How to modify a js variable within a shiny server (htmlwidgets/R) - d3.js

Question

This question is more conceptual than explicit to a particular piece of code so I guess there's no toy-code this time.
I have a rather large shiny app that uses some d3.js scripts. The D3 objects access a few global variables when their update functions are called. I'd like to control these variables from the Shiny server - is this possible?

Answer 1

This app assumes the global variable was assigned to the window. Then it uses a custom message handler to create the original variable (#1) and another to manipulate the value of that variable (#2).
Edited to have message sent back to R with the state of the global variable.
R Code:
library(shiny)
ui <- fluidPage(
# Application title
titlePanel("Global Variable Manipulation"),
mainPanel(
sliderInput("data", "Element", min = 0, max = 100, value = 0),
actionButton("go", "GO"),
textOutput("var"),
singleton(
tags$head(tags$script(src = "message-handler.js"))
),
singleton(
tags$head(tags$script(src = "message-handler2.js"))
),
singleton(
tags$head(tags$script(src = "back-to-shiny.js"))
)
)
)
server <- function(input, output, session) {
observe({
session$sendCustomMessage(type = 'testmessage',
message = list(a = 1, b = 'text',
controller = input$data))
})
observeEvent(input$go,{
session$sendCustomMessage(type = 'changeMessage',
message = list(2))
})
output$var <- renderPrint({
input$jsvalue
})
}
# Run the application
shinyApp(ui = ui, server = server)
message-handler.js code:
Shiny.addCustomMessageHandler("testmessage",
function(message) {
window.globalVar = message.controller
alert(JSON.stringify(window.globalVar));
}
);
message-handler2.js code:
Shiny.addCustomMessageHandler("changeMessage",
function(message) {
window.globalVar = window.globalVar-message;
alert(JSON.stringify(window.globalVar));
}
);
back-to-shiny.js code:
setInterval(function(){
var message = window.globalVar;
Shiny.onInputChange("jsvalue", message);
},0);