[Edit: I was able to figure out a workaround for this, see below.]
我正在尝试从S3流式传输多个远程MP4剪辑，将它们按顺序播放为一个连续视频（以便在剪辑内和剪辑之间进行清理），不会出现任何卡顿现象，而无需先将其明确下载到设备中 . 但是，我发现剪辑缓冲区非常缓慢（即使在快速的网络连接上），也无法找到解决问题的适当方法 .
我一直在尝试使用 AVPlayer ，因为 AVPlayer 与 AVMutableComposition 播放提供的视频轨道作为一个连续的轨道（不像 AVQueuePlayer ，我收集它分别播放每个视频，因此不支持剪辑之间的连续擦洗） .
当我将其中一个资产直接粘贴到 AVPlayerItem 并播放（没有 AVMutableComposition ）时，它会快速缓冲 . 但是使用 AVMutableComposition ，视频在第二个剪辑上开始出现非常糟糕的情况（我的测试用例有6个剪辑，每个剪辑大约6秒），而音频一直在继续 . 在它播放一次之后，如果我倒回到开头它会非常顺利地播放，所以我认为问题在于缓冲 .
我目前试图解决这个问题的尝试让人感到费解，因为这似乎是 AVPlayer 的一个相当基本的用例 - 我希望有很多想法 .
这是设置 AVMutableComposition 的主要代码：
// Build an AVAsset for each of the source URIs
- (void)prepareAssetsForSources:(NSArray *)sources
{
NSMutableArray *assets = [[NSMutableArray alloc] init]; // the assets to be used in the AVMutableComposition
NSMutableArray *offsets = [[NSMutableArray alloc] init]; // for tracking buffering progress
CMTime currentOffset = kCMTimeZero;
for (NSDictionary* source in sources) {
bool isNetwork = [RCTConvert BOOL:[source objectForKey:@"isNetwork"]];
bool isAsset = [RCTConvert BOOL:[source objectForKey:@"isAsset"]];
NSString *uri = [source objectForKey:@"uri"];
NSString *type = [source objectForKey:@"type"];
NSURL *url = isNetwork ?
[NSURL URLWithString:uri] :
[[NSURL alloc] initFileURLWithPath:[[NSBundle mainBundle] pathForResource:uri ofType:type]];
AVURLAsset *asset = [AVURLAsset URLAssetWithURL:url options:nil];
currentOffset = CMTimeAdd(currentOffset, asset.duration);
[assets addObject:asset];
[offsets addObject:[NSNumber numberWithFloat:CMTimeGetSeconds(currentOffset)]];
}
_clipAssets = assets;
_clipEndOffsets = offsets;
}
// Called with _clipAssets
- (AVPlayerItem*)playerItemForAssets:(NSMutableArray *)assets
{
AVMutableComposition* composition = [AVMutableComposition composition];
for (AVAsset* asset in assets) {
CMTimeRange editRange = CMTimeRangeMake(CMTimeMake(0, 600), asset.duration);
NSError *editError;
[composition insertTimeRange:editRange
ofAsset:asset
atTime:composition.duration
error:&editError];
}
AVPlayerItem* playerItem = [AVPlayerItem playerItemWithAsset:composition];
return playerItem; // this is used to initialize the main player
}
我最初的想法是：由于它使用香草 AVPlayerItem 快速缓冲，为什么不保持一个单独的缓冲播放器依次加载每个资产（没有 AVMutableComposition ）来缓冲主要播放器的资产？
- (void)startBufferingClips
{
_bufferingPlayerItem = [AVPlayerItem playerItemWithAsset:_clipAssets[0]
automaticallyLoadedAssetKeys:@[@"tracks"]];
_bufferingPlayer = [AVPlayer playerWithPlayerItem:_bufferingPlayerItem];
_currentlyBufferingIndex = 0;
}
// called every 250 msecs via an addPeriodicTimeObserverForInterval on the main player
- (void)updateBufferingProgress
{
// If the playable (loaded) range is within 100 milliseconds of the clip
// currently being buffered, load the next clip into the buffering player.
float playableDuration = [[self calculateBufferedDuration] floatValue];
CMTime totalDurationTime = [self playerItemDuration :_bufferingPlayer];
Float64 totalDurationSeconds = CMTimeGetSeconds(totalDurationTime);
bool bufferingComplete = totalDurationSeconds - playableDuration < 0.1;
float bufferedSeconds = [self bufferedSeconds :playableDuration];
float playerTimeSeconds = CMTimeGetSeconds([_player currentTime]);
__block NSUInteger playingClipIndex = 0;
// find the index of _player's currently playing clip
[_clipEndOffsets enumerateObjectsUsingBlock:^(id offset, NSUInteger idx, BOOL *stop) {
if (playerTimeSeconds < [offset floatValue]) {
playingClipIndex = idx;
*stop = YES;
}
}];
// TODO: if bufferedSeconds - playerTimeSeconds <= 0, pause the main player
if (bufferingComplete && _currentlyBufferingIndex < [_clipAssets count] - 1) {
// We're done buffering this clip, load the buffering player with the next asset
_currentlyBufferingIndex += 1;
_bufferingPlayerItem = [AVPlayerItem playerItemWithAsset:_clipAssets[_currentlyBufferingIndex]
automaticallyLoadedAssetKeys:@[@"tracks"]];
_bufferingPlayer = [AVPlayer playerWithPlayerItem:_bufferingPlayerItem];
}
}
- (float)bufferedSeconds:(float)playableDuration {
__block float seconds = 0.0; // total duration of clips already buffered
if (_currentlyBufferingIndex > 0) {
[_clipEndOffsets enumerateObjectsUsingBlock:^(id offset, NSUInteger idx, BOOL *stop) {
if (idx + 1 >= _currentlyBufferingIndex) {
seconds = [offset floatValue];
*stop = YES;
}
}];
}
return seconds + playableDuration;
}
- (NSNumber *)calculateBufferedDuration {
AVPlayerItem *video = _bufferingPlayer.currentItem;
if (video.status == AVPlayerItemStatusReadyToPlay) {
__block float longestPlayableRangeSeconds;
[video.loadedTimeRanges enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop) {
CMTimeRange timeRange = [obj CMTimeRangeValue];
float seconds = CMTimeGetSeconds(CMTimeRangeGetEnd(timeRange));
if (seconds > 0.1) {
if (!longestPlayableRangeSeconds) {
longestPlayableRangeSeconds = seconds;
} else if (seconds > longestPlayableRangeSeconds) {
longestPlayableRangeSeconds = seconds;
}
}
}];
Float64 playableDuration = longestPlayableRangeSeconds;
if (playableDuration && playableDuration > 0) {
return [NSNumber numberWithFloat:longestPlayableRangeSeconds];
}
}
return [NSNumber numberWithInteger:0];
}
它最初看起来像一个魅力，但后来我切换到另一组测试剪辑，然后缓冲再次非常缓慢（缓冲播放器帮助，但还不够） . 似乎加载到缓冲播放器中的资产的 loadedTimeRange 与 AVMutableComposition 中的相同资产的 loadedTimeRange 不匹配：即使在加载到缓冲播放器中的每个项目的 loadedTimeRange s表示整个资产已经存在缓冲，主要播放器的视频继续口吃（而音频无缝播放到最后） . 同样，一旦主要播放器通过所有剪辑一次，重绕后回放是无缝的 .
我希望这个问题的答案，无论它是什么，都将成为其他iOS开发人员试图实现这个基本用例的起点 . 谢谢！
Edit: Since I posted this question, I made the following workaround for this. Hopefully this will save whoever runs into this some headache.
我最终做的是保持两个缓冲播放器（两个 AVPlayer ）开始缓冲前两个剪辑，然后移动到最低索引的无缓冲剪辑，之后 loadedTimeRanges 指示缓冲当前剪辑已完成 . 我根据当前缓冲的剪辑和缓冲播放器的 loadedTimeRanges 进行了逻辑暂停/取消暂停播放，加上小幅度 . 这需要一些记账变量，但并不太复杂 .
这就是缓冲播放器的初始化方式（我在这里省略了簿记逻辑）：
- (void)startBufferingClips
{
_bufferingPlayerItemA = [AVPlayerItem playerItemWithAsset:_clipAssets[0]
automaticallyLoadedAssetKeys:@[@"tracks"]];
_bufferingPlayerA = [AVPlayer playerWithPlayerItem:_bufferingPlayerItemA];
_currentlyBufferingIndexA = [NSNumber numberWithInt:0];
if ([_clipAssets count] > 1) {
_bufferingPlayerItemB = [AVPlayerItem playerItemWithAsset:_clipAssets[1]
automaticallyLoadedAssetKeys:@[@"tracks"]];
_bufferingPlayerB = [AVPlayer playerWithPlayerItem:_bufferingPlayerItemB];
_currentlyBufferingIndexB = [NSNumber numberWithInt:1];
_nextIndexToBuffer = [NSNumber numberWithInt:2];
} else {
_nextIndexToBuffer = [NSNumber numberWithInt:1];
}
}
此外，我需要确保视频和音频轨道没有被合并，因为它们被添加到 AVMutableComposition ，因为这显然干扰了缓冲（可能他们没有收到新数据） . 这是从 NSAsset 数组构建 AVMutableComposition 的代码：
- (AVPlayerItem*)playerItemForAssets:(NSMutableArray *)assets
{
AVMutableComposition* composition = [AVMutableComposition composition];
AVMutableCompositionTrack *compVideoTrack = [composition addMutableTrackWithMediaType:AVMediaTypeVideo
preferredTrackID:kCMPersistentTrackID_Invalid];
AVMutableCompositionTrack *compAudioTrack = [composition addMutableTrackWithMediaType:AVMediaTypeAudio
preferredTrackID:kCMPersistentTrackID_Invalid];
CMTime timeOffset = kCMTimeZero;
for (AVAsset* asset in assets) {
CMTimeRange editRange = CMTimeRangeMake(CMTimeMake(0, 600), asset.duration);
NSError *editError;
NSArray *videoTracks = [asset tracksWithMediaType:AVMediaTypeVideo];
NSArray *audioTracks = [asset tracksWithMediaType:AVMediaTypeAudio];
if ([videoTracks count] > 0) {
AVAssetTrack *videoTrack = [videoTracks objectAtIndex:0];
[compVideoTrack insertTimeRange:editRange
ofTrack:videoTrack
atTime:timeOffset
error:&editError];
}
if ([audioTracks count] > 0) {
AVAssetTrack *audioTrack = [audioTracks objectAtIndex:0];
[compAudioTrack insertTimeRange:editRange
ofTrack:audioTrack
atTime:timeOffset
error:&editError];
}
if ([videoTracks count] > 0 || [audioTracks count] > 0) {
timeOffset = CMTimeAdd(timeOffset, asset.duration);
}
}
AVPlayerItem* playerItem = [AVPlayerItem playerItemWithAsset:composition];
return playerItem;
}
使用这种方法，使用 AVMutableComposition 为主播放器缓冲工作很好，速度很快，至少在我的设置中 .