https://www.cnblogs.com/zyl910/archive/2013/06/07/objcdoc.html
https://blog.csdn.net/dolacmeng/article/details/45588293

https://onevcat.com/2012/08/advanced-collection-view/

//
// EOCCollectionViewLayout.m
// CollectionCustomLayoutDemo_01
//
// Created by yong on 2018/5/9.
// Copyright © 2018年 yong. All rights reserved.
//

import "EOCCollectionViewLayout.h"

@interface EOCCollectionViewLayout ()

@end

/** 有效距离:当item的中间x距离屏幕的中间x在HMActiveDistance以内,才会开始放大, 其它情况都是缩小 */
static CGFloat const ActiveDistance = 1024;

/** 缩放因素: 值越大, item就会越大 */
static CGFloat const ScaleFactor = 0.4;

@implementation EOCCollectionViewLayout

• (instancetype)init
  {
  self = [super init];
  if (self) {
  self.minimumLineSpacing = -140.0;//cell的最小间隔
  self.scrollDirection = UICollectionViewScrollDirectionHorizontal;
  }
  return self;
  }

//- (void)prepareLayout {
// [super prepareLayout];
//}

//- (CGSize)collectionViewContentSize {
//}

• (NSArray<UICollectionViewLayoutAttributes *> *)layoutAttributesForElementsInRect:(CGRect)rect {
  NSArray *array = [super layoutAttributesForElementsInRect:rect];
  CGRect visibleRect;
  visibleRect.origin = self.collectionView.contentOffset;
  visibleRect.size = self.collectionView.bounds.size;

  //遍历所有布局属性
  for (UICollectionViewLayoutAttributes* attributes in array) {
  //如果cell在屏幕上则进行缩放
  if (CGRectIntersectsRect(attributes.frame, rect)) {
  CGFloat distance = CGRectGetMidX(visibleRect) - attributes.center.x;//距离中点的距离
  CGFloat normalizedDistance = distance / ActiveDistance;
  if (ABS(distance) < ActiveDistance) {
  CGFloat zoom = 1 + ScaleFactor*(1 - ABS(normalizedDistance));
  attributes.transform3D = CATransform3DMakeScale(zoom, zoom, 1.0);
  attributes.zIndex = (1 - ABS(distance));
  }
  }
  }
  return array;
  }

//- (UICollectionViewLayoutAttributes *)layoutAttributesForItemAtIndexPath:(NSIndexPath *)indexPath {
// UICollectionViewLayoutAttributes *attributes = [UICollectionViewLayoutAttributes layoutAttributesForCellWithIndexPath:indexPath];
//
// return attributes;
//}

• (CGPoint)targetContentOffsetForProposedContentOffset:(CGPoint)proposedContentOffset withScrollingVelocity:(CGPoint)velocity {
  //proposedContentOffset是没有设置对齐时本应该停下的位置（collectionView落在屏幕左上角的点坐标）
  CGFloat offsetAdjustment = MAXFLOAT;//初始化调整距离为无限大
  CGFloat horizontalCenter = proposedContentOffset.x + (CGRectGetWidth(self.collectionView.bounds) / 2.0);//collectionView落在屏幕中点的x坐标
  CGRect targetRect = CGRectMake(proposedContentOffset.x, 0.0, self.collectionView.bounds.size.width, self.collectionView.bounds.size.height);//collectionView落在屏幕的大小
  NSArray* array = [super layoutAttributesForElementsInRect:targetRect];//获得落在屏幕的所有cell的属性

  //对当前屏幕中的UICollectionViewLayoutAttributes逐个与屏幕中心进行比较，找出最接近中心的一个
  for (UICollectionViewLayoutAttributes* layoutAttributes in array) {
  CGFloat itemHorizontalCenter = layoutAttributes.center.x;
  if (ABS(itemHorizontalCenter - horizontalCenter) < ABS(offsetAdjustment)) {
  offsetAdjustment = itemHorizontalCenter - horizontalCenter;
  }
  }
  //调整
  return CGPointMake(proposedContentOffset.x + offsetAdjustment, proposedContentOffset.y);
  }

/**

• 只要显示的边界发生改变就重新布局:
  内部会重新调用prepareLayout和layoutAttributesForElementsInRect方法获得所有cell的布局属性
  */

• (BOOL)shouldInvalidateLayoutForBoundsChange:(CGRect)newBounds
  {
  return YES;
  }

@end