset.bottom - rect.size.height; //给rect的y赋新值，因为在最后消失的临界点要跟谁消失，所以取小 rect.origin.y = MIN(maxY,headerMissingY); //给header的结构信息的frame重新赋值 attributes.frame = rect; //如果按照正常情况下,header离开屏幕被系统回收，而header的层次关系又与cell相等，如果不去理会，会出现cell在header上面的情况 //通过打印可以知道cell的层次关系zIndex数值为0，我们可以将header的zIndex设置成1，如果不放心，也可以将它设置成非常大，这里随便填了个7 attributes.zIndex = 7; } } for (UICollectionViewLayoutAttributes *attr in self.decorationViewAttrs) { if (CGRectIntersectsRect(rect, attr.frame)) { [superArray addObject:attr]; } } //转换回不可变数组，并返回 return [superArray copy]; } //return YES;表示一旦滑动就实时调用上面这个layoutAttributesForElementsInRect:方法 - (BOOL) shouldInvalidateLayoutForBoundsChange:(CGRect)newBound { return YES; } #pragma mark =============== - (void)prepareLayout { [super prepareLayout]; [self.decorationViewAttrs removeAllObjects]; NSInteger numberOfSections = [self.collectionView numberOfSections]; id delegate = self.collectionView.delegate; if (!numberOfSections || ![delegate conformsToProtocol:@protocol(XHCollectionViewDelegateFlowLayout)]) { return; } for (NSInteger section = 0; section < numberOfSections; section++) { NSInteger numberOfItems = [self.collectionView numberOfItemsInSection:section]; if (numberOfItems <= 0) { continue; } UICollectionViewLayoutAttributes *firstItem = [self layoutAttributesForItemAtIndexPath:[NSIndexPath indexPathForItem:0 inSection:section]]; UICollectionViewLayoutAttributes *lastItem = [self layoutAttributesForItemAtIndexPath:[NSIndexPath indexPathForItem:numberOfItems - 1 inSection:section]]; if (!firstItem || !lastItem) { continue; } UIEdgeInsets sectionInset = [self sectionInset]; if ([delegate respondsToSelector:@selector(collectionView:layout:insetForSectionAtIndex:)]) { UIEdgeInsets inset = [delegate collectionView:self.collectionView layout:self insetForSectionAtIndex:section]; sectionInset = inset; } CGRect sectionFrame = CGRectUnion(firstItem.frame, lastItem.frame); sectionFrame.origin.x -= sectionInset.left; sectionFrame.origin.y -= sectionInset.top; if (self.scrollDirection == UICollectionViewScrollDirectionHorizontal) { sectionFrame.size.width += sectionInset.left + sectionInset.right; sectionFrame.size.height = self.collectionView.frame.size.height; } else { sectionFrame.size.width = self.collectionView.frame.size.width; sectionFrame.size.height += sectionInset.top + sectionInset.bottom; } // 2、定义 XHCollectionViewLayoutAttributes *attr = [XHCollectionViewLayoutAttributes layoutAttributesForDecorationViewOfKind:XHCollectionViewSectionBackground withIndexPath:[NSIndexPath indexPathForItem:0 inSection:section]]; attr.frame = sectionFrame; attr.zIndex = -1; attr.backgroundColor = [delegate collectionView:self.collectionView layout:self backgroundColorForSection:section]; [self.decorationViewAttrs addObject:attr]; } } - (nullable UICollectionViewLayoutAttributes *)layoutAttributesForDecorationViewOfKind:(NSString*)elementKind atIndexPath:(NSIndexPath *)indexPath { if ([elementKind isEqualToString:XHCollectionViewSectionBackground]) { return [self.decorationViewAttrs objectAtIndex:indexPath.section]; } return [super layoutAttributesForDecorationViewOfKind:elementKind atIndexPath:indexPath]; } @end