#import <UIKit/UIKit.h>

@interface ViewController : UIViewController<UICollectionViewDataSource,UICollectionViewDelegateFlowLayout>{

}
@property (strong, nonatomic)UICollectionView *collectionView;

@end

ViewController.m

#import "ViewController.h"

@interface ViewController ()

@end

@implementation ViewController

- (void)viewDidLoad
{
  [super viewDidLoad];
  
  //确定是水平滚动，还是垂直滚动
  UICollectionViewFlowLayout *flowLayout=[[UICollectionViewFlowLayout alloc] init];
  [flowLayout setScrollDirection:UICollectionViewScrollDirectionVertical];
  
  self.collectionView=[[UICollectionView alloc] initWithFrame:CGRectMake(0, 64, 320, 200) collectionViewLayout:flowLayout];
  self.collectionView.dataSource=self;
  self.collectionView.delegate=self;
  [self.collectionView setBackgroundColor:[UIColor clearColor]];
  
  //注册Cell，必须要有
  [self.collectionView registerClass:[UICollectionViewCell class] forCellWithReuseIdentifier:@"UICollectionViewCell"];
  
  [self.view addSubview:self.collectionView];
}


#pragma mark -- UICollectionViewDataSource

//定义展示的UICollectionViewCell的个数
-(NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSection:(NSInteger)section
{
  return 10;
}

//定义展示的Section的个数
-(NSInteger)numberOfSectionsInCollectionView:(UICollectionView *)collectionView
{
  return 2;
}

//每个UICollectionView展示的内容
-(UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath
{
  static NSString * CellIdentifier = @"UICollectionViewCell";
  UICollectionViewCell * cell = [collectionView dequeueReusableCellWithReuseIdentifier:CellIdentifier forIndexPath:indexPath];
  
  cell.backgroundColor = [UIColor colorWithRed:((10 * indexPath.row) / 255.0) green:((20 * indexPath.row)/255.0) blue:((30 * indexPath.row)/255.0) alpha:1.0f];
  UILabel *label = [[UILabel alloc] initWithFrame:CGRectMake(0, 0, 20, 20)];
  label.textColor = [UIColor redColor];
  label.text = [NSString stringWithFormat:@"%d",indexPath.row];
 
  for (id subView in cell.contentView.subviews) {
    [subView removeFromSuperview];
  }
  [cell.contentView addSubview:label];
  return cell;
}

#pragma mark --UICollectionViewDelegateFlowLayout

//定义每个Item 的大小
- (CGSize)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout*)collectionViewLayout sizeForItemAtIndexPath:(NSIndexPath *)indexPath
{
  return CGSizeMake(60, 60);
}

//定义每个UICollectionView 的 margin
-(UIEdgeInsets)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout *)collectionViewLayout insetForSectionAtIndex:(NSInteger)section
{
  return UIEdgeInsetsMake(5, 5, 5, 5);
}

#pragma mark --UICollectionViewDelegate

//UICollectionView被选中时调用的方法
-(void)collectionView:(UICollectionView *)collectionView didSelectItemAtIndexPath:(NSIndexPath *)indexPath
{
  UICollectionViewCell * cell = (UICollectionViewCell *)[collectionView cellForItemAtIndexPath:indexPath];
  //临时改变个颜色，看好，只是临时改变的。如果要永久改变，可以先改数据源，然后在cellForItemAtIndexPath中控制。（和UITableView差不多吧！O(∩_∩)O~）
  cell.backgroundColor = [UIColor greenColor];
  NSLog(@"item======%d",indexPath.item);
  NSLog(@"row=======%d",indexPath.row);
  NSLog(@"section===%d",indexPath.section);
}

//返回这个UICollectionView是否可以被选择
-(BOOL)collectionView:(UICollectionView *)collectionView shouldSelectItemAtIndexPath:(NSIndexPath *)indexPath
{
  return YES;
}

@end