Added stereoPan property to allow audio panning

[rafaelrpinto]

This PR adds the channel property to Video that allows playing the audio on the left channel, right channel or both (default).

I only added the behavior for Android. Since I use linux I'm not able to implement the IOS part but should be a similar logic using the pan property

Related issue: #1050

Update: see the comments below to check the final solution

[rafaelrpinto]

I'm not very acquainted with media jargon so channel might not be the best name. Instead I could change to just pan or panPosition and instead of both use center.

[cobarx]

Very neat concept, what are you using this for you if you don't mind my asking.

I would call it joint or stereo, that's the term they use in mp3s.

Rather than making this a binary choice of left or right, what about allowing you to choose a balance:
leftChannelVolume and rightChannelVolume (default to 1.0 for both)
That way you can do an actual pan, shifting from left to right and back again.

[rafaelrpinto]

@cobarx
Thanks for checking the code.

I'm planning on using in on an app that performs hearing tests. So we need to identify some level of hearing loss on each ear.

Maybe instead of leftChannelVolume and rightChannelVolume we could use the same argument as IOS pan attribute, an integer from -1.0 to 1.0 where -1.0 is full left, 0.0 is center and 1.0 is full right.

I think this way its easier to use <Video ... volume={1} pan={-0.5} /> and for IOS the usage would be seamless.

What do you think?

[cobarx]

That's a neat application.

Yeah I like your idea, one prop is definitely better than two. I wonder if we should call it stereoPan to distinguish from other pan related terms like the PanResponder.

[rafaelrpinto]

Cool. stereoPan is fine by me.

This weekend I will setup an osx vm and make the change for both platforms to make the feature consistent.

[rafaelrpinto]

@cobarx
I updated the code with the behavior we discussed. Now stereoPan optionally receives a value between -1.0 and 1.0 and works like the IOS pan attribute.

I will limit this PR just for Android. Setting up an osx vm will be a pain and anyone with a mac can easily implement this part on IOS because there is no volume calculation required.

[cobarx]

@rafaelrpinto That's great!

I checked and ExoPlayer doesn't support changing the volume per channel yet:
google/ExoPlayer#2659
So unless I'm missing something, I don't believe the ExoPlayer code you added is valid, can you remove that.

I have the iOS dev environment setup, but that property only exists on the AVAudioPlayer, but not a generic AVPlayer that plays video. So I think we'll have to push that down the road.

[rafaelrpinto]

@cobarx
You are right, I misunderstood the ExoPlayer part. I reverted the relevant changes.

[cobarx]

Tested and it's working great! Merging. Thanks for making the changes and adding the docs.

[rafaelrpinto]

Thanks!