首先，在 viewcontroller 上先加个UICollectionViewDelegateFlowLayout
如下：
class TemplateCollectionViewController: UICollectionViewController,UICollectionViewDelegateFlowLayout {

}

实现下面两个layout 的 delegate 方法即可

func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {
    let padding: CGFloat =  20
    let collectionViewSize = collectionView.frame.size.width - 3*padding
    
    return CGSize(width: collectionViewSize/2, height: **)
}

func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, insetForSectionAt section: Int) -> UIEdgeInsets {
    return UIEdgeInsetsMake(0, 20, 0, 20)
}