Async image loading after some actions on this image - image changes to wrong image while scrolling

Question

I know that there is similar question: Async image loading from url inside a UITableView cell - image changes to wrong image while scrolling

But I do not only load images from URL. I have them, perform some actions on them, and then assign them to UIImageView WITHIN UITableViewCell .

This is how I do this:

1. Within cellForRowAtIndexPath:

    cell.configureCellWithComment(comment) 

2. These are methods inside my cell:

    func configureCellWithComment(comment: WLComment) { if let attachmentUrl = comment.attachmentUrl, let fileName = NSURL(string: attachmentUrl)?.lastPathComponent { if !NSFileManager.defaultManager().fileExistsAtPath(comment.destinationPathForAttachment().URLByAppendingPathComponent(fileName).path!) { WLFileClient.sharedClient().downloadFileFrom(attachmentUrl, destinationPath: comment.destinationPathForAttachment(), fileName: fileName, progress: nil, completionBlock: { error in self.attachImageToImageView() }) } else { attachImageToImageView() } } } 

Private method:

private func attachImageToImageView() {

    if let attachmentUrl = comment.attachmentUrl, let fileName = NSURL(string: attachmentUrl)?.lastPathComponent {

        dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), {

            let image = WLFileClient.sharedClient().previewImageForFileAtUrl(self.comment.destinationPathForAttachment().URLByAppendingPathComponent(fileName))

            dispatch_async(dispatch_get_main_queue(), {
                self.previewAttachmentButton.enabled = true
                self.attachmentPreviewImageView.image = image
            });
        });
    }
}

What should I do to fix this?

Answer 1

your downloadFileFrom is an asyc method. so by the time this task completes, the cell may have been reused for another row. so before you call self.attachImageToImageView() , check if the current cell is serving the same row as it was when the method started.

in cellForRowAtIndexPath , set the tag of the cell to the current row number.

cell.tag = indexPath.row

Now in your configureCellWithComment method, first store the tag into a variable and then check if the variable is still the same after the async task:

var tagCache:int = self.tag;

WLFileClient.sharedClient().downloadFileFrom(attachmentUrl, destinationPath: comment.destinationPathForAttachment(), fileName: fileName, progress: nil, completionBlock: {

    if(tagCache==self.tag)
      self.attachImageToImageView()
})

PS: I'm more used to Objective-C, not swift, so forgive syntax mistakes if any.