问题：
在使用UICollectionView时，经常会碰到删除多个cell的问题。问题可归结为：二元数组中删除多个数据。
struct VideoModel {
let path: String
}
var collection = [[VideoModel]]()
删除方法：
func delete(indexPaths: [IndexPath]) {}
简单暴力的方法：新建集合，过滤要删除的
var newCollection = [[VideoModel]]()
for section in (0..<videoCollection.count) {
var videos = videoCollection[section]
if indexPaths.first(where: { $0.section == section }) == nil {
newCollection.append(videos)
continue
}
for row in (0..<videos.videos.count) {
if indexPaths.first(where: { $0.row == row }) != nil { continue }
videos.append(videos[row])
}
}
videoCollection = newCollection
占位法：利用option特性，将删除的部分设置为nil，过滤
var collection = [[VideoModel?]?]()
indexPaths.forEach { (indexPath) in
collection[indexPath.section]?[indexPath.item] = nil
}
collection = collection.map { (items) -> [VideoModel?]? in
items?.filter { $0 != nil }
}
排序法：删除由大到小
indexPaths.sorted { $0 > $1 }.forEach { collection[$0.section].remove(at: $0.item) }
