Improve support for multiple metadata tracks

[phhusson]

[REQUIRED] Searched documentation and issues

I've browsed https://github.com/google/exoplayer/issues?utf8=%E2%9C%93&q=is%3Aissue+metadata

[REQUIRED] Question

As far as I understand, ExoPlayer currently supports only one metadata track at the same time.
My usecase is to have EIT and TDT DVB tracks (respectively EPG and current time) reported back to the application through onMetadata. So what I want is to select all metadata tracks, instantiate related decoders (based on various mime-types), and have all those metadata sent to a single MetadataOutput.
I don't know if there are cases where we don't want all tracks (there is the case when there are multiple PMTs in the stream, but that's already not well handled by ExoPlayer anyway)
EIT and TDT are just examples, DVB has many other metadata on various tracks. (AIT, BAT, NIT, ...)

What is the recommendation to do that?
Currently i'm thinking of instantiating multiple MetadataRenderer (though it needs to be a constant number, that'll need to be magically set), and changing selectTracks to allow assigning multiple renderers, but that's not really clean...

[AquilesCanta]

As things are now you'll need one Metadata renderer per metadata track you want to handle. The number of renderers depends on the formats you want to handle. For example, if you need to handle ID3 and SCTE35, you'll need to inject two renderers. You will also need to implement track selection so that each of the tracks gets mapped to a different metadata renderer.

In the case of PSI and TS tables in general, I think you can just inject custom payload readers (See TsPayloadReader.Factory) but keep in mind that those payload readers run on a worker thread, so keep cross-thread communication in mind.

I'll use this issue to track better support for multiple metadata tracks.

[tonihei]

For the point of using multiple renderers and making it simpler to set them up: There is also #6589 where this question has been asked for audio renderers and #6652 for video renderers (duped to the other one).

[tonihei]

This requested enhancement was implemented by #6678, so closing this issue. If there is something left to implement, please file a new issue for tracking. Thanks!