Question about how sendCustomMessage converts data.frames in lists to JSON

[Carl Ganz]

Hello,

I thought that when you use session$sendCustomMessage to pass info from the server to the UI that it converts the object to JSON using jsonlite::toJSON, but that doesn't appear to be the case. Here is an example:

df <- list(mydf=data.frame(v1="Hello",v2="World",

                 stringsAsFactors=FALSE))

library(shiny)

ui <- shinyUI(fluidPage(

  singleton(tags$script(HTML(

    "Shiny.addCustomMessageHandler('test-json',function(object) {

      console.log(JSON.stringify(object));

    })"

    ))),

  actionButton("test","Press me")  

  ))

server <- shinyServer(function(input, output, session) {

 

  observeEvent(input$test,{

    print(jsonlite::toJSON(df))

    session$sendCustomMessage('test-json',df)

  })

  

})

# Run the application

shinyApp(ui = ui, server = server)

When I print jsonlite::toJSON(df) I get this:

{ mydf: [ {v1: "Hello", v2: "World"} ] }

But when I pass the object to the UI and print the JSON to the console I get:

{ mydf: { v1: ["Hello"], v2: ["World"] } }

The placement of the brackets is different, and it is causing me some problems. 

Is there a way to ensure that when a data.frame in a list is passed to the UI that the JSON wraps the data.frame in brackets?

Thanks

[Joe Cheng]

I believe if you call jsonlite::toJSON and pass the result to sendCustomMessage, it'll be smart enough not to double-JSON-encode.

Or if you're developing an htmlwidget you can use this JS function to do the conversion for you: http://www.htmlwidgets.org/develop_advanced.html#htmlwidgets.dataframetod3

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