dash callback_context in R? #208

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1. rpkyle commented on Jun 3, 2020

   

   rpkyle

   on Jun 3, 2020

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   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.

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2. rpkyle commented on Jun 3, 2020

   

   rpkyle

   on Jun 3, 2020

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   @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
3. FerusAndBeyond commented on Jun 3, 2020

   

   FerusAndBeyond

   on Jun 3, 2020

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

   🚀React with 🚀1Reacted by Ryan Patrick Kyle
4. FerusAndBeyond

   closed this as completedon Jun 3, 2020