UICollectionView查询出队

Question

I have an UICollectionView that makes a grid. Each cell to make the grid has a UIImage in it (created in IB).

I am using reuseable cells to keep the requests down. How can I use this cell and the UIImage ? Is there someway of storing it in an array before it goes away ? I have created a tag but I don't if this will help ? If i created each cell manually then there will be around 100 @IBOutlets in my controller !! Here is my code to get the cells displayed..

Any ideas would be brilliant. I am trying to get the UIImage inside the cell, so I can hide it and also name it before the cell dequeues.

 func collectionView(collectionView: UICollectionView, cellForItemAtIndexPath indexPath: NSIndexPath) -> UICollectionViewCell {
        let cell = collectionView.dequeueReusableCellWithReuseIdentifier("LetterCell", forIndexPath: indexPath) as UICollectionViewCell

        cell.tag = indexPath.row
        return cell
    }

Answer 1

Images take up a surprising amount of memory. As a result, you generally do not want an architecture that requires you to hold all of images (or worse, cells) in memory at one time. You want your collection view cells to be reused and you want to retrieve the images from persistent storage in a just-in-time manner (aka, "lazy" image loading).

To minimize the memory footprint of your app, so your model would generally contain the minimal amount of information, for example just references to those images (such as filenames). Only load the images only when they're really needed by the UI.

For example, let's assume that the images were files in the “Application Support” folder of the device, then you might have an array of filenames (called imageNames in my example below), and you might do something like:

var imageNames = [String]()   // this is populated elsewhere, perhaps `viewDidLoad`

func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
    let cell = collectionView.dequeueReusableCell(withReuseIdentifier: reuseIdentifier, for: indexPath) as! CustomCell

    let imageURL = try! FileManager.default
        .url(for: .applicationSupportDirectory, in: .userDomainMask, appropriateFor: nil, create: true)
        .appendingPathComponent(imageNames[indexPath.item])

    cell.imageView.image = UIImage(contentsOfFile: imageURL.path)

    return cell
}

---

If you really wanted to hold those images in memory (for example, for even smoother response time), you might use a NSCache , but make sure this cache empties itself upon receiving memory pressure. For example:

var imageCache = ImageCache()

func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
    let cell = collectionView.dequeueReusableCell(withReuseIdentifier: reuseIdentifier, for: indexPath) as! CustomCell

    let imageName = imageNames[indexPath.item]
    if let image = imageCache[imageName] {
        cell.imageView.image = image
    } else {
        let imageURL = try! FileManager.default
            .url(for: .applicationSupportDirectory, in: .userDomainMask, appropriateFor: nil, create: true)
            .appendingPathComponent(imageName)
        let image = UIImage(contentsOfFile: imageURL.path)
        imageCache[imageName] = image
        cell.imageView.image = image
    }

    return cell
}

Where

class ImageCache: NSCache<NSString, UIImage> {

    var observer: NSObjectProtocol?

    override init() {
        super.init()

        // empty queue upon memory pressure

        observer = NotificationCenter.default.addObserver(forName: UIApplication.didReceiveMemoryWarningNotification, object: nil, queue: .main) { [unowned self] notification in
            self.removeAllObjects()
        }
    }

    deinit {
        NotificationCenter.default.removeObserver(observer!)
    }

    subscript(key: String) -> UIImage? {
        get { return object(forKey: key as NSString) }
        set { setValue(newValue, forKey: key) }
    }
}

There are other optimizations that one might consider, too. For example, if these images are large, you might make sure that you're loading the image view with images resized to something optimal for the collection view cell. But hopefully this illustrates some of the basic concepts when dealing with images in a UICollectionView .