标签：#avfoundation

A user picks a video from Photos. The app turns that video into a file the server can accept. Then it uploads the file. On the surface, this sounds like a simple upload flow. In practice, iOS makes you answer a much more specific question: How do you reliably turn a video from the user's Photo Library into an uploadable MP4 file? At first, I used AVAssetExportPresetPassthrough . It looked like the right default. It preserves the original media as much as possible, avoids unnecessary re-encoding, and is fast when it works. No quality loss, less CPU usage, less battery cost. But some videos failed during export. The error was usually in the AVFoundationErrorDomain Code=-11838 family. Apple describes this error as operationNotSupportedForAsset : an operation was attempted that is not supported for the asset. At first, I suspected an iCloud download issue, a Photos permission edge case, or something retryable inside PHImageManager . That was not the real problem. The actual problem was more fundamental: I was trying to write an iPhone MOV asset into an MP4 file using a passthrough export. The Original Flow The original code was roughly shaped like this: PHImageManager . default () . requestExportSession ( forVideo : asset , options : options , exportPreset : AVAssetExportPresetPassthrough ) { exportSession , info in exportSession ? . outputURL = outputURL exportSession ? . outputFileType = . mp4 exportSession ? . exportAsynchronously { // upload } } The intention was reasonable: Ask Photos for an export session for the PHAsset . Use AVAssetExportPresetPassthrough to preserve the original quality. Write the result as an .mp4 file for upload. Many videos exported successfully this way. That is what made the bug confusing. Some videos worked. Some did not. The hidden assumption was: Any iPhone video can become an MP4 file through a passthrough export. That assumption is not always true. iPhone Videos Are Usually MOV Files To a user, it is just "a video." Internally, an iPh