Can I save the old value of a reactive object when it changes?

Question

Note: After coming up with the answer I reworded the question to make if clearer.

Sometimes in a shiny app. I want to make use of a value s

Answer 1

Seeing as the session flush event method seems to be broken for this purpose, here is an alternative way to do it using an observeEvent construct and a reactive variable.

library(shiny)

ui <- fluidPage(
  h1("Memory"),
  sidebarLayout(
    sidebarPanel(
      numericInput("val", "Next Value", 10)
    ),
    mainPanel(
      verbatimTextOutput("curval"),
      verbatimTextOutput("lstval")
    )
  )
)

server <- function(input,output,session) {
  rv <- reactiveValues(lstval=0,curval=0)

  observeEvent(input$val, {rv$lstval <- rv$curval; rv$curval <- input$val})

  curre <- reactive({req(input$val);  input$val; rv$curval})
  lstre <- reactive({req(input$val);  input$val; rv$lstval})

  output$curval <- renderPrint({sprintf("cur:%d",curre())})
  output$lstval <- renderPrint({sprintf("lst:%d",lstre())})
}
options(shiny.reactlog = TRUE)
shinyApp(ui, server)

Yielding: