ios - AVURLAsset loadValuesAsynchronously 从不失败

Question

我正在阅读和尝试 Apple 关于加载视频 Assets 的示例代码。

但我注意到 AVURLAsset 上的 loadValuesAsynchronously 函数永远不会失败，即使我在设备上打开飞行模式或在我的 mac 上关闭 wifi。任何人都知道在什么情况下 loadValuesAsynchronously 将无法加载 key ？

这是相对的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)
    }
}

Answer 1

终于想通了，回答我自己的问题:我一直在使用 HLS 视频，URL 的格式为 https://www.foo.com/master.m3u8 。 loadValuesAsynchronously 不会执行任何网络操作来确定 URL 是否包含有效的 HLS 文件。这就是“可播放”键始终有效的原因:

static let assetKeysRequiredToPlay = [
    "playable",
    "hasProtectedContent"
]

但是，如果我还添加了一些播放信息，例如“duration”和“tracks”，那么如果没有任何网络连接，它将失败，因为它需要实际加载信息。更多细节引用这个答案:https://stackoverflow.com/a/31418812/1035008