Swift sizeForItemAt失效不起做用 ，自定义Layout计算cell 高度方法不调用

1.重写layout 自动计算cell 宽高 继承UICollectionViewFlowLayout

// 主要计算cell 宽高的方法
    override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
        super.layoutAttributesForElements(in: rect)
        let array: Array = super.layoutAttributesForElements(in: rect) ?? NSArray.init() as! [UICollectionViewLayoutAttributes]
            for idx in 1 ..< array.count {
                let attribute: UICollectionViewLayoutAttributes = array[idx]
                let attribute2: UICollectionViewLayoutAttributes = array[idx - 1]

                if (attribute.indexPath.section == attribute2.indexPath.section) && attribute.frame.origin.x != 0 {
                    let maximumSpacing: Int = 10
                    let orgin: CGFloat = attribute2.frame.maxX

                    let leftX = orgin + CGFloat(maximumSpacing) + attribute.frame.size.width
                    let rightX = self.collectionView!.frame.size.width - 10

                    if leftX <= rightX {
                        var frame: CGRect = attribute.frame
                        frame.origin.x = orgin + CGFloat(maximumSpacing)
                        attribute.frame = frame
                    } else {
                        var frame: CGRect = attribute.frame
                        frame.origin.x = CGFloat(maximumSpacing)
                        frame.origin.y = attribute2.frame.maxY + CGFloat(maximumSpacing)
                        attribute.frame = frame
                    }
                }
            }
        return array
    }
override func shouldInvalidateLayout(forBoundsChange newBounds: CGRect) -> Bool {
        return true
    }

使用sizeForItemAt 给出cell 的宽高的时候 不生效的缘由是 没有引用**

UICollectionViewDelegateFlowLayout

**
示例：

class ViewController: UIViewController, UICollectionViewDataSource, UICollectionViewDelegate, UICollectionViewDelegateFlowLayout {

便可使用sizeForItemAt 找不到这个方法 点协议进去可查