UI collection view

- (void)viewDidLoad
{
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
    UICollectionViewFlowLayout *layout=[[UICollectionViewFlowLayout alloc] init];
    _collectionView=[[UICollectionView alloc] initWithFrame:CGRectMake(130, 100, 780, 1024) collectionViewLayout:layout];
    _collectionView.autoresizesSubviews= YES;
    [_collectionView setDataSource:self];
    [_collectionView setDelegate:self];
    _collectionView.backgroundColor = [UIColor clearColor];

    [_collectionView registerClass:[UICollectionViewCell class] forCellWithReuseIdentifier:@"cellIdentifier"];

    [self.view addSubview:_collectionView];
    for (int i=0; i<12; i+=2) {
        NSIndexPath *index = [NSIndexPath indexPathForItem:i inSection:0];
        NSIndexPath *index1 = [NSIndexPath indexPathForItem:i+1 inSection:0];
        [_collectionView moveItemAtIndexPath:index toIndexPath:index1];
    }
    for (int i=2; i<12; i+=2) {
        NSIndexPath *index = [NSIndexPath indexPathForItem:i inSection:0];
        NSIndexPath *index1 = [NSIndexPath indexPathForItem:i+1 inSection:0];
        [_collectionView moveItemAtIndexPath:index toIndexPath:index1];
    }


}

- (void)didReceiveMemoryWarning
{
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}
- (NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSection:(NSInteger)section
{
    return 12;
}
-(NSInteger)numberOfSectionsInCollectionView:(UICollectionView *)collectionView
{
    return 1;
}
- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath
{
    UICollectionViewCell *cell=[collectionView dequeueReusableCellWithReuseIdentifier:@"cellIdentifier" forIndexPath:indexPath];

    // code for customising the cell....
    cell.layer.shadowColor = [[UIColor blackColor]CGColor];
    cell.layer.shadowOffset = CGSizeMake(20, 20);
    cell.layer.borderColor = [[UIColor blackColor] CGColor];
    cell.layer.borderWidth =1;
    cell.alpha = 0;



    return cell;
}
-(CGFloat)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout *)collectionViewLayout minimumLineSpacingForSectionAtIndex:(NSInteger)section
{
    return 10;
}
- (CGSize)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout*)collectionViewLayout sizeForItemAtIndexPath:(NSIndexPath *)indexPath
{
    return CGSizeMake(377, 172);
}