Implemented basic async volume transition for media_player

[PetePriority]

Description:

This PR implements a simple volume_transition service for the media_player platform. The service calls async_volume_transition which starts a coroutine _async_transition_helper.

Since I'm new with asyncio, I'd appreciate feedback on my implementation. One problem I see is that when the service gets called while another transition is already running, they'd interfere. Is there a way to cancel an already running coroutine? Another solution would be to have a class-owned threading.Timer which calls itself after the interval, but this would require __init__ for the platform and super-calls from all platform components.

Pull request in home-assistant.github.io with documentation (if applicable): home-assistant/home-assistant.io#2319

[mention-bot]

@PetePriority, thanks for your PR! By analyzing the history of the files in this pull request, we identified @balloob, @armills and @fabaff to be potential reviewers.

[emlove]

Also we'd need to abort the transition if the user changes the volume externally during a transition. We have the same issues here as implementing software dimmer transitions. There's a thread about this somewhere, but I can't seem to find it.

Much later edit, for posterity: #656

[pvizeli]

I don't think we should add logic on base stuff. At last we only represent a device a not extend it. For that we have automation. I don't think we should do that. CC @balloob ?

[PetePriority]

This branch shows an example of cancelable transitions. I had to make changes to the set_volume_level and __init__ of the mpd component, which might be a bit too invasive, as this would have to be done with every component.

Alternatively, we could move the functionality to a separate "fader" module that calls the set_volume service of specified media players. Maybe that would be a better approach?

[balloob]

I'm not a fan of this either. @armills is right. It will add too much complexity and eventually will end up with a pile of workarounds and hacks.

Some alternatives to achieve this:

• Create a script that calls set volume with incremental volumes.
• Create a custom component that calls the services for you
• Add an optional transition parameter to async_set_volume_level. It is then up to the platforms to implement transition. They can choose to do this with a generic volume helper transition method. That way we leave the power to the platforms to create this the way they want it.

[pvizeli]

@PetePriority I close this PR. You can choise a way they Paulus describe in his comments.