SwiftUI中PhotosPicker的loadTransferable无法响应取消，如何实现加载超时？

核心问题 #

你的代码逻辑本身没问题，但loadTransferable(type:)底层的照片加载操作不响应任务取消：当超时任务触发group.cancelAll()后，图片加载任务仍会继续执行，而withTaskGroup必须等待所有添加的任务完成才会退出，导致整个函数卡在withTaskGroup块中，后续的switch代码永远无法执行。

可行解决方案 #

方案1：使用官方Task.withTimeout（iOS 16+/macOS 13+）

苹果在iOS 16后提供了原生超时API，代码最简洁，且能保证超时后立即返回结果（即使底层加载任务未终止，也不会阻塞当前函数）：

func loadImage(_ image: PhotosPickerItem, timeout milliseconds: Int) async throws -> UIImage {
    try await Task.withTimeout(nanoseconds: UInt64(milliseconds) * 1_000_000) {
        guard let imageData = try await image.loadTransferable(type: Data.self),
              let uiImage = UIImage(data: imageData) else {
            throw MyError.imageCouldNotBeLoaded(details: "Image transfer or decoding issues.")
        }
        return uiImage
    } onTimeout: {
        throw MyError.imageCouldNotBeLoaded(details: "Timeout occurred.")
    }
}

方案2：async let + 并行任务（iOS 15+兼容）

通过async let启动并行任务，利用任务优先级实现超时逻辑，同样能保证函数及时返回：

func loadImage(_ image: PhotosPickerItem, timeout milliseconds: Int) async throws -> UIImage {
    // 图片加载任务
    async let loadTask: UIImage = {
        guard let imageData = try await image.loadTransferable(type: Data.self),
              let uiImage = UIImage(data: imageData) else {
            throw MyError.imageCouldNotBeLoaded(details: "Image transfer or decoding issues.")
        }
        return uiImage
    }()
    
    // 超时任务
    async let timeoutTask: Error = {
        try await Task.sleep(nanoseconds: UInt64(milliseconds) * 1_000_000)
        throw MyError.imageCouldNotBeLoaded(details: "Timeout occurred.")
    }()
    
    do {
        // 优先等待加载任务完成，成功则取消超时任务
        let image = try await loadTask
        timeoutTask.cancel()
        return image
    } catch {
        // 加载失败，取消超时任务并抛出错误
        timeoutTask.cancel()
        throw error
    }
}

方案3：使用PHAsset主动控制加载（真正终止任务）

如果需要彻底终止底层加载操作（避免后台资源浪费），可以直接使用Photos框架的PHImageManager，通过请求ID主动取消加载：

import Photos

func loadImage(_ image: PhotosPickerItem, timeout milliseconds: Int) async throws -> UIImage {
    // 从PhotosPickerItem获取PHAsset
    guard let assetID = image.itemIdentifier,
          let asset = PHAsset.fetchAssets(withLocalIdentifiers: [assetID], options: nil).firstObject else {
        throw MyError.imageCouldNotBeLoaded(details: "Failed to get PHAsset.")
    }
    
    let semaphore = DispatchSemaphore(value: 0)
    var result: Result<UIImage, Error>!
    var requestID: PHImageRequestID!
    
    let options = PHImageRequestOptions()
    options.deliveryMode = .highQualityFormat
    options.isNetworkAccessAllowed = true // 允许网络加载高清图
    
    // 发起图片加载请求
    requestID = PHImageManager.default().requestImage(
        for: asset,
        targetSize: PHImageManagerMaximumSize,
        contentMode: .aspectFit,
        options: options
    ) { image, info in
        defer { semaphore.signal() }
        
        if let error = info?[PHImageErrorKey] as? Error {
            result = .failure(error)
            return
        }
        
        result = image.map(.success) ?? .failure(MyError.imageCouldNotBeLoaded(details: "Failed to load image."))
    }
    
    // 启动超时计时器，到点取消加载请求
    DispatchQueue.global().asyncAfter(deadline: .now() + .milliseconds(milliseconds)) {
        PHImageManager.default().cancelImageRequest(requestID)
        result = .failure(MyError.imageCouldNotBeLoaded(details: "Timeout occurred."))
        semaphore.signal()
    }
    
    semaphore.wait()
    
    switch result {
    case .success(let image): return image
    case .failure(let error): throw error
    case .none: throw MyError.imageCouldNotBeLoaded(details: "Unknown error.")
    }
}

注意：此方案需要在Info.plist中添加NSPhotoLibraryUsageDescription权限描述。

内容的提问来源于stack exchange，提问作者Sebastian