Support lazy preparation in ConcatenatingMediaSource for long playlists

[Khang-NT]

Change request

I found this class is very useful DynamicConcatenatingMediaSource.DeferredTimeline, I think it is reusable, just like DeferredMediaPeriod class. Please make it public.

I need to use DeferredTimeline to create a class called DeferredMediaSource: https://gist.github.com/Khang-NT/985e12e1d73454a79a989165c9bd13fa . This class solves my problem when using DynamicConcatenatingMediaSource, because DynamicConcatenatingMediaSource prepare all child media source immediately, for instance, I have a playlist contains 100 DASH media, DynamicConcatenatingMediaSource will prepare/download 100 manifest file, but I just need to prepare current playing item, and prepare the next position for pre-buffering (similar with #3400)

Version of ExoPlayer being used

2.7.0

[tonihei]

I think we can do this as it will also clean up some of the code in the already long DynamicConcatenatingMediaSource class.

More generally, the problem you are trying to solve is actually a useful feature especially for long playlists of DASH, HLS or SmoothStreaming media items where all manifests are downloaded immediately. The downside of not downloading the manifests is that the duration of the item stays unknown until you are beginning to play it. I think it would still be helpful to provide that as an option in the constructor of the DynamicConcatenatingMediaSource (which defaults to immediate child preparation). Assuming we would add such a feature, would you still need access to the DeferredTimeline class? Or would it be enough to leave it internal in this case?

[Khang-NT]

The problem above is my only use case of DeferredTimeline. It would be nice if DynamicConcatenatingMediaSource supports to solve that problem officially, then keep DeferredTimeline class as internal up to your design.

[Khang-NT]

DynamicConcatenatingMediaSource is deprecated from 2.8.0. @tonihei, Does it support "lazy preparing" now?

[tonihei]

Not yet, sorry. It has just been merged with the existing ConcatenatingMediaSource because they had an almost identical feature set.

[tonihei]

Now available with the commit referenced above.