Force shiny to render plot in loop - r

Question

I have a shiny app that runs a simulation. The goal is to show the user the calculation steps in between as a plot.
How do I force shiny to update the plot?
An MWE would look like this
library(shiny)
server <- function(input, output, session) {
# base plot as a placeholder
output$myplot <- renderPlot(plot(1:1, main = "Placeholder"))
# wait until the button is triggered
observeEvent(input$run, {
print("Do some calculations in 3 steps")
for (i in seq_len(3)) {
print("Do some calculations")
# ...
x <- seq_len(i * 100)
y <- (x + 1)^2 - 1 # this will do for now
print("Plot the data ")
# ISSUE HERE!
# this should render the current step of the simulation, instead it
# renders only after the whole code is run (i.e., after step 3)
output$myplot <- renderPlot(plot(x, y, main = sprintf("Round %i", i), type = "l"))
print("Wait for 1 second for the user to appreciate the plot...")
Sys.sleep(1)
}
})
}
ui <- fluidPage(
actionButton("run", "START"),
plotOutput("myplot")
)
shinyApp(ui = ui, server = server)
The issue is, that shiny runs the code and produces one plot at the end of the simulation, however, I want to get a plot at each simulation step (displayed for at least one second).
Any help/hint is greatly appreciated.
Appendix
I have looked at this post, but replacing the text with a plot/renderPlot doesn't yield the correct results.

Answer 1

You could nest an observer into an observeEvent to make it work. Based on Jeff Allen's code from the SO topic you linked.
Crucial part:
observeEvent(input$run, {
rv$i <- 0
observe({
isolate({
rv$i <- rv$i + 1
})
if (isolate(rv$i) < maxIter){
invalidateLater(2000, session)
}
})
})
Full code:
library(shiny)
server <- function(input, output, session) {
rv <- reactiveValues(i = 0)
maxIter <- 3
output$myplot <- renderPlot( {
if(rv$i > 0) {
x <- seq_len(rv$i * 100)
y <- (x + 1)^2 - 1 # this will do for now
plot(x, y, main = sprintf("Round %i", rv$i), type = "l")
} else {
plot(1:1, main = "Placeholder")
}
})
observeEvent(input$run, {
rv$i <- 0
observe({
isolate({
rv$i <- rv$i + 1
})
if (isolate(rv$i) < maxIter){
invalidateLater(2000, session)
}
})
})
}
ui <- fluidPage(
actionButton("run", "START"),
plotOutput("myplot")
)
shinyApp(ui = ui, server = server)