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:

Answer 2

Update This answer was posted before the advent of the reactiveValues/observeEvent model in shiny. I think that @MikeWise 's answer is the better way to do this.

After some playing around this is what I came up with. The ui.r is nothing special

ui.r

library(shiny)

ui <- shinyUI(fluidPage(
  sidebarLayout(
    sidebarPanel(
      selectizeInput(inputId="XX", label="Choose a letter",choices=letters[1:5])
    ),
    mainPanel(
      textOutput("Current"),
      textOutput("old")
    )
  ) 
))  

"Current" will display the current selection and "old" displays the previous selection.

In the server.r I made use of three key functions: reactiveValues, isolate and session$onFlush.

server.r

library(shiny)

server <- function(input, output,session) {

  Values<-reactiveValues(old="Start")

  session$onFlush(once=FALSE, function(){
    isolate({ Values$old<-input$XX })
  })


  output$Current <- renderText({paste("Current:",input$XX)})

  output$old <- renderText({ paste("Old:",Values$old) })
}  

The server.r works like this.

First, Values$old is created using the reactiveValues function. I gave it the value "Start" to make it clear what was happening on load up.

Then I added a session$onFlush function. Note that I have session as an argument in my server function. This will run every time that shiny flushes the reactive system - such as when the selectizeInput is changed by the user. What is important is that it will run before input$XX gets a new value - so the value has changed at the selectizeInput but not at XX.

Inside the session$onFlush I then assign the outgoing value of XX to Values$old. This is done inside an isolate() as this will prevent any problems with input$XX gets updated with the new values. I can then use input$XX and Values$old in the renderText() functions.