Swift: download image from Internet and cache them doesn't work properly. need suggestions

Question

I am new to swift, and I am building an App that download images from the Internet and display in a UICollectionView, I can achieve this function successfully, however, it seems like overtime you scroll the screen, it downloaded the images again and again, which may causes a lot of data of the users. I found that I can use cache to solve this problem, but it didn't work, I think I did everything right, but seems the data didn't stored in the cache. here is the code, anyone can help me solve this? thanks

var imageCache = [String: UIImage]()
class ViewController: UIViewController {
override func viewDidLoad() {
    super.viewDidLoad()
    // Do any additional setup after loading the view, typically from a nib.
}

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

extension ViewController: UICollectionViewDataSource {

func numberOfSectionsInCollectionView(collectionView: UICollectionView) -> Int {
    return 1
}

func collectionView(collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
    return 5
}

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

    Alamofire.request(.GET, "http://localhost:8888/wordpress/wp-json/wp/v2/posts").responseJSON { (response) in
        if let jsonFile = response.result.value {

            var arrayOfPosts = [Posts]()

            for post in jsonFile as! NSArray {

                let postObj = Posts()
                postObj.postThumbnailUrlString = post.valueForKeyPath("featured_image_thumbnail_url") as! String

                arrayOfPosts.append(postObj)
            }


            let imageUrlString: String = arrayOfPosts[indexPath.row].postThumbnailUrlString

            let imageUrl: NSURL = NSURL(string: imageUrlString)!







            if let imageFromCache = imageCache[imageUrlString] {

                cell.imageView.image = imageFromCache
                return
            }

            let request = NSURLRequest(URL: imageUrl)
            let session = NSURLSession.sharedSession()
            let dataTask = session.dataTaskWithRequest(request) { (data, response, error) in

                let imageToCache = UIImage(data: data!)

                imageCache[imageUrlString] = imageToCache

                dispatch_async(dispatch_get_main_queue(), {



                    cell.imageView.image = imageToCache
                })

            }

            dataTask.resume()



        }

    }


   return cell

}}

I also have a class of Posts and a customized collectionviewcell. this stocked me a long time, really hope anyone can help me solve this problem. thanks!!

Answer 1

Use AlamofireImage

Swift 2.2 or 2.3

Add --> pod 'AlamofireImage', '~> 2.5' in your pod file.

Swift 3.1

Add --> pod 'AlamofireImage', '~> 3.3' in your pod file.

Put the following line in your code which will automatically cache the image and download it as well.

override func viewDidLoad() {
     super.viewDidLoad()
     //hit your web service here to get all url's
 }

func collectionView(collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int
{
   return arrImageURL.count
}


func collectionView(collectionView: UICollectionView, cellForItemAtIndexPath indexPath: NSIndexPath) -> UICollectionViewCell
{ 
   cell.imgViewPhoto.af_setImageWithURL(arrImageURL[indexPath.row], placeholderImage: UIImage())
}

Answer 2

Kingfisher is a lightweight and pure Swift implemented library for downloading and caching image from the web. This project is heavily inspired by the popular SDWebImage. And it provides you a chance to use pure Swift alternative in your next app. kingfisher