NSImageView上的NSCollectionViewItem选择，使用CALayer设置图像周围的边框

// 获取NSCollectionViewItem中的NSImageView实例
func collectionView(_ collectionView: NSCollectionView, itemForRepresentedObjectAt indexPath: IndexPath) -> NSCollectionViewItem {
    let item = collectionView.makeItem(withIdentifier: NSUserInterfaceItemIdentifier("CollectionViewItem"), for: indexPath)
    
    if let imageView = item.viewWithTag(1) as? NSImageView {
        // 创建CALayer实例并设置边框属性
        let borderLayer = CALayer()
        borderLayer.borderColor = NSColor.red.cgColor
        borderLayer.borderWidth = 2.0
        borderLayer.cornerRadius = 5.0
        
        // 将CALayer添加到NSImageView的layer中
        imageView.layer = borderLayer
        imageView.wantsLayer = true
    }
    
    return item
}