R shiny updateCheckboxInput

Question

I have a R shiny application in which I have drop down box defined in ui.R as lib whose values are c("X","Y","Z") and also a check box called spcheck which is defined as

checkboxInput("spcheck","label1",value = FALSE))

Other conditions makes this checkbox available only when input$lib=Z and checking it would make few things appear. When users select a different library, say X, I want to make the spcheck value FALSE, So I added the following code to server.R

observe({
          if (input$lib %in% c("X","Y") )  
          {cat("uncheck called 1 : ",input$spcheck,'\n')
           updateCheckboxInput(session,"spcheck","label1,value = FALSE)
           cat("uncheck called 2 : ",input$spcheck,'\n')
          }
          else return()                            
        })

The text displayed at console is :

 uncheck called 1 :  TRUE 
 uncheck called 2 :  TRUE 

Why is it not making the spcheck value FALSE? May be I'm missing something very trivial, but I couldn't figure it out. Any help??

Answer 1

Because it takes a few milliseconds since you tell shiny to update the input until it actually happens. When you call the update method, shiny has to send a message to JavaScript to change the value of the input, and once that's done, JavaScript sends a message back to R saying that the input has a new value. But the input variable does not change instantly when you make the call, it'll only have an updated value next time a reactive statement uses the input$spcheck variable