How to “remember” information from the previous iteration when using reactiveTimer in Shiny?

Question

I am trying (as a simple example) to make a Shiny app that graphs a single point whose coordinates update periodically using reactiveTimer() . Here is what I have:

library(shiny)
library(ggplot2)


server <- shinyServer(function(input, output, session) {
            autoInvalidate <- reactiveTimer(4000, session)

            output$myplot <- renderPlot({
             autoInvalidate()
             coords_curr <- as.data.frame(t((runif(2))))

             ggplot(coords_curr, aes(V1,V2)) + 
              geom_point() +
              xlim(0,1) + ylim(0,1) 
           })
         })

ui <- shinyUI(fluidPage(
       mainPanel(plotOutput("myplot"))
      ))


shinyApp(ui, server)

Is there a way to have this iteration's coordinates depend on the coordinates from the previous iteration? So, for instance, I would like to have something like:

coords_curr <- coords_prev + as.data.frame(t((runif(2, min=0, max=0.25))))

where coords_prev are the previous iteration's coords_curr . Is there a way to save information from the previous iteration?

Thank you!

Answer 1

One way of doing it is by using global assignment operator <<- .

rm(list = ls())
library(shiny)
library(ggplot2)

server <- shinyServer(function(input, output, session) {
  autoInvalidate <- reactiveTimer(500, session)
  # Initiate the values
  coords_prev  <<- 0

  output$myplot <- renderPlot({
    autoInvalidate()
    coords_curr <- coords_prev  + as.data.frame(t((runif(2, min=0, max=0.25))))
    # Add previous value to it
    coords_prev <<- coords_curr
    ggplot(coords_curr, aes(V1,V2))+ geom_point() + xlim(0,1) + ylim(0,1) 
  })
})

ui <- fluidPage(mainPanel(plotOutput("myplot")))
shinyApp(ui, server)

Answer 2

You can use reactiveValues :

library(shiny)
library(ggplot2)
server <- shinyServer(function(input, output, session) {
  autoInvalidate <- reactiveTimer(500, session)

  myReactives  <- reactiveValues(myCords = c(0,0))

  output$myplot <- renderPlot({

    autoInvalidate()
    coords_curr <- isolate({
      myReactives$myCords  + as.data.frame(t((runif(2, min=0, max=0.25))))
    })
    # Add previous value to it
    myReactives$myCords <- coords_curr
    ggplot(coords_curr, aes(V1,V2))+ geom_point() + xlim(0,1) + ylim(0,1) 
  })
})

ui <- fluidPage(mainPanel(plotOutput("myplot")))
shinyApp(ui, server)