Debug shiny render* output - r

Question

I'm trying to debug my shiny dashboard
For several render* function, I need to debug them with some log (with print or cat) but I can't use those function inside a renderDataTable() / renderText()
for example:
output$selectedData = renderDataTable(
myCsv[which(myCsv[[myCase_id]]==input$process_tokens),]
)
I would like to print something to the console before and after the instruction of renderDataTable() but
output$selectedData = renderDataTable(
cat("rendering...")
myCsv[which(myCsv[[myCase_id]]==input$process_tokens),]
cat("rendered")
)
How can I do this ?

Answer 1

Here is a possible solution to the problem. First I use a variable called data to assingn any calculations to, in your case
data<-myCsv[which(myCsv[[myCase_id]]==input$process_tokens),]. This is used inside the render function and will be created when the output is rendered since it relies on this. I then use an observe function that requires the variable data to be created before printing the second "rendered" to the console. That works once on startup, and will work fine if your data is constant. If you have changing data, for my example the data changes with a user selection, we will have to re-render the table. Since the render function is reactive and you are using input$process_tokens, the render function will re-run when the input changes. In this example it runs when input$select changes. When it runs it resets the variable data to NULL, and we trigger a separate observeEvent that monitors changes to input$select(input$process_tokens). This observeEvent also requires data before continuing, and since the render function set it to null it will not print the second "rendered" until data is created, just as in the first case.
library(shiny)
library(DT)
ui <- fluidPage(
sidebarLayout(
sidebarPanel(
selectizeInput("select","select",choices=(c(1,2,3,4)))
),
mainPanel(
dataTableOutput("selectedData")
)
))
server <- function(input, output, session) {
data<-reactive({data.frame(input$select,4,5)})
output$selectedData <- renderDataTable({
data<-NULL
print("rendering..")
data<-datatable(data())
})
#Observe inital rendering (only needed if no change to data)
observe({
req(data)
print("rendered!")
})
#Observe Changes to data
observeEvent(input$select,{
req(data)
print("rendered!")
})
}
shinyApp(ui, server)
Specific code for you:
server <- function(input, output, session) {
output$selectedData <- renderDataTable({
data<-NULL
print("rendering..")
data<- myCsv[which(myCsv[[myCase_id]]==input$process_tokens),]
})
#Observe inital rendering (only needed if no change to data)
observe({
req(data)
print("rendered!")
})
#Observe Changes to data
observeEvent(input$process_tokens,{
req(data)
print("rendered!")
})
}
shinyApp(ui, server)
Note that you will get two "rendered" printouts when the program initially starts, this is b/c both the observe and observeEvent run since both conditions are met. If your data does change with input$process_tokens, then you can get rid of the observe function, and only use the observeEvent. If your data does not change and the table is only rendered once, then get rid of the observeEvent. I was trying to cover all bases.