LiedouYouxuan/YouHuiProject/YouHui/Home/ViewController/Today/View/XLPlainFlowL.../XLPlainFlowL...

//
//  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