#import "ViewController.h"

@interface ViewController ()<UICollectionViewDelegate,UICollectionViewDataSource>
@property (nonatomic, strong) UICollectionView *collectionView;
@end

@implementation ViewController
static NSString * const reuseIdentifier = @"cell";
static CGFloat const minInteritemSpacing = 50;
static CGFloat const minLineSpacing = 20;
static CGFloat const TBInset = 50;
static CGFloat const LRInset = 20;
static CGFloat const horCount = 3.0;
static CGFloat const verCount = horCount;
- (void)viewDidLoad {
    [super viewDidLoad];

    UICollectionViewFlowLayout *flow = [[UICollectionViewFlowLayout alloc]init];
    flow.scrollDirection = UICollectionViewScrollDirectionHorizontal;
    flow.sectionInset = UIEdgeInsetsMake(TBInset, LRInset, TBInset, LRInset);
    flow.minimumInteritemSpacing = minInteritemSpacing;
    flow.minimumLineSpacing = minLineSpacing;
//此处完全根据flow.sectionInset、flow.minimumInteritemSpacing、flow.minimumLineSpacing设置。这里还用到了floor()函数，他的作用是：如果是小数，则求最大的整数但不大于本身. 
    flow.itemSize = CGSizeMake((self.view.frame.size.width - 2 * minLineSpacing - 2 * LRInset)/horCount, floor((self.view.frame.size.height - 2 * minInteritemSpacing - 2 * TBInset)/verCount));
    UICollectionView *collectionView = [[UICollectionView alloc]initWithFrame:self.view.bounds collectionViewLayout:flow];
    collectionView.backgroundColor = [UIColor whiteColor];
    collectionView.pagingEnabled = YES;
    self.collectionView = collectionView;
    collectionView.delegate = self;
    collectionView.dataSource = self;
    [self.view addSubview:collectionView];
    [collectionView registerClass:[UICollectionViewCell class] forCellWithReuseIdentifier:reuseIdentifier];
}

#pragma mark - UICollectionViewDataSource

- (NSInteger)numberOfSectionsInCollectionView:(UICollectionView *)collectionView {
    return 2;
}

- (NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSection:(NSInteger)section {
    return 8;
}

- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath {
    UICollectionViewCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:reuseIdentifier forIndexPath:indexPath];
    UIView *con = [[UIView alloc]initWithFrame:cell.contentView.bounds];
    [cell.contentView addSubview:con];
    
    UILabel *label = [[UILabel alloc]initWithFrame:con.bounds];
    label.backgroundColor = [UIColor greenColor];
    label.textAlignment = NSTextAlignmentCenter;
    label.text = [NSString stringWithFormat:@"%zd",indexPath.item];
    [con addSubview:label];
    return cell;
}

@end

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