ViewController.m#

//
//  ViewController.m
//  CustomFlowLayout

//

#import "ViewController.h"
#import "TestCollectionViewCell.h"
#import "CustomFlowLayout.h"
@interface ViewController ()<UICollectionViewDataSource,UICollectionViewDelegate>

@end

@implementation ViewController

NSString *identifier = @"cell";

- (void)viewDidLoad {
    [super viewDidLoad];
    //创建布局对象
    CustomFlowLayout *flowLayout = [[CustomFlowLayout alloc] init];
    
    UICollectionView *collectionView = [[UICollectionView alloc] initWithFrame:CGRectMake(0, 20, 414, 200) collectionViewLayout:flowLayout];
    collectionView.delegate = self;
    collectionView.dataSource = self;
    [collectionView registerClass:[TestCollectionViewCell class] forCellWithReuseIdentifier:identifier];
    [self.view addSubview:collectionView];
    
    
    
}




- (NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSection:(NSInteger)section{
    return 20;
}


- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath{
    TestCollectionViewCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:identifier forIndexPath:indexPath];
    
    cell.imgView.image = [UIImage imageNamed:@"10.png" ];
    return cell;
}

- (void)didReceiveMemoryWarning {
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}

@end

TestCollectionViewCell.h#

//
//  TestCollectionViewCell.h
//  CustomFlowLayout
//

//

#import <UIKit/UIKit.h>

@interface TestCollectionViewCell : UICollectionViewCell

@property(nonatomic,strong)UIImageView *imgView;

@end

TestCollectionViewCell.m#

//
//  TestCollectionViewCell.m
//  UI_21_CustomFlowLayout
//

//

#import "TestCollectionViewCell.h"

@implementation TestCollectionViewCell

-(instancetype)initWithFrame:(CGRect)frame{
    if (self = [super initWithFrame:frame]) {
        
        self.imgView = [[UIImageView alloc] initWithFrame:self.contentView.bounds];
        
        [self.contentView addSubview:_imgView];
    }
    
    return self;
}


@end

CustomFlowLayout.m#

//
//  CustomFlowLayout.m
//  CustomFlowLayout
.
//

#import "CustomFlowLayout.h"

#define kItem  100

@implementation CustomFlowLayout


//做好一些准备工作,初始化
//准备布局
-(void)prepareLayout{
    
    //设置大小
    self.itemSize = CGSizeMake(kItem, kItem);
    //设置滚动方向
    self.scrollDirection = UICollectionViewScrollDirectionHorizontal;
    //最小行间距
    self.minimumLineSpacing = 50;
    
    
}

//是否时刻改变并且重新布局
-(BOOL)shouldInvalidateLayoutForBoundsChange:(CGRect)newBounds{
    return YES;
}


//数组里放置的是每一个item设置的属性(例如: frame等等)
-(NSArray<UICollectionViewLayoutAttributes *> *)layoutAttributesForElementsInRect:(CGRect)rect{
    //从父类里获取item放置属性的数组
    NSArray *array = [super layoutAttributesForElementsInRect:rect];
    //获取屏幕中心点的x坐标
    CGFloat centerX = self.collectionView.frame.size.width/2 + self.collectionView.contentOffset.x;
    //遍历
    for (UICollectionViewLayoutAttributes *attrs in array) {
        
        //item的中心点
        CGFloat itemCenterX = attrs.center.x;
        //缩放比例
        CGFloat scale = 1 + 0.5*(1 - ABS(itemCenterX - centerX))/200;
        //
        attrs.transform3D = CATransform3DMakeScale(scale, scale, 1);
    }

    return array;
}

@end