Swift 如何模拟可以立即返回和或与Delivercawait同步返回的CompletionButtons风格代码

Swift并发让我们显式地标记旨在从特定全局参与者调用的函数和类型，并且编译器将保证它们始终与该参与者隔离.在大多数情况下，这足以提供流畅的UI体验并减少不必要的演员 skip .因此，我们可以将该函数和图像缓存重写为一个参与者隔离的类:

@MainActor
final class ImageDownloader {
    
    enum CacheEntry {
        case inProgress(Task<UIImage?, Never>)
        case complete(UIImage)
    }
    
    private var cache: [URL: CacheEntry] = [:]
    
    func fetchImage(from url: URL) async -> UIImage? {
        if let cached = cache[url] {
            switch cached {
            case .complete(let image):
                return image
            case .inProgress(let task):
                return await task.value
            }
        }
        
        let task = Task {
            await downloadImage(from: url)
        }
        
        cache[url] = .inProgress(task)
        let image = await task.value
        
        if let image {
            cache[url] = .complete(image)
        }
        
        return image
    }
    
    private nonisolated func downloadImage(from url: URL) async -> UIImage? {
        do {
            let response = try await URLSession.shared.data(from: url)
            return UIImage(data: response.0)
        } catch {
            print(error)
            return nil
        }
    }
    
}

请注意缓存如何跟踪下载状态.在问题中的基于回调的版本中，可能会在初始下载进行时启动额外的下载.该技术基于https://developer.apple.com/wwdc21/10133?time=536中的代码(缓存示例在页面上，但不在视频中，但值得观看视频.)

现在，在视图控制器中，当您配置单元格时，您可以从顶级任务调用此方法:

final class ViewController: UIViewController, UICollectionViewDelegate, UICollectionViewDataSource {

    @IBOutlet private weak var collectionView: UICollectionView!
    
    private let items = ListItem.sampleListItems()
    private let imageDownloader = ImageDownloader()
    
    ...
    
    func collectionView(_ collectionView: UICollectionView, 
                        cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        let listItem = items[indexPath.item]
        let imageCell = collectionView.dequeueReusableCell(
            withReuseIdentifier: "ImageCell",
            for: indexPath
        ) as! ImageCell
        let title = listItem.title
        print("Cell dequeued for: \(title)")
        
        // UIViewControllers are always MainActor-isolated, which
        // means this Task is on the MainActor, so there's no need
        // to call `MainActor.run`.
        Task {
            let img = await imageDownloader.fetchImage(from: listItem.url)
            imageCell.configure(text: title, image: img)
            print("Cell configured for item: \(title)")
        }

        print("Returning cell for: \(title)")
        return imageCell
    }
    
}

只有在downloadImage(from:)中调用URL会话时，这才会跳下并返回到MainActor.

它还解决了基于回调的版本中潜在的错误来源:基于回调的方法只有在从主队列调用时才能正常工作，但没有什么可以阻止我们从其他队列调用它.如果从另一个线程调用它，代码可能会因同时访问缓存而崩溃.

尽管cellForRowAt中的任务始终在MainActor上执行，但它可能要在MainActor上执行其他任务之后才会执行.这可能会导致您看到的一些闪烁.如果是这种情况，我们可以在ImageDownloader中添加一些包装代码:

extension ImageDownloader {
    private func cachedImaged(from url: URL) -> UIImage? {
        guard case let .complete(img) = cache[url] else {
            return nil
        }
        return img
    }
    
    func fetchImage(from url: URL, completion: @escaping (UIImage?) -> Void) {
        if let image = cachedImaged(from: url) {
            completion(image)
        } else {
            //No need for explicit MainActor switching, since it's an isolated class.
            Task {
                let img = await fetchImage(from: url)
                completion(img)
            }
        }
    }
}

视图控制器中的一个:

func collectionView(_ collectionView: UICollectionView, 
                    cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
    let listItem = items[indexPath.item]
    let imageCell = collectionView.dequeueReusableCell(
        withReuseIdentifier: "ImageCell",
        for: indexPath
    ) as! ImageCell
    let title = listItem.title
    print("Cell dequeued for: \(title)")
    
    imageDownloader.fetchImage(from: listItem.url) { img in
        imageCell.configure(text: title, image: img)
        print("Cell configured for item: \(title)")
    }
    
    print("Returning cell for: \(title)")
    return imageCell
}

这最终看起来与您的原始版本相似，但通过将ImageDownloader及其缓存隔离到MainActor，我们消除了不当使用导致竞争条件的可能性.