使用组合的 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，苹果没有为如此常见的东西提供原生的解决方案。无论如何，我发现Sundell 的博客非常有趣，并从中汲取了其中的优点来创建我自己的 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()
        }
    }
}

正如您所看到的，发布者提供了一个AnyPublisher<UIImage, Error>. 我不完全确定如何ImageLoader在MyImageView下面显示的内容中使用它：

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UIImage从ImageLoader返回 实例的发布者中提取AnyPublisher<UIImage, Error>？

Answer 1

您必须使用ObservableObjectImageLoader 提供的订阅者。

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)
            }
    }
}