Add <optgroup> support to selectInput()/selectizeInput()

[yihui]

A test case:

states = setNames(state.abb, state.name)
runApp(list(
  ui = fluidPage(
    selectInput('x1', 'X1', states),
    checkboxGroupInput('x2', 'X2', states, inline=TRUE),
    radioButtons('x3', 'X3', states),
    selectizeInput('x4', 'X4', list(
      Eastern = c(`New York` = 'NY', `New Jersey` = 'NJ'),
      Western = c(`California` = 'CA', `Washington` = 'WA')
    ), multiple = TRUE),
    verbatimTextOutput('y1'),
    verbatimTextOutput('y2'),
    verbatimTextOutput('y3'),
    verbatimTextOutput('y4')
  ),
  server = function(input, output, session) {
    updateSelectInput(session, 'x1', 'X One',
                      c(states, `District of Columbia` = 'DC'),
                      selected = 'IA')
    updateCheckboxGroupInput(session, 'x2', choices = states[1:10])
    updateRadioButtons(session, 'x3', choices=sample(states, 5), inline=TRUE)
    updateSelectizeInput(session, 'x4', choices=list(
      Eastern = c(`Rhode Island` = 'RI', `New Jersey` = 'NJ'),
      Western = c(`Oregon` = 'OR', `Washington` = 'WA'),
      Middle = c(Iowa = 'IA')
    ))
    output$y1 = renderPrint(input$x1)
    output$y2 = renderPrint(input$x2)
    output$y3 = renderPrint(input$x3)
    output$y4 = renderPrint(input$x4)
  }
))

[yihui]

Okay, this PR should be complete now.

For those who want to use updateSelectizeInput() with <optgroup>, it may require substantial knowledge about selectize.js. I may be able to add an example to the article in shiny dev center when I have got time.

[yihui]

I have made two examples for this PR.

• basic usage of optgroup: https://demo.shinyapps.io/024-optgroup-selectize/
• optgroup for server-side selectize: https://demo.shinyapps.io/023-optgroup-server/ I'm not going to consider the possibility of dynamically generating optgroups, since I think in most cases, the size of the optgroups data should be relatively small, so it does not hurt if the user just provides all groups as part of the configuration like what I did in this example

I do not have anything else to add to this PR except a news item. These two examples will be added to shiny dev center later.

[wch]

For the update*Input functions, did they stop working after the other changes? That is, why does the HTML need to be generated on the server side?

[wch]

This part looks too tricky to me - I'd really prefer a more straightforward way of accomplishing the same goal.

[yihui]

The more straightforward way is probably to copy and paste, which is less desirable.

[yihui]

I have tested the update*Input functions, and all of them are still working (the relevant commit was d2fc851). See the test cases in the description of this PR. The reason to send raw HTML was due to this optgroup feature: I thought it would be difficult to generate optgroups in shiny.js. Since we have R code that generates the HTML code, we can just use it instead of repeating this task in JavaScript. The downside is the size of data passed through WebSockets (the JSON object of choices should be much smaller than the pre-generated HTML string). I talked about this to @jcheng5 and he thought we could optimize this later if necessary.

[wch]

OK, that sounds like a good reason to generate the HTML on the server side.