如何解决UICollectionView bounces无效问题

#import "WJ_VoteViewController.h"
#import "WJ_VoteCollectionViewCell.h"
#import "Masonry.h"


@interface WJ_VoteViewController ()
{
    UICollectionView *mainCollectionView;
}

@property(nonatomic, assign) CGFloat itemWidth;

@end

@implementation WJ_VoteViewController

-(void)viewWillAppear:(BOOL)animated{
    [self.navigationController setNavigationBarHidden:NO animated:animated];
    [UIApplication sharedApplication].statusBarStyle = UIStatusBarStyleLightContent;
    
}

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view.
    self.itemWidth = (IPHONE_WIDTH - 5 * 10) / 4;
    
    [self setNavUI];
    
    [self setBottomUI];
}


//设置导航栏
-(void)setNavUI
{
    UIBarButtonItem *backItem = [[UIBarButtonItem alloc] initWithTitle:@"" style:UIBarButtonItemStylePlain target:nil action:nil];
    [self.navigationItem setBackBarButtonItem:backItem];
    self.title = @"下周菜单投票";
}


//导航栏下面的UI
-(void)setBottomUI
{
    self.view.backgroundColor = [UIColor whiteColor];
    
    //1.初始化layout
    UICollectionViewFlowLayout *layout = [[UICollectionViewFlowLayout alloc] init];
    //设置collectionView滚动方向
    [layout setScrollDirection:UICollectionViewScrollDirectionVertical];
    //设置headerView的尺寸大小
    layout.headerReferenceSize = CGSizeMake(self.view.frame.size.width, 100);
    
    //2.初始化collectionView
    mainCollectionView = [[UICollectionView alloc] initWithFrame:self.view.bounds collectionViewLayout:layout];
    [self.view addSubview:mainCollectionView];
    //大view的背景图
    mainCollectionView.backgroundColor = [UIColor clearColor];
    mainCollectionView.bounces = YES;
    //3.注册collectionViewCell
    //注意，此处的ReuseIdentifier 必须和 cellForItemAtIndexPath 方法中 一致 均为 cellId
    [mainCollectionView registerClass:[WJ_VoteCollectionViewCell class] forCellWithReuseIdentifier:@"cellId"];
    
    //注册headerView  此处的ReuseIdentifier 必须和 cellForItemAtIndexPath 方法中 一致  均为reusableView
    [mainCollectionView registerClass:[UICollectionReusableView class] forSupplementaryViewOfKind:UICollectionElementKindSectionHeader withReuseIdentifier:@"reusableView"];
    
    //4.设置代理
    mainCollectionView.delegate = self;
    mainCollectionView.dataSource = self;

}


#pragma mark collectionView代理方法
//返回section个数
- (NSInteger)numberOfSectionsInCollectionView:(UICollectionView *)collectionView
{
    return 2;
}

//每个section的item个数
- (NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSection:(NSInteger)section
{
    return 12;
}

- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath
{
    
    WJ_VoteCollectionViewCell *cell = (WJ_VoteCollectionViewCell *)[collectionView dequeueReusableCellWithReuseIdentifier:@"cellId" forIndexPath:indexPath];
    
//    cell.botlabel.text = [NSString stringWithFormat:@"{%ld,%ld}",(long)indexPath.section,(long)indexPath.row];
    
    cell.backgroundColor = [UIColor blueColor];
    
    return cell;
}

//设置每个item的尺寸
- (CGSize)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout *)collectionViewLayout sizeForItemAtIndexPath:(NSIndexPath *)indexPath
{
    return CGSizeMake(self.itemWidth, 94);
}

//footer的size
- (CGSize)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout *)collectionViewLayout referenceSizeForFooterInSection:(NSInteger)section
{
    return CGSizeMake(10, 10);
}

//header的size
- (CGSize)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout *)collectionViewLayout referenceSizeForHeaderInSection:(NSInteger)section
{
    return CGSizeMake(10, 10);
}

//设置每个item的UIEdgeInsets
- (UIEdgeInsets)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout *)collectionViewLayout insetForSectionAtIndex:(NSInteger)section
{
    return UIEdgeInsetsMake(10, 10, 10, 10);
}

//设置每个item水平间距
- (CGFloat)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout *)collectionViewLayout minimumInteritemSpacingForSectionAtIndex:(NSInteger)section
{
    return 10;
}


//设置每个item垂直间距
- (CGFloat)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout *)collectionViewLayout minimumLineSpacingForSectionAtIndex:(NSInteger)section
{
    return 5;
}




//点击item方法
- (void)collectionView:(UICollectionView *)collectionView didSelectItemAtIndexPath:(NSIndexPath *)indexPath
{
    WJ_VoteCollectionViewCell *cell = (WJ_VoteCollectionViewCell *)[collectionView cellForItemAtIndexPath:indexPath];
//    NSString *msg = cell.botlabel.text;
//    NSLog(@"%@",msg);
}

#pragma mark - 监听自定导航栏按钮事件
-(void)navBtnClick:(UIButton *)btn{
    [self.navigationController popViewControllerAnimated:YES];
}

collectionView点击bounces.gif

从运行效果上看垂直方向并不能滚动，快快想个解决办法呀！
还好大金哥帮我想了一个办法
mainCollectionView.bounces = YES;换成这句
mainCollectionView.alwaysBounceVertical = YES;

效果出来如图所示：

collectionView点击bounces1.gif