Getting started with effects managers and WebRTC

[Gábor Varga]

Hello folks,

I would like to use WebRTC from Elm. 

I did not find any prior work on that, so I believe I will have to write it my self -- which sounds like fun for now.

To me this seems like an exercise where I should write an effects manager as well as some native JS.

Where shall I get started with effects managers? Are there any secret blog posts, or not-yet-published documentation on this?

If not, which elm package/module would be the most educative: perhaps Websocket in core?

Thanks,

Gabor

[Alex Lew]

If you're willing to put up with potential inaccuracies / incompleteness, I have an unpublished (and probably to-remain-unpublished) blog post draft here with some observations about effect managers: https://medium.com/@alex.lew/8e87fd14332c. This was really written out of curiosity & in case others were curious, not as a how-to-write-effect-managers guide, but you may find it helpful in making sense of existing effect manager code. Good luck! :)

[Pablo Bollansée]

Did you ever finish this? As I was looking to try WebRTC in Elm as well.

[Martin Janiczek]

I think the problem with WebRTC is the non-existence of universal signaling protocol.

Disregarding how the signaling server code would work, the API would probably have to look something like:

type ConnectionStatus
  = Disconnected
  | ConnectedNotReady
  | ConnectedReady


listenOnly : String -> (ConnectionStatus -> msg) -> (Json.Decode.Value -> msg) -> Sub msg
listenOnly connectionName setConnectionStatusMsg getDataMsg = ...


sendOnly : String -> (ConnectionStatus -> msg) -> Sub msg
sendOnly connectionName setConnectionStatusMsg = ...


listenAndSend : String -> (ConnectionStatus -> msg) -> (Json.Decode.Value -> msg) -> Sub msg
listenAndSend connectionName setConnectionStatus getDataMsg = ...


send : String -> Json.Decode.Value -> Cmd msg
send connectionName value = ...

I am creating an Elm app with three WebRTC peers (library, controller, output) and the graph like this:

- library -> controller

- library -> output

- controller -> output

So that would translate into (controller):

type Peer
  = Library
  | Output


type Msg
  = SetPeerStatus Peer WebRTC.ConnectionStatus
  | SendDataToOutput -- sent periodically or onClick of a button
  | GetDataFromLibrary Json.Decode.Value

type alias DataFromLibrary =
  Bool

type alias DataForOutput =
  Int

type alias Model =
  { webRtcStatus : List (Peer, WebRTC.ConnectionStatus)
  , dataFromLibrary : Maybe DataFromLibrary
  , dataForOutput : DataForOutput
  }

update msg model =
  case msg of
    SetPeerStatus peer status ->
      ( { model | webRtcStatus =
           List.Extra.updateIf
             (\(oldPeer, _) -> oldPeer == peer)
             (\_ -> (peer, status))
             model.webRtcStatus
        }
      , Cmd.none
      )


    SendDataToOutput ->
      ( model
      , WebRTC.send "output" model.data
      )

    GetDataFromLibrary json ->
      ( model -- do something with JSON, eg. Json.Decode.decodeValue libraryDataDecoder. This part is big so I did not include it. But it can be done :)
      , Cmd.none
      )


subscriptions model =
  Sub.batch
    [ WebRTC.listenOnly "library" (SetPeerStatus Library) GetDataFromLibrary
    , WebRTC.sendOnly "output" (SetPeerStatus Output)
    ]

I don't know how/whether to handle reconnections, error values (as in WebSocket.LowLevel), ...

Does this kind of API make sense? Gabor, did you make any progress with this effect manager?

[Gábor Varga]

Sorry guys for the late answer. 

I never got too far with this, so I do not have anything to share. Currently we use our in-house JS lib to take care of the WebRTC/media related stuff, and since it mostly get's the job done, I do not see it getting replaced in the near term future.