ios - 如何在 objective-c 中访问子类中父类 Collection View 的不可见单元格

Question

当我尝试通过子类调用其方法访问父类单元格时它崩溃了，因为只有 collectionview 的可见单元格在队列中，但 cellForItemAtIndexPath 不可见单元格为 nil

这是我的子类的代码:

 func scrollViewDidEndDecelerating(scrollView: UIScrollView) {
    print(self.collectionView.visibleCells())

    for cell in self.collectionView.visibleCells() {
        let collectionViewCell = cell as! PSMediaCell
        let indexPath:NSIndexPath = self.collectionView.indexPathForCell(collectionViewCell)!
        let  imageObj:NSMutableDictionary = imageArr.objectAtIndex(indexPath.item) as! NSMutableDictionary

        // this is calling parent class method
        mediavc.mymethodforback(indexPath, vc: self)

        if imageObj.objectForKey("image_url")?.rangeOfString("mov").length > 0 {
        } else {
            var imageView:UIImageView = collectionViewCell.contentView.viewWithTag(122) as! UIImageView
             imagev = imageView
            let indicatorView:UIActivityIndicatorView = collectionViewCell.contentView.viewWithTag(233) as! UIActivityIndicatorView
            var videoIcon:UIImageView = cell.contentView.viewWithTag(133) as! UIImageView
            var imageOverlay:UIImageView = collectionViewCell.contentView.viewWithTag(234) as! UIImageView
            //var videoIcon:UIImageView = collectionViewCell.contentView.viewWithTag(133) as! UIImageView
            var downloadingFilePath = NSTemporaryDirectory().stringByAppendingPathComponent(imageObj.objectForKey("image_url") as! String)
            let imageN = UIImage(contentsOfFile:downloadingFilePath as String )
            if imageN == nil {
                var downloadRequest = AWSS3TransferManagerDownloadRequest()
                var downloadingFilePath1 = NSTemporaryDirectory().stringByAppendingPathComponent(imageObj.objectForKey("image_url") as! String)
                let downloadingFileURL = NSURL(fileURLWithPath: downloadingFilePath1)
                downloadRequest.bucket = "photosharebucket1"
                downloadRequest.key = imageObj.objectForKey("image_url") as! String
                downloadRequest.downloadingFileURL = downloadingFileURL
                self.download(downloadRequest, ImageObj:imageObj)
                indicatorView.hidden = false
                imageOverlay.hidden  = false
            } else {
                imageView.image = imageN
                indicatorView.hidden = true
                imageOverlay.hidden  = true
                videoIcon.hidden  = true
            }
         }
     }
 }

这是父类的代码:

func mymethodforback(images:NSIndexPath , vc :PSUserSentPhotoDetailsVC) {
    print( images.row)
    print(PSMediaDetailVC.collection.visibleCells().count)
    var cell :PSMediaCollectionViewCell = PSMediaDetailVC.collection.cellForItemAtIndexPath(images) as! PSMediaCollectionViewCell  

    // here i m getting nil because cell for particular indexpath is not visible in parent class
    var imageView:UIImageView = cell.imagevieww as UIImageView
    PSMediaDetailVC.imageve = imageView
    print("xcxcxcxc")
    print( PSMediaDetailVC.imageve)
    print(PSMediaDetailVC.collection)
    print(self)
    self.ysl_addTransitionDelegate(PSMediaDetailVC.collection)
    self.navigationController(PSMediaDetailVC.nvc, animationControllerForOperation: UINavigationControllerOperation.Push, fromViewController: self, toViewController:vc )
    // self.ysl_pushTransitionAnimationWithToViewControllerImagePointY(0, animationDuration: 0.3)        
}

Answer 1

UICollectionView 中的单元格显示来自底层数据模型的信息，它们本身并不存储信息。因此，不需要将当前不在屏幕上的单元格保存在内存中；在需要它们之前调用 cellForItemAtIndexPath 可以很容易地重新创建它们。这使 Collection View 能够重用单元格对象并减少其内存占用。

所有这一切的结果是，您无法通过在 Collection View 上调用 cellForItemAtIndexPath 来获取当前未显示的单元格。