Implement support for clientside callbacks in Dash for R

[rpkyle]

This PR proposes modifications to support clientside callbacks in Dash for R, as they currently exist in Dash for Python (plotly/dash/issues/266, plotly/dash/pull/672, plotly/dash-renderer/pull/143).

Specifically, the following changes are proposed:

• add a function called clientsideFunction which allows app developers to reference a JavaScript function and its namespace
• modify insertIntoCallbackMap to accept either an R function or a JavaScript function
• modify assert_valid_callbacks to permit either a function to be passed into the callback_map, or a reference to a JavaScript function (provided that the object's names are namespace and function_name)
• modify the _dash-dependencies route handler to add clientside_function to the JSON payload

Closes #129.

[rpkyle]

Sample app (cf. https://community.plot.ly/t/dash-0-41-0-released/22131):

library(dash)
library(dashCoreComponents)
library(dashHtmlComponents)

app <- Dash$new()

app$layout(htmlDiv(list(
    dccInput(id='input'),
    htmlDiv(id='output-clientside'),
    htmlDiv(id='output-serverside')
    )
    )
)

app$callback( 
  output(id = "output-serverside", property = "children"),
  params = list(
    input(id = "input", property = "value")
  ),
  function(value) {
    sprintf("Server says %s", value)
  }
)

app$clientside_callback(
  output('output-clientside', 'children'),
  params=list(input('input', 'value')),
  clientsideFunction(
    namespace = 'clientside',
    function_name = 'display'
    )
)

app$run_server(showcase = TRUE, debug = TRUE)

[alexcjohnson]

What if we allow app$callback to accept either an R function or a clientsideFunction, determine which it is based on type, and do away with app$clientside_callback? It's a little different from Python, but that's just because of Python's decorators vs R's anonymous functions. It's still plenty clear, I think, as you've still got an explicit call to clientsideFunction.

[rpkyle]

I wondered about that as I was working on this PR; using the same syntax for both seems perfectly fine to me. I'll go ahead and make this change.

[rpkyle]

fixed in e4fffaf

[alexcjohnson]

Looks great! just one small suggestion https://github.com/plotly/dashR/pull/130/files#r330202703 and trailing whitespace, then 💃

[rpkyle]

Looks great! just one small suggestion https://github.com/plotly/dashR/pull/130/files#r330202703 and trailing whitespace, then 💃

Right, sorry about that. RStudio has been configured to 🔪 trailing whitespace, but my vim settings weren't working as effectively. I had configured

autocmd FileType R,Rd,py,js autocmd BufWritePre <buffer> %s/\s\+$//e

Using a keystroke to make the change manually before save is working, so will use that going forward:

nnoremap <F5> :let _s=@/<Bar>:%s/\s\+$//e<Bar>:let @/=_s<Bar><CR>