R shiny observe for row deselection dataTable - r

Question

I have a shiny app that has a DT::renderDataTable, the user can select a row in the datatable.
The following bit of code will only print FALSE (when a row is selected):
observeEvent(input$segment_library_datatable_rows_selected, {
print(is.null(input$segment_library_datatable_rows_selected))
})
How do I get it to print when a row is also deselected? (The print value would be TRUE)

Answer 1

observeEvent(input$selected,ignoreNULL = FALSE,{...})
ignoreNULL is defaulted to TRUE. Set to FALSE to have the event observed upon deselection.

Answer 2

As I understand a working minimal example would be the following (sel() reactive is TRUE if a row in datatable is selected):
library(shiny)
library(DT)
ui <- fluidPage(
DT::dataTableOutput("datatable"),
textOutput("any_rows_selected")
)
server <- function(input, output) {
# Output iris dataset
output$datatable <- DT::renderDataTable(iris, selection = "single")
# Reactive function to determine if a row is selected
sel <- reactive({!is.null(input$datatable_rows_selected)})
# Output result of reactive function sel
output$any_rows_selected <- renderText({
paste("Any rows selected: ", sel())
})
}
shinyApp(ui, server)

Answer 3

Alternatively you can use observe() which will respond to any hits to the input$datatable_rows_selected, including NULL.
To repurpose Kristoffer W. B.'s code:
library(shiny)
library(DT)
ui <- fluidPage(
DT::dataTableOutput("testtable")
)
server <- function(input, output) {
# Output iris dataset
output$testtable<- DT::renderDataTable(iris, selection = "single")
# Observe function that will run on NULL values
an_observe_func = observe(suspended=T, {
input$testtable_rows_selected
isolate({
#do stuff here
print(input$testtable_rows_selected)
})
})
#start the observer, without "suspended=T" the observer
# will start on init instead of when needed
an_observe_func$resume()
shinyApp(ui, server)
A few things to note:
1) I found it best to start the observer in suspended mode, that way it doesn't start when the program initializes. You can turn it on whenever you want it to... observe... (such as after you render the datatable, or before you'd like to start tracking selections).
2) Use isolate to stop the observer from tracking multiple elements. in this case the observer should only react to input$testtable_rows_selected, instead of everything else occurring. The symptom of this problem is that your observer fires multiple times on a single change.