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我center的collectionview单元的viewWIllTransition功能如下

var center = CGPoint()
  override func viewWillTransition(to size: CGSize, with coordinator: UIViewControllerTransitionCoordinator) {
    super.viewWillTransition(to: size, with: coordinator)
    coordinator.animate(alongsideTransition: { (context) -> Void in
    if (UIDevice.current.orientation == UIDeviceOrientation.portrait) {
    if let indexPath = self.categoryCV.indexPathsForSelectedItems?.first
    {
        print("Portrait")
        let attributes: UICollectionViewLayoutAttributes? = self.categoryCV.layoutAttributesForItem(at: indexPath)
        self.center = (attributes?.center)!
        print("Center: \((attributes?.center)!)")
    }
        }
   else
        {
    if let indexPath = self.categoryCV.indexPathsForSelectedItems?.first
    {
        print("Landscape")
        let attributes: UICollectionViewLayoutAttributes? = self.categoryCV.layoutAttributesForItem(at: indexPath)
        self.center = (attributes?.center)!
        print("Center: \((attributes?.center)!)")
      }
        }
        })}

我知道在设备旋转之前，上述内容不会得到实施。center但我喜欢didSelectItemAt indexpath 传递 transition.start = self.center

func collectionView(_ collectionView: UICollectionView, didSelectItemAt indexPath: IndexPath) {

   let detailViewController = self.storyboard?.instantiateViewController(withIdentifier: "TVC") as? ThirdViewController
// other code
    transition.start = self.center
    print("outside center: \(self.center)")

}

这是因为我希望过渡从单元格中心开始，但在旋转设备之前不会调用 viewWillTransition 函数。那么有没有一种方法可以调用它viewWillTransition，didSelectItemAt indexpath以便它何时被调用cell取决于selected它所orientation在的位置？

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