UICollectionView在单元格中显示错误的图像

Question

I am building a UITableView that is going to have cells with different layouts in them. The cell I am having issues with has a UICollectionView embedded in it that is generated from an API.

The category name and id populate in the cell correctly, but the images in the UICollectionView do not. The images load, but they are not the right ones for that category. Screen capture of how the collection is loading currently

Some of the things I've tried:

• Hard-coding the ids for each one of the categories instead of dynamically generating them. When I do this, the correct images load (sometimes but not always) ... and if they do load correctly, when I scroll the images change to wrong ones

• The prepareForReuse() function ... I'm not exactly sure where I would put it and what I would reset in it (I have code I believe already kind of nils the image out [code included below])

I have spent a few hours trying to figure this out, but I am stuck ... any suggestions would be appreciated.

My View Controller:

class EcardsViewController: BaseViewController {

@IBOutlet weak var categoryTable: UITableView!

var categories = [CategoryItem]()

override func viewDidLoad() {
    super.viewDidLoad()

    // Do any additional setup after loading the view.
    self.categoryTable.dataSource! = self
    self.categoryTable.delegate! = self

    DispatchQueue.main.async {
        let jsonUrlString = "https://*********/******/category"
        guard let url = URL(string: jsonUrlString) else { return }

        URLSession.shared.dataTask(with: url) { (data, response, err) in
            guard let data = data else { return }

            if err == nil {
                do {
                    let decoder = JSONDecoder()
                    let ecardcategory = try decoder.decode(Category.self, from: data)
                    self.categories = ecardcategory.category
                    self.categories.sort(by: {$0.title < $1.title})
                    self.categories = self.categories.filter{$0.isFeatured}
                } catch let err {
                    print("Err", err)
                }

                DispatchQueue.main.async {
                    self.categoryTable.reloadData()
                }
            }
        }.resume()
    }
}

override func didReceiveMemoryWarning() {
    super.didReceiveMemoryWarning()
    // Dispose of any resources that can be recreated.
}

}

extension EcardsViewController: UITableViewDelegate, UITableViewDataSource {
    func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
        return categories.count
    }

    func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
            let cell = tableView.dequeueReusableCell(withIdentifier: "EcardsCategoriesTableViewCell", for: indexPath) as! EcardsCategoriesTableViewCell
            cell.categoryName.text = ("\(categories[indexPath.row].title)**\(categories[indexPath.row].id)")
            cell.ecardCatId = String(categories[indexPath.row].id)
            return cell

    }

}

My Table Cell:

class EcardsCategoriesTableViewCell: UITableViewCell {

@IBOutlet weak var categoryName: UILabel!
@IBOutlet weak var thisEcardCollection: UICollectionView!


var ecardCatId = ""
var theseEcards = [Content]()
let imageCache = NSCache<NSString,AnyObject>()

override func awakeFromNib() {
    super.awakeFromNib()

    // Initialization code
    self.thisEcardCollection.dataSource! = self
    self.thisEcardCollection.delegate! = self

    DispatchQueue.main.async {
        let jsonUrlString = "https://**********/*******/content?category=\(self.ecardCatId)"
        guard let url = URL(string: jsonUrlString) else { return }
        URLSession.shared.dataTask(with: url) { (data, response, err) in
            guard let data = data else { return }

            if err == nil {
                do {
                    let decoder = JSONDecoder()
                    let ecards = try decoder.decode(Ecards.self, from: data)
                    self.theseEcards = ecards.content
                    self.theseEcards = self.theseEcards.filter{$0.isActive}

                } catch let err {
                    print("Err", err)
                }

                DispatchQueue.main.async {
                    self.thisEcardCollection.reloadData()
                }
            }
        }.resume()
    }
}

override func setSelected(_ selected: Bool, animated: Bool) {
    super.setSelected(selected, animated: animated)

    // Configure the view for the selected state

}
}

extension EcardsCategoriesTableViewCell: UICollectionViewDelegate, UICollectionViewDataSource {

func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
    return theseEcards.count > 7 ? 7 : theseEcards.count
}

func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
    let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "EcardCategoriesCollectionViewCell", for: indexPath) as! EcardCategoriesCollectionViewCell
    cell.ecardImage.contentMode = .scaleAspectFill
    let ecardImageLink = theseEcards[indexPath.row].thumbSSL
    cell.ecardImage.downloadedFrom(link: ecardImageLink)
    return cell
}

}

Collection View Cell:

class EcardCategoriesCollectionViewCell: UICollectionViewCell {
    @IBOutlet weak var ecardImage: UIImageView!
}

Extension to "download" image:

extension UIImageView {

func downloadedFromReset(url: URL, contentMode mode: UIViewContentMode = .scaleAspectFit, thisurl: String) {
    contentMode = mode
    self.image = nil
    // check cache
    if let cachedImage = ImageCache.shared.image(forKey: thisurl) {
        self.image = cachedImage
        return
    }

    URLSession.shared.dataTask(with: url) { data, response, error in
        guard
            let httpURLResponse = response as? HTTPURLResponse, httpURLResponse.statusCode == 200,
            let mimeType = response?.mimeType, mimeType.hasPrefix("image"),
            let data = data, error == nil,
            let image = UIImage(data: data)
            else { return }
        ImageCache.shared.save(image: image, forKey: thisurl)
        DispatchQueue.main.async() {
            self.image = image
        }
        }.resume()
}
func downloadedFrom(link: String, contentMode mode: UIViewContentMode = .scaleAspectFit) {
    guard let url = URL(string: link) else { return }
    downloadedFromReset(url: url, contentMode: mode, thisurl: link)
}
}

Answer 1

Both UICollectionViewCell and UITableViewCell are reused. As one scrolls off the top of the screen, it is reinserted below the visible cells as the next cell that will appear on screen. The cells retain any data that they have during this dequeuing/requeuing process. prepareForReuse exists to give you a point to reset the view to default values and to clear any data from the last time it was displayed. This is especially important when working with asynchronous processes, such as network calls, as they can outlive the amount of time that a cell is displayed. Additionally, you're doing a lot of non-setup work in awakeFromNib . This method is not called every time a cell is displayed, it is only called the FIRST time a cell is displayed. If that cell goes off screen and is reused, awakeFromNib is not called. This is likely a big reason that your collection views have the wrong data, they're never making their network request when they appear on screen.

EcardsCategoriesTableViewCell:

prepareForReuse should be implemented. A few things need to occur in this method:

• theseEcards should be nilled. When a table view scrolls off screen, you want to get rid of the collection view data or else the next time that cell is displayed, it will show the collection view data potentially for the wrong cell.
• You should keep a reference to the dataTask that runs in awakeFromNib and then call cancel on this dataTask in prepareForReuse. Without doing this, the cell can display, disappear, then get reused before the dataTask completes. If that is the case, it may replace the intended values with the values from the previous dataTask (the one that was supposed to run on the cell that was scrolled off screen).

Additionally, the network call needs to be moved out of awakeFromNib :

• You are only ever making the network call in awakeFromNib . This method only gets called the first time a cell is created. When you reuse a cell, it is not called. This method should be used to do any additional setup of views from the nib, but is not your main entry point in adding data to a cell. I would add a method on your cell that lets you set the category id. This will make the network request. It will look something like this:

   func setCategoryId(_ categoryId: String) { DispatchQueue.main.async { let jsonUrlString = "https://**********/*******/content?category=\\(categoryId)" guard let url = URL(string: jsonUrlString) else { return } URLSession.shared.dataTask(with: url) { (data, response, err) in guard let data = data else { return } if err == nil { do { let decoder = JSONDecoder() let ecards = try decoder.decode(Ecards.self, from: data) self.theseEcards = ecards.content self.theseEcards = self.theseEcards.filter{$0.isActive} } catch let err { print("Err", err) } DispatchQueue.main.async { self.thisEcardCollection.reloadData() } } }.resume() } } 

This will be called in the cellForRowAt dataSource method in EcardsViewController .

EcardCategoriesCollectionViewCell :

This cell has similar issues. You are setting images asynchronously, but are not clearing the images and cancelling the network requests when the cell is going to be reused. prepareForReuse should be implemented and the following should occur within it:

• The image on the image view should be cleared or set to a default image.
• The image request should be cancelled. This is going to take some refactoring to accomplish. You need to hold a reference to the dataTask in the collection view cell so that you can cancel it when appropriate.

---

After implementing these changes in the cells, you'll likely notice that the tableview and collection view feel slow. Data isn't instantly available. You'll want to cache the data or preload it some way. That is a bigger discussion than is right for this thread, but it will be your next step.