关于.loadTransferable(type:completion:)进度追踪失效的问题咨询

使用PhotosPickerItem的loadTransferable(type:completionHandler:)方法返回的NSProgress对象无法正常更新：

• KVO无法触发进度变更通知
• 用定时器轮询观察，日志始终输出如下内容，进度无任何变化：

  <NSProgress: 0x600002c2d300> : Parent: 0x0 (portion: 0) / Fraction completed: 0.0000 / Completed: 0 of 0
  

相关代码：

private func onSelection() {
    guard let item = self.selection else {
        return
    }
    
    self.progress = item.loadTransferable(type: Data.self) { result in
        /// ...
    }
    
    self.observeProgress()
}

private func observeProgress() {
    guard let progress = self.progress else { return }
    let _ = Timer.scheduledTimer(withTimeInterval: 0.1, repeats: true) { timer in
        print(progress)
        
        if progress.isFinished {
            timer.invalidate()
        }
    }
}

1. 先确认进度对象的触发条件 #

loadTransferable返回的NSProgress只有在资源需要从iCloud下载时才会逐步更新进度。如果资源已经在本地缓存，任务会直接完成，进度对象可能不会有中间状态的变化。

可以先在completion回调里确认传输状态：

self.progress = item.loadTransferable(type: Data.self) { result in
    switch result {
    case .success(let data):
        print("传输完成，数据大小：\(data?.count ?? 0)")
    case .failure(let error):
        print("传输失败：\(error.localizedDescription)")
    }
}

2. 正确用KVO观察进度 #

如果确实需要监听进度，要保证KVO的实现符合规范：

• 必须在主线程添加观察者（NSProgress的KVO通知默认在主线程发送）
• 正确实现observeValue(forKeyPath:of:change:context:)方法
• 记得在任务结束后移除观察者，避免内存泄漏

示例代码：

private var progressObservationContext = 0

private func onSelection() {
    guard let item = self.selection else { return }
    
    self.progress = item.loadTransferable(type: Data.self) { [weak self] result in
        // 处理结果后移除观察者
        self?.progress?.removeObserver(self!, forKeyPath: "fractionCompleted", context: &self!.progressObservationContext)
    }
    
    progress?.addObserver(self, forKeyPath: "fractionCompleted", options: [.initial, .new], context: &progressObservationContext)
}

override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
    guard context == &progressObservationContext else {
        super.observeValue(forKeyPath: keyPath, of: object, change: change, context: context)
        return
    }
    
    if keyPath == "fractionCompleted", let progress = object as? NSProgress {
        print("当前进度：\(progress.fractionCompleted)")
        if progress.isFinished {
            progress.removeObserver(self, forKeyPath: "fractionCompleted", context: &progressObservationContext)
        }
    }
}

3. 用Combine替代KVO（Swift 5.1+） #

如果项目使用Combine框架，监听进度会更简洁：

import Combine

private var cancellables = Set<AnyCancellable>()

private func onSelection() {
    guard let item = self.selection else { return }
    
    self.progress = item.loadTransferable(type: Data.self) { result in
        // 处理结果
    }
    
    progress?.publisher(for: \.fractionCompleted)
        .receive(on: DispatchQueue.main)
        .sink { [weak self] fraction in
            print("当前进度：\(fraction)")
            if self?.progress?.isFinished == true {
                self?.cancellables.removeAll()
            }
        }
        .store(in: &cancellables)
}

4. 检查相册权限配置 #

确保Info.plist中添加了必要的权限声明：

• NSPhotoLibraryUsageDescription（访问本地相册权限）
• NSPhotoLibraryAddUsageDescription（保存到相册权限）

内容的提问来源于stack exchange，提问作者sadel