如何下载网址列表的图像，并在使用Alamofire进行此类操作结束时收到通知？

Question

简单地说，我有一个字符串数组，如：

let images = ["http://www.a.pl/1.jpg", "http://www.a.pl/2.jpg", "http://www.a.pl/3.jpg"]

如何为每个任务创建任务并在完成所有任务后收到通知？

Answer 1

这是一种非常简单的方法，但是：

func didFinishLoadingImages(_ images: [String: UIImage]) {
    print(images)
    PlaygroundPage.current.needsIndefiniteExecution = false
}

let images = ["http://www.a.pl/1.jpg", "http://www.a.pl/2.jpg", "http://www.a.pl/3.jpg"]
var downloadedImages = [String: UIImage]()

let session = URLSession.shared
var downloadsRemaining = images.count
for imagePath in images {
    guard let url = URL(string: imagePath) else {
        downloadsRemaining -= 1
        continue
    }

    session.dataTask(with: url) { (data, _, _) in
        var image: UIImage?
        if let imageData = data {
            // Create the image in the background thread to avoid UI performance issues
            image = UIImage(data: imageData)
        }

        DispatchQueue.main.async {
            downloadedImages[imagePath] = image
            downloadsRemaining -= 1
            if downloadsRemaining == 0 {
                didFinishLoadingImages(downloadedImages)
            }
        }
    }
        .resume()
}

Answer 2

这是一个简单的解决方案：

1. 创建ImageAsset模型和两个别名：

   class ImageAsset {
       var url: String
       var key: String
       var image: UIImage?
       // MARK: - Initialization
       init(url: String, key: String) {
           self.url = url
           self.key = key
       }
   }
   
   typealias ImageAssetHandler = ([ImageAsset]) -> Void
   typealias ImageHandler = (UIImage?) -> Void

2. 创建将接受图像资源数组的函数：

   func downloadImages(assets: [ImageAsset], completion: @escaping ImageAssetHandler) {
       let group = DispatchGroup()
       var newAssets = [ImageAsset]()
       for asset in assets {
           group.enter()
           executor.downloadImage(with: asset) { image in
               asset.image = image
               newAssets.append(asset)
               group.leave()
           }
       }
       group.notify(queue: .main) {
       completion(newAssets)
       }
    }

3. 某些执行程序的方法应如下所示：

   func downloadImage(with asset: ImageAsset, completion: @escaping ImageHandler) {
        Alamofire.request(asset.url).responseImage { response in
            completion(response.result.value)
        }
   }

4. 就这样;）

用法示例：

    let images = [
        "https://i.pinimg.com/originals/d9/07/ec/d907ec461fbfa0f172a1098a0d9e9324.jpg",
        "https://c402277.ssl.cf1.rackcdn.com/photos/906/images/story_full_width/sumatran-tiger-circle_44312149.jpg?1345533208",
        "https://upload.wikimedia.org/wikipedia/commons/4/41/Siberischer_tiger_de_edit02.jpg"]
    network.downloadImages(assets: images.map { ImageAsset(url: $0, key: "hejkey") }) { imageAssets in
        print("completed")
        for a in imageAssets {
            print(a.url)
            print(a.key)
            print(a.image)
            print(a.image?.size)
        }
    }

和控制台中的结果：

completed
https://c402277.ssl.cf1.rackcdn.com/photos/906/images/story_full_width/sumatran-tiger-circle_44312149.jpg?1345533208
hejkey
Optional( size {333.33333333333331, 200} orientation 0 scale 3.000000)
Optional((333.3333333333333, 200.0))
https://upload.wikimedia.org/wikipedia/commons/4/41/Siberischer_tiger_de_edit02.jpg
hejkey
Optional( size {533.33333333333337, 400} orientation 0 scale 3.000000)
Optional((533.3333333333334, 400.0))
https://i.pinimg.com/originals/d9/07/ec/d907ec461fbfa0f172a1098a0d9e9324.jpg
hejkey
Optional( size {853.33333333333337, 640} orientation 0 scale 3.000000)
Optional((853.3333333333334, 640.0))