防止在Swift中的可重用单元格上重新加载

Question

I am new to Swift language and I am facing a problem...

In the "My profile" tableview, I query (to Parse) for all currentUser()'s posts with an includeKey of array of pointers that point to tagged objects from an other Class.

For each post, I have to set different colors for each object that is tagged according to its "type" (using AttributedStrings to concatenated strings)

I discovered that cells are reloading each time I scroll down but the thing is the code is becoming quite heavy and slow.

Do you have an idea how I could fix this or how I could prevent cells from being reused ?

Thanks a lot.

This is my code from the cellForRowAtIndexPath() :

    cell.selectionStyle = UITableViewCellSelectionStyle.None

    // pour activer la selection de cellules
    // cell.selectionStyle = UITableViewCellSelectionStyleBlue;(By Default)
    // cell.selectionStyle = UITableViewCellSelectionStyleGray;

    var imageToLoad = self.images[indexPath.row] as PFFile
    var imageCaption = self.imageCaptions[indexPath.row] as String
    var imageDate = self.imageDates[indexPath.row] as String
    var postKooleqts: AnyObject = self.kooleqts[indexPath.row] as AnyObject

            imageToLoad.getDataInBackgroundWithBlock{(imageData, error) -> Void in
        if (error == nil){

            var finalizedImage = UIImage(data:imageData!)
            cell.postImage.image = finalizedImage

        }
    }

    cell.postCaption.text = imageCaption
    cell.postDate.text = imageDate
    cell.nbKooleqts.text = "\(postKooleqts)"

    var x = 0
     var htmlString = ""

        if (linkedPages[indexPath.row].count > 0){
        while x < linkedPages[indexPath.row].count {

            println("indexx : \([indexPath.row])")
            println("stringg : \(htmlString)")

            let pageToAdd = linkedPages[indexPath.row][x]["name"] as! String

            //let test = linkedPages[indexPath.row][x]

            //println("resultat \([indexPath.row]) : \([test])")

            if (linkedPages[indexPath.row][x]["type"] as! String == "obj"){
            stringToAdd = "<span style=\"font-family : HelveticaNeue; font-size : 15;  color: white;\"><span style=\"font-weight: bold; color: #c00b0b\">\(pageToAdd) </span>- -</span>"
            }
            else if (linkedPages[indexPath.row][x]["type"] as! String == "bra"){
            stringToAdd = "<span style=\"font-family : HelveticaNeue; font-size : 15;  color: white;\"><span style=\"font-weight: bold; color: #0099ff\">\(pageToAdd)</span>- -</span>"
            }
            else if (linkedPages[indexPath.row][x]["type"] as! String == "peo"){                let stringToAdd = "<span style=\"font-family : HelveticaNeue; font-size : 15; color: white;\"><span style=\"font-weight: bold; color: #0bdf6a\">\(pageToAdd)</span>- -</span>"
            }
            else if (linkedPages[indexPath.row][x]["type"] as! String == "eve"){
            stringToAdd = "<span style=\"font-family : HelveticaNeue; font-size : 15;  color: white;\"><span style=\"font-weight: bold; color: #cc99ff\">\(pageToAdd)</span>- -</span>"
            }
            else if (linkedPages[indexPath.row][x]["type"] as! String == "pla"){
            stringToAdd = "<span style=\"font-family : HelveticaNeue; font-size : 15;  color: white;\"><span style=\"font-weight: bold; color: #e16818\">\(pageToAdd)</span>- -</span>"
            } else {stringToAdd = ""}

            htmlString = htmlString + "\(stringToAdd)"

            var encodedData = htmlString.dataUsingEncoding(NSUTF8StringEncoding)!
            var attributedOptions = [NSDocumentTypeDocumentAttribute: NSHTMLTextDocumentType]
            let attributedString = NSAttributedString(data: encodedData, options: attributedOptions, documentAttributes: nil, error: nil)

            cell.linkedPages.attributedText = attributedString

            x++

            }

        }else {

            println("empty")

            cell.linkedPages.text = ""

    }

    return cell
}

UPDATE : The linkedPages seems to be the main lag issue as if I remove it, it no longer lags

UPDATE2 : Here is the working code to have the before/after, no lags anymore :

 var x = 0
     var htmlString = ""
    var attributedString = NSMutableAttributedString()

    var attrs:[String : AnyObject]?

        if (linkedPages[indexPath.row].count > 0){

        while x < linkedPages[indexPath.row].count {

            //println("indexx : \([indexPath.row])")
            //println("stringg : \(htmlString)")

            var pageToAdd = linkedPages[indexPath.row][x]["name"] as! String

            attributedString = NSMutableAttributedString(attributedString: attributedString)
            var addComma = NSMutableAttributedString(string: ", ")

            var paraStyle = NSMutableParagraphStyle()
            paraStyle.paragraphSpacing = 15.0

            if (linkedPages[indexPath.row][x]["type"] as! String == "obj"){
                attrs = [NSFontAttributeName : UIFont.boldSystemFontOfSize(15.0), NSForegroundColorAttributeName : UIColor(red: 192.0/255.0, green: 11.0/255.0, blue: 11.0/255.0, alpha: 1.0), NSParagraphStyleAttributeName : paraStyle]
            }
            else if (linkedPages[indexPath.row][x]["type"] as! String == "bra"){
                attrs = [NSFontAttributeName : UIFont.boldSystemFontOfSize(15.0), NSForegroundColorAttributeName : UIColor(red: 12.0/255.0, green: 153.0/255.0, blue: 255.0/255.0, alpha: 1.0), NSParagraphStyleAttributeName : paraStyle]
            }
            else if (linkedPages[indexPath.row][x]["type"] as! String == "peo"){
            attrs = [NSFontAttributeName : UIFont.boldSystemFontOfSize(15.0), NSForegroundColorAttributeName : UIColor(red: 11.0/255.0, green: 223.0/255.0, blue: 106.0/255.0, alpha: 1.0), NSParagraphStyleAttributeName : paraStyle]
            }
            else if (linkedPages[indexPath.row][x]["type"] as! String == "eve"){
            attrs = [NSFontAttributeName : UIFont.boldSystemFontOfSize(15.0), NSForegroundColorAttributeName : UIColor(red: 204.0/255.0, green: 153.0/255.0, blue: 255.0/255.0, alpha: 1.0), NSParagraphStyleAttributeName : paraStyle]
            }
            else if (linkedPages[indexPath.row][x]["type"] as! String == "pla"){
            attrs = [NSFontAttributeName : UIFont.boldSystemFontOfSize(15.0), NSForegroundColorAttributeName : UIColor(red: 225.0/255.0, green: 90.0/255.0, blue: 1.0/255.0, alpha: 1.0), NSParagraphStyleAttributeName : paraStyle]
            }


            var coloredString = NSMutableAttributedString(string:"\(pageToAdd)", attributes:attrs)

            attributedString.appendAttributedString(coloredString)
            attributedString.appendAttributedString(addComa)


            cell.linkedPages.attributedText = attributedString

            x++

            }

        }else {

            println("empty")

            cell.linkedPages.text = ""

    }

            return cell

Answer 1

The likely reason is your call to getData() for the images in each cell. As far as I know there is no way to not reuse cells, as that's exaclty what the method is designed to do. The reason this is getting so slow is that the OS is trying to wait for each image to be loaded in order to display the cell, and doing this will bog it down very quickly. Instead what you need to do is set a default image for the cell, and then call var imageData = imageToLoad.getDataInBackground({}) . This will allow the cells to be returned while scrolling, and then the images will be populated after they've loaded instead of waiting to draw the cell until the image is retrieved.

Answer 2

Why don't you implement an image cache, instead of downloading every time the cell is created, check and see if you haven't already downloaded the image, and if you have use the previously downloaded copy.

You would have to tune the size of your cache to your memory requirements and eject images that are far away from your current position in the table.

You code also has another issue in that if you scroll down really fast it will queue up lots and lots of downloads. Ideally you need to either have a way to cancel downloads for cells that are no longer in view, or to stop the background downloads until the table stops scrolling.