使用 AVFoundation 保留 hls 播放列表，下载两个不同的视频轨道，使下载大小达到 go 高于预期

问题描述

我正在尝试通过解析特定比特率/分辨率来下载 hls 播放列表视频轨道。

下载工作非常好。 但是我们需要从清单中下载特定的视频轨道变体。 所以我通过AVAssetDownloadTaskMinimumRequiredMediaBitrateKey和AVAssetDownloadTaskMinimumRequiredPresentationSizeKey解析比特率和分辨率。

但是AVAggregateAssetDownloadTask总是下载视频轨道段的特定（已解析）比特率变体以及较低分辨率的视频轨道段。 这使得下载大小为 go 高于预期。

这是预期的行为还是我做错了什么？

我尝试了来自https://developer.apple.com/documentation/avfoundation/offline_playback_and_storage/using_avfoundation_to_play_and_persist_http_live_streams的示例源，它具有相同的行为。

这就是我的清单文件的样子。

#EXT-X-VERSION:7
## Created with Unified Streaming Platform  (version=1.11.3-24483)
#EXT-X-SESSION-KEY:METHOD=SAMPLE-AES,URI="skd://xxxxxxx",KEYFORMAT="com.apple.streamingkeydelivery",KEYFORMATVERSIONS="1"

# AUDIO groups
#EXT-X-MEDIA:TYPE=AUDIO,GROUP-ID="audio-aacl-96",LANGUAGE="en",NAME="eng",DEFAULT=YES,AUTOSELECT=YES,CHANNELS="2",URI="vod-idx-audio_eng=96000.m3u8"
#EXT-X-MEDIA:TYPE=AUDIO,GROUP-ID="audio-aacl-160",LANGUAGE="en",NAME="eng",DEFAULT=YES,AUTOSELECT=YES,CHANNELS="2",URI="vod-idx-audio_eng=160000.m3u8"
#EXT-X-MEDIA:TYPE=AUDIO,GROUP-ID="audio-ec-3-192",LANGUAGE="en",NAME="eng",DEFAULT=YES,AUTOSELECT=YES,CHANNELS="6",URI="vod-idx-audio_eng_1=192000.m3u8"
#EXT-X-MEDIA:TYPE=AUDIO,GROUP-ID="audio-aacl-320",LANGUAGE="en",NAME="eng",DEFAULT=YES,AUTOSELECT=YES,CHANNELS="6",URI="vod-idx-audio_eng_2=320000.m3u8"

# SUBTITLES groups
#EXT-X-MEDIA:TYPE=SUBTITLES,GROUP-ID="textstream",LANGUAGE="da",NAME="Dansk",DEFAULT=YES,AUTOSELECT=YES,URI="vod-idx-textstream_dan=1000.m3u8"
#EXT-X-MEDIA:TYPE=SUBTITLES,GROUP-ID="textstream",LANGUAGE="fi",NAME="Suomi",AUTOSELECT=YES,URI="vod-idx-textstream_fin=1000.m3u8"
#EXT-X-MEDIA:TYPE=SUBTITLES,GROUP-ID="textstream",LANGUAGE="no",NAME="Norsk",AUTOSELECT=YES,URI="vod-idx-textstream_nor=1000.m3u8"
#EXT-X-MEDIA:TYPE=SUBTITLES,GROUP-ID="textstream",LANGUAGE="sv",NAME="Svenska",AUTOSELECT=YES,URI="vod-idx-textstream_swe=1000.m3u8"

# variants
#EXT-X-STREAM-INF:BANDWIDTH=416000,CODECS="mp4a.40.2,avc1.4D4015",RESOLUTION=480x270,FRAME-RATE=25,VIDEO-RANGE=SDR,AUDIO="audio-aacl-96",SUBTITLES="textstream",CLOSED-CAPTIONS=NONE
vod-idx-video=300000.m3u8
#EXT-X-STREAM-INF:BANDWIDTH=799000,CODECS="mp4a.40.2,avc1.4D401E",RESOLUTION=640x360,FRAME-RATE=25,VIDEO-RANGE=SDR,AUDIO="audio-aacl-160",SUBTITLES="textstream",CLOSED-CAPTIONS=NONE
vod-idx-video=600000.m3u8
#EXT-X-STREAM-INF:BANDWIDTH=1385000,CODECS="mp4a.40.2,avc1.4D401E",RESOLUTION=768x432,FRAME-RATE=25,VIDEO-RANGE=SDR,AUDIO="audio-aacl-320",SUBTITLES="textstream",CLOSED-CAPTIONS=NONE
vod-idx-video=1000000.m3u8
#EXT-X-STREAM-INF:BANDWIDTH=2021000,CODECS="mp4a.40.2,avc1.4D401F",RESOLUTION=960x540,FRAME-RATE=25,VIDEO-RANGE=SDR,AUDIO="audio-aacl-320",SUBTITLES="textstream",CLOSED-CAPTIONS=NONE
vod-idx-video=1600000.m3u8
#EXT-X-STREAM-INF:BANDWIDTH=3505000,CODECS="mp4a.40.2,avc1.64001F",RESOLUTION=1280x720,FRAME-RATE=25,VIDEO-RANGE=SDR,AUDIO="audio-aacl-320",SUBTITLES="textstream",CLOSED-CAPTIONS=NONE
vod-idx-video=3000000.m3u8
#EXT-X-STREAM-INF:BANDWIDTH=5837000,CODECS="mp4a.40.2,avc1.640032",RESOLUTION=1920x1080,FRAME-RATE=25,VIDEO-RANGE=SDR,AUDIO="audio-aacl-320",SUBTITLES="textstream",CLOSED-CAPTIONS=NONE
vod-idx-video=5200000.m3u8
#EXT-X-STREAM-INF:BANDWIDTH=513000,CODECS="ec-3,avc1.4D4015",RESOLUTION=480x270,FRAME-RATE=25,VIDEO-RANGE=SDR,AUDIO="audio-ec-3-192",SUBTITLES="textstream",CLOSED-CAPTIONS=NONE
vod-idx-video=300000.m3u8
#EXT-X-STREAM-INF:BANDWIDTH=831000,CODECS="ec-3,avc1.4D401E",RESOLUTION=640x360,FRAME-RATE=25,VIDEO-RANGE=SDR,AUDIO="audio-ec-3-192",SUBTITLES="textstream",CLOSED-CAPTIONS=NONE
vod-idx-video=600000.m3u8
#EXT-X-STREAM-INF:BANDWIDTH=1255000,CODECS="ec-3,avc1.4D401E",RESOLUTION=768x432,FRAME-RATE=25,VIDEO-RANGE=SDR,AUDIO="audio-ec-3-192",SUBTITLES="textstream",CLOSED-CAPTIONS=NONE
vod-idx-video=1000000.m3u8
#EXT-X-STREAM-INF:BANDWIDTH=1891000,CODECS="ec-3,avc1.4D401F",RESOLUTION=960x540,FRAME-RATE=25,VIDEO-RANGE=SDR,AUDIO="audio-ec-3-192",SUBTITLES="textstream",CLOSED-CAPTIONS=NONE
vod-idx-video=1600000.m3u8
#EXT-X-STREAM-INF:BANDWIDTH=3375000,CODECS="ec-3,avc1.64001F",RESOLUTION=1280x720,FRAME-RATE=25,VIDEO-RANGE=SDR,AUDIO="audio-ec-3-192",SUBTITLES="textstream",CLOSED-CAPTIONS=NONE
vod-idx-video=3000000.m3u8
#EXT-X-STREAM-INF:BANDWIDTH=5707000,CODECS="ec-3,avc1.640032",RESOLUTION=1920x1080,FRAME-RATE=25,VIDEO-RANGE=SDR,AUDIO="audio-ec-3-192",SUBTITLES="textstream",CLOSED-CAPTIONS=NONE
vod-idx-video=5200000.m3u8

# variants
#EXT-X-STREAM-INF:BANDWIDTH=98000,CODECS="mp4a.40.2",AUDIO="audio-aacl-96",SUBTITLES="textstream"
vod-idx-audio_eng=96000.m3u8
#EXT-X-STREAM-INF:BANDWIDTH=163000,CODECS="mp4a.40.2",AUDIO="audio-aacl-160",SUBTITLES="textstream"
vod-idx-audio_eng=160000.m3u8
#EXT-X-STREAM-INF:BANDWIDTH=325000,CODECS="mp4a.40.2",AUDIO="audio-aacl-320",SUBTITLES="textstream"
vod-idx-audio_eng_2=320000.m3u8
#EXT-X-STREAM-INF:BANDWIDTH=195000,CODECS="ec-3",AUDIO="audio-ec-3-192",SUBTITLES="textstream"
vod-idx-audio_eng_1=192000.m3u8

# keyframes
#EXT-X-I-FRAME-STREAM-INF:BANDWIDTH=40000,CODECS="avc1.4D4015",RESOLUTION=480x270,VIDEO-RANGE=SDR,URI="keyframes/vod-idx-video=300000.m3u8"
#EXT-X-I-FRAME-STREAM-INF:BANDWIDTH=80000,CODECS="avc1.4D401E",RESOLUTION=640x360,VIDEO-RANGE=SDR,URI="keyframes/vod-idx-video=600000.m3u8"
#EXT-X-I-FRAME-STREAM-INF:BANDWIDTH=133000,CODECS="avc1.4D401E",RESOLUTION=768x432,VIDEO-RANGE=SDR,URI="keyframes/vod-idx-video=1000000.m3u8"
#EXT-X-I-FRAME-STREAM-INF:BANDWIDTH=212000,CODECS="avc1.4D401F",RESOLUTION=960x540,VIDEO-RANGE=SDR,URI="keyframes/vod-idx-video=1600000.m3u8"
#EXT-X-I-FRAME-STREAM-INF:BANDWIDTH=398000,CODECS="avc1.64001F",RESOLUTION=1280x720,VIDEO-RANGE=SDR,URI="keyframes/vod-idx-video=3000000.m3u8"
#EXT-X-I-FRAME-STREAM-INF:BANDWIDTH=689000,CODECS="avc1.640032",RESOLUTION=1920x1080,VIDEO-RANGE=SDR,URI="keyframes/vod-idx-video=5200000.m3u8"

这就是我创建下载任务的方式

let preferredMediaSelection = asset.urlAsset.preferredMediaSelection
if #available(iOS 14.0, *) {
      guard let task =
          assetDownloadURLSession.aggregateAssetDownloadTask(with: asset.urlAsset,
                                    mediaSelections: [preferredMediaSelection],
                                    assetTitle: asset.stream.name,
                                    assetArtworkData: nil,
                                    options:
                                    [AVAssetDownloadTaskMinimumRequiredMediaBitrateKey:5200000, AVAssetDownloadTaskMinimumRequiredPresentationSizeKey: CGSize(width: 1920, height: 1080)])
      else { return }

      task.taskDescription = asset.stream.name
      task.resume()
    }

任何帮助将不胜感激。 谢谢