Swift collectionview单元格动画仅在滚动时运行

Question

我正在尝试为列表和网格布局之间的集合视图设置动画。

它在两个布局之间动画很好，但我不知道如何动画一些约束。

例如：

func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "cell", for: indexPath) as! PhoneCollectionViewCell

        if collectionView.collectionViewLayout == listLayout {
            cell.leadingConst.constant = -20
        } else {
            cell.leadingConst.constant = 10
        }

        return cell
    }

这只会在我开始滚动时更改约束，并且屏幕上会出现新项目。我想在更改布局时更改约束。

更改布局的方法：

@IBAction func gridButtonDidTouch(_ sender: AnyObject) {        
        if collectionView.collectionViewLayout == gridLayout {
            // list layout
            UIView.animate(withDuration: 0.1, animations: {
                self.collectionView.collectionViewLayout.invalidateLayout()
                self.collectionView.setCollectionViewLayout(self.listLayout, animated: false)
            })
        } else {
            // grid layout
            UIView.animate(withDuration: 0.1, animations: {
                self.collectionView.collectionViewLayout.invalidateLayout()
                self.collectionView.setCollectionViewLayout(self.gridLayout, animated: false)
            })
        }

    }


override func viewWillTransition(to size: CGSize, with coordinator: UIViewControllerTransitionCoordinator) {
        super.viewWillTransition(to: size, with: coordinator)
        collectionView.collectionViewLayout.invalidateLayout()
    }

class GridLayout: UICollectionViewFlowLayout {

    var numberOfColumns: Int = 3

    init(numberOfColumns: Int) {
        super.init()
        minimumLineSpacing = 1
        minimumInteritemSpacing = 1

        self.numberOfColumns = numberOfColumns
    }

    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }

    override var itemSize: CGSize {
        get {
            if let collectionView = collectionView {
                let itemWidth: CGFloat = (collectionView.frame.width/CGFloat(self.numberOfColumns)) - self.minimumInteritemSpacing
                let itemHeight: CGFloat = 260.0
                return CGSize(width: itemWidth, height: itemHeight)
            }

            // Default fallback
            return CGSize(width: 100, height: 100)
        }
        set {
            super.itemSize = newValue
        }
    }

    override func targetContentOffset(forProposedContentOffset proposedContentOffset: CGPoint) -> CGPoint {
        return proposedContentOffset
    }

}


class ListLayout: UICollectionViewFlowLayout {

    var itemHeight: CGFloat = 180

    init(itemHeight: CGFloat) {
        super.init()
        minimumLineSpacing = 1
        minimumInteritemSpacing = 1

        self.itemHeight = itemHeight
    }

    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }

    override var itemSize: CGSize {
        get {
            if let collectionView = collectionView {
                let itemWidth: CGFloat = collectionView.frame.width
                return CGSize(width: itemWidth, height: self.itemHeight)
            }

            // Default fallback
            return CGSize(width: 100, height: 100)
        }
        set {
            super.itemSize = newValue
        }
    }

    override func targetContentOffset(forProposedContentOffset proposedContentOffset: CGPoint) -> CGPoint {
        return proposedContentOffset
    }

}