subview 다 지우는 법

for view in self.view.subviews {
    view.removeFromSuperview()
}

CollectionView Register

self.cvMain.register(CollectionViewCell.self, forCellWithReuseIdentifier: "CollectionViewCell")
self.cvMain.delegate = self
self.cvMain.dataSource = self

let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "CollectionViewCell", for: indexPath) as! CollectionViewCell