Although `UICollectionViewCell`s is displayed, `[collectionView visibleCells]` returns nil.

```
- (void)viewDidLoad{
    self.collectionView = [[UICollectionView alloc]initWithFrame:CGRectMake(0, 0, 320, 568) collectionViewLayout:flowLayout];
    self.collectionView.delegate = self;
    self.collectionView.dataSource = self;
    [self.collectionView registerClass:[CustomCell class] forCellWithReuseIdentifier:@"Cell"];
    [self.view addSubview:self.collectionView];

    NSLog(@"%d", [[self.collectionView visibleCells] count]);
}

- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath
{
    CustomCell* cell = [collectionView dequeueReusableCellWithReuseIdentifier:@"Cell" forIndexPath:indexPath];
    return cell;
}

```

Do you have any idea?