Shiny record past input values

past_input.R

#' Record the past input values 'only for a session'. Persistent store approach is different. 
#' This snippet has example for selecting rows in datatable from the DT package. Reference: 
#' https://stackoverflow.com/a/41199134

library(shiny)
library(DT)

ui <- fluidPage(
  dataTableOutput("dt")
)

server <- function(input, output, session) {
  output$dt <- renderDataTable(mtcars, selection = list(mode = 'single'))
  
  row <- reactiveValues(pastval = 0, curval = 0)
  
  observeEvent(input$dt_rows_selected, {
    row$prevval <- row$curval
    row$curval <- input$dt_rows_selected
  })
  
  row.current <- reactive({
    req(input$dt_rows_selected)
    input$dt_rows_selected
    row$curval
  })
  
  row.prev <- reactive({
    req(input$dt_rows_selected)
    input$dt_rows_selected
    row$prevval
  })
  
  # print debug:
  observe({
    print(paste("input.row:", input$dt_rows_selected))
    print(sprintf("row.current:%d", row.current()))
    print(sprintf("row.prev:%d", row.prev()))
    print(paste("---"))
  })
}

shinyApp(ui, server)