How to fetch data from REST API periodically? - r

Question

Background
I am currently building a „Data-Logger“ - App using R Shiny. I do have an REST - API, which returns a value, that changes over time. My goal is to create an Shiny App, in which an user can click on an actionbutton to start writing the values fetched from the api periodically (e.g. every 60 seconds) to a dataframe. The logging of the data also should be stopped, when the user clicks on another actionbutton.
Problem
My problem is writing a function that starts executing when a button is pressed, executes periodically after that and stops executing when another button is pressed.
Previous Ideas
I previously tried using invalidateLater(), but i could not achieve what i desire.
Can you guys help me out with a clever thought or idea?
Thanks in advance!

Answer 1

This should show how it works. invalidateLater() is the right choice. The start/stop buttons change a reactive expression that determines whether polling is on or off. That way, the reactive RestPoll expression gets notified every time it gets switched on/off and, of course, after 500 ms as long as Running() == TRUE.
library(shiny)
ui <- fluidPage(
actionButton("btnStart", "Start"),
actionButton("btnStop", "Stop"),
textOutput("outTime")
)
server <- function(input, output, session) {
Running <- reactiveVal(FALSE)
observeEvent(input$btnStart, {
Running(TRUE)
})
observeEvent(input$btnStop, {
Running(FALSE)
})
RestPoll <- reactive({
if (Running()) # IS called every time `Running` changes
invalidateLater(500, session)
# Add any REST calls here, process the results
return(Sys.time()) # deliver results
})
output$outTime <- renderText({
req(RestPoll())
})
}
shinyApp(ui, server)
You could also do it with a reactiveTimer but that would also poll and use resources when polling is not required.