UICollectionViewCell将不会被调用

Question

我试图做一篇不像大象那样吃RAM的UICollectionView篇文章。因此，UICollectionView由包含一个图像的全屏单元格组成。如果出现UICollectionViewCell，则会加载内容。这是代码：

- (void)viewDidLoad {
    UICollectionViewFlowLayout *layout=[[UICollectionViewFlowLayout alloc] init];
    layout.scrollDirection = UICollectionViewScrollDirectionHorizontal;
    layout.itemSize = CGSizeMake(self.view.frame.size.width, self.view.frame.size.height);
    layout.minimumInteritemSpacing = 0;
    layout.minimumLineSpacing = 0;

    _collectionView = [[UICollectionView alloc] initWithFrame:self.view.frame collectionViewLayout:layout];
    [_collectionView setDataSource:self];
    [_collectionView setDelegate:self];
    _collectionView.pagingEnabled = YES;
    [_collectionView registerClass:[UICollectionViewCell class] forCellWithReuseIdentifier:@"cellIdentifier"];
    [_collectionView setBackgroundColor:[UIColor redColor]];

    [self.view addSubview:_collectionView];
}

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

    //add UIImageView here

    //add UITextView with tag 123 here

    return cell;
}

- (void)collectionView:(UICollectionView *)collectionView willDisplayCell:(UICollectionViewCell *)cell forItemAtIndexPath:(NSIndexPath *)indexPath {
    UICollectionViewCell *cell = [_collectionView cellForItemAtIndexPath:indexPath];

    UITextView *textView = [cell viewWithTag:123];

    textView.text = @"Testing this out.";
}

然而，这并没有做任何事情。没有错误，它只是没有做任何事情。我做错了什么？