使用 combine's Publisher 在 SwiftUI Image 中从远程 URL 异步加载图像

Question

我一直在寻找从远程服务器图像 URL 异步加载图像的良好解决方案。网上有很多解决方案。遗憾的是，Apple 没有为如此常见的东西提供原生服务。不管怎样，我发现Sundell's blog 真的很有趣，并从中汲取了一些好处，创建了我自己的 ImageLoader，如下所示：

import Combine

class ImageLoader {

    private let urlSession: URLSession
    private let cache: NSCache<NSURL, UIImage>

    init(urlSession: URLSession = .shared,
         cache: NSCache<NSURL, UIImage> = .init()) {
        self.urlSession = urlSession
        self.cache = cache
    }

    func publisher(for url: URL) -> AnyPublisher<UIImage, Error> {
        if let image = cache.object(forKey: url as NSURL) {
            return Just(image)
                .setFailureType(to: Error.self)
                .receive(on: DispatchQueue.main)
                .eraseToAnyPublisher()
        } else {
            return urlSession
                .dataTaskPublisher(for: url)
                .map(\.data)
                .tryMap { data in
                    guard let image = UIImage(data: data) else {
                        throw URLError(.badServerResponse, userInfo: [
                            NSURLErrorFailingURLErrorKey: url
                        ])
                    }
                    return image
                }
                .receive(on: DispatchQueue.main)
                .handleEvents(receiveOutput: { [cache] image in
                    cache.setObject(image, forKey: url as NSURL)
                })
                .eraseToAnyPublisher()
        }
    }
}

如您所见，发布者提供了AnyPublisher<UIImage, Error> 的实例。我不完全确定如何在我的MyImageView 中使用这个ImageLoader，如下所示：

struct MyImageView: View {

    var url: URL
    var imageLoader = ImageLoader()

    @State private var image = #imageLiteral(resourceName: "placeholder")

    var body: some View {
        Image(uiImage: image)
            .onAppear {
                let cancellable = imageLoader.publisher(for: url).sink(receiveCompletion: { failure in
                    print(failure) // doesn't print
                }, receiveValue: { image in
                    self.image = image // not getting executed
                })
                cancellable.cancel() // tried with and without this line.
            }
    }
}

如何从返回AnyPublisher<UIImage, Error> 实例的ImageLoader 发布者中提取UIImage？

Answer 1

您必须使用ObservableObject 订阅 ImageLoader 提供的发布者。

class ImageProvider: ObservableObject {
    @Published var image = UIImage(named: "icHamburger")!
    private var cancellable: AnyCancellable?
    private let imageLoader = ImageLoader()

    func loadImage(url: URL) {
        self.cancellable = imageLoader.publisher(for: url)
            .sink(receiveCompletion: { failure in
            print(failure)
        }, receiveValue: { image in
            self.image = image
        })
    }
}

struct MyImageView: View {
    var url: URL
    @StateObject var viewModel = ImageProvider()
    var body: some View {
        Image(uiImage: viewModel.image)
            .onAppear {
                viewModel.loadImage(url: url)
            }
    }
}

Answer 2

因为我不想对一个简单的ImageLoader 进行太多隔离，所以我让它符合ObservableObject。所以，我只是在修改@sElanthiaiyan 提供的答案。此外，通过更多研究，我发现发布者需要在需要时存储，并在不再使用时释放。这是修改后的代码

图像加载器：

class ImageLoader: ObservableObject {

    @Published var image: UIImage
    private var bag = Set<AnyCancellable>()
    //...

    init(placeholder: UIImage = UIImage(),
         urlSession: URLSession = .shared,
         cache: NSCache<NSURL, UIImage> = .init()) {
        self.image = placeholder
        //...
    }

    //...

    func load(from url: URL) {
        publisher(for: url)
            .sink(receiveCompletion: { _ in })
            { image in
                self.image = image
            }
            .store(in: &bag)
    }
}

我的图像视图：

struct MyImageView: View {

    var url: URL
    @StateObject var imageLoader = ImageLoader(placeholder: UIImage(named: "placeholder")!) // or @ObservedObject if iOS 13 support is required

    var body: some View {
        Image(uiImage: imageLoader.image)
            .onAppear {
                imageLoader.load(from: url)
            }
    }
}

Answer 3

iOS 15+

struct ContentView: View {
    var body: some View {
        if #available(iOS 15.0, *) {
            AsyncImage(url: URL(string: "https://unsplash.com/photos/_ce9iqvtF90/download?force=true"), transaction: .init(animation: .spring())) { phase in
                         switch phase {
                         case .empty:
                             Color.green
                             .opacity(0.2)
                             .transition(.opacity.combined(with: .scale))
                         case .success(let image):
                           image
                             .resizable()
                             .aspectRatio(contentMode: .fill)
                             .transition(.opacity.combined(with: .scale))
                         case .failure(let error):
                             Color.red
                         @unknown default:
                             Color.yellow
                         }
                       }
                       .frame(width: 400, height: 266)
                       .mask(RoundedRectangle(cornerRadius: 16))
        } else {
            // Fallback on earlier versions
        }

    }
}