flowLayout.swift

    override func viewDidLoad() {
        super.viewDidLoad()
        //cell and cell spacing
        let itemSpace: CGFloat = 3
        //how many colume in a row
        let columeCount: CGFloat = 3
        //將collectionViewLayout轉型成UICollectionViewFlowLayout(向下捲動)
        let flowLayout = collectionViewLayout as? UICollectionViewFlowLayout
        //計算cell寬度，利用畫面的寬減去要的間距然後再除以要拆分成幾格(這樣可以避開不同的型號的iphone算出對應的寬度
        let width = floor((collectionView.bounds.width-itemSpace*(columeCount-1))/columeCount)
        //設定cell的大小
        flowLayout?.itemSize = CGSize(width: width, height: width)
        //設成zero才會根據itemSize來設定cell大小，不然會被autoLayout條件影響(如果有額外設定cell autoLayout)
        flowLayout?.estimatedItemSize = CGSize.zero
        //定義cell間的間距(左右間距)
        flowLayout?.minimumInteritemSpacing = itemSpace
        //定義colume間的間距(上下間距)
        flowLayout?.minimumLineSpacing = itemSpace
        //定義cell到編框間的間距(上下左右)
//        flowLayout?.sectionInset = UIEdgeInsets(top: 0, left: 5, bottom: 0, right: 5)
    }