dash callback_context in R?

[FerusAndBeyond]

In python dash.callback_context can be used to see what triggers the callback, how can this be done in dashR?

[rpkyle]

Hi @FerusAndBeyond! Thanks for your interest in Dash for R. callback_context is indeed available (since #87).

All that's required is to access app$callback_context() from within a callback -- if you try to call this method from outside a callback, Dash for R will present a helpful reminder.

Here's a demo, apologies that it isn't very interesting, but it will give you the general idea:

library(dash)
library(dashHtmlComponents)

app <- Dash$new()

app$layout(
  htmlDiv(list(
    htmlButton('Button 1', id='btn-1'),
    htmlButton('Button 2', id='btn-2'),    
    htmlButton('Button 3', id='btn-3'),
    htmlDiv(id='container')
  )
  )
)

app$callback(output=list(id='container', property='children'),
             params=list(
               input(id='btn-1', property='n_clicks'),
               input(id='btn-2', property='n_clicks'),
               input(id='btn-3', property='n_clicks')),
             function(btn1, btn2, btn3)
             {
               ctx <- app$callback_context()
               
               if (is.null(ctx$triggered[["value"]][[1]])) {
                 msg <- 'None of the buttons have been clicked yet'
               } else {
                 button_num <- ctx$triggered[["prop_id"]]
                 msg <- sprintf('Button %s was most recently clicked', unlist(strsplit(button_num, split = "[.]"))[1])
               }
               
               ctx_msg <- jsonlite::prettify(dash:::to_JSON(ctx))
               
               return(htmlDiv(list(
                 htmlDiv(sprintf('btn1 : %s', btn1)),
                 htmlDiv(sprintf('btn2 : %s', btn2)),
                 htmlDiv(sprintf('btn3 : %s', btn3)),
                 htmlDiv(msg),
                 htmlPre(ctx_msg)
               )))
             }
)

app$run_server()

Let me know if this helps or you have other questions -- if this answered your question, I'll go ahead and close the issue.

[rpkyle]

@FerusAndBeyond There's also a bit about this feature in our online documentation for R, in case it's helpful:

https://dashr.plotly.com/advanced-callbacks

[FerusAndBeyond]

@rpkyle Thanks! Must have missed that page and nothing came up when googling. I'll close it.