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我在閱讀&嘗試關於加載視頻資產的Apple示例代碼。AVURLAsset loadValues異步永不會失敗
但我注意到AVURLAsset上的loadValuesAsynchronously功能永遠不會失效，即使我打開設備上的飛行模式或關閉我的mac上的wifi。有人知道在什麼情況下loadValuesAsynchronously將無法加載密鑰？
這是相對sample code：
asset.loadValuesAsynchronously(forKeys: PlayerViewController.assetKeysRequiredToPlay) {
/*
The asset invokes its completion handler on an arbitrary queue.
To avoid multiple threads using our internal state at the same time
we'll elect to use the main thread at all times, let's dispatch
our handler to the main queue.
*/
DispatchQueue.main.async() {
/*
This method is called when the `AVAsset` for our URL has
completed the loading of the values of the specified array
of keys.
*/
/*
Test whether the values of each of the keys we need have been
successfully loaded.
*/
for key in PlayerViewController.assetKeysRequiredToPlay {
var error: NSError?
if asset.statusOfValue(forKey: key, error: &error) == .failed {
let stringFormat = NSLocalizedString("error.asset_%@_key_%@_failed.description", comment: "Can't use this AVAsset because one of it's keys failed to load")
let message = String.localizedStringWithFormat(stringFormat, title, key)
self.handleError(with: message, error: error)
return
}
}
// We can't play this asset.
if !asset.isPlayable || asset.hasProtectedContent {
let stringFormat = NSLocalizedString("error.asset_%@_not_playable.description", comment: "Can't use this AVAsset because it isn't playable or has protected content")
let message = String.localizedStringWithFormat(stringFormat, title)
self.handleError(with: message)
return
}
/*
We can play this asset. Create a new AVPlayerItem and make it
our player's current item.
*/
self.loadedAssets[title] = asset
let name = (thumbnailResourceName as NSString).deletingPathExtension
let type = (thumbnailResourceName as NSString).pathExtension
let path = Bundle.main.path(forResource: name, ofType: type)!
let thumbnail = UIImage(contentsOfFile: path)!
self.assetTitlesAndThumbnails[asset.url] = (title, thumbnail)
}
}