Notifying dependent functions by using reactive values in Shiny - r

Question

I understand that reactive values notifies any reactive functions that depend on that value as per the description here
based on this I wanted to make use of this property and create a for loop that assigns different values to my reactive values object, and in turn I am expecting another reactive function to re-execute itself as the reactive values are changing inside the for loop. Below is a simplified example of what i am trying to do:
This is the ui.R
library(shiny)
# Define UI
shinyUI(pageWithSidebar(
titlePanel("" ,"For loop with reactive values"),
# Application title
headerPanel(h5(textOutput("Dummy Example"))),
sidebarLayout(
#Sidebar
sidebarPanel(
textInput("URLtext", "Enter csv of urls", value = "", width = NULL, placeholder = "Input csv here"),
br()
),
# Main Panel
mainPanel(
h3(textOutput("caption"))
)
)
))
This is the server file:
library(shiny)
shinyServer(function(input, output) {
values = reactiveValues(a = character())
reactive({
url_df = read.table(input$URLtext)
for (i in 1:5){
values$a = as.character(url_df[i,1])
Sys.sleep(1)
}
})
output$caption <- renderText(values$a)
})
This does not give the expected result. Actually when I checked the content of values$a
it was null. Please help!

Answer 1

Rather than using a for loop, try using invalidateLater() with a step counter. Here's a working example that runs for me with an example csv found with a quick google search (first column is row index 1-100).
library(shiny)
# OP's ui code
ui <- pageWithSidebar(
titlePanel("" ,"For loop with reactive values"),
headerPanel(h5(textOutput("Dummy Example"))),
sidebarLayout(
sidebarPanel(
textInput("URLtext", "Enter csv of urls", value = "", width = NULL, placeholder = "Input csv here"),
br()
),
mainPanel(
h3(textOutput("caption"))
)
)
)
server <- function(input, output, session) {
# Index to count to count through rows
values = reactiveValues(idx = 0)
# Create a reactive data_frame to read in data from URL
url_df <- reactive({
url_df <- read.csv(input$URLtext)
})
# Reset counter (and url_df above) if the URL changes
observeEvent(input$URLtext, {values$idx = 0})
# Render output
output$caption <- renderText({
# If we have an input$URLtext
if (nchar(req(input$URLtext)) > 5) {
# Issue invalidation command and step values$idx
if (isolate(values$idx < nrow(url_df()))) {
invalidateLater(0, session)
isolate(values$idx <- values$idx + 1)
}
}
# Sleep 0.5-s, so OP can see what this is doing
Sys.sleep(0.5)
# Return row values$idx of column 1 of url_df
as.character(url_df()[values$idx, 1])
})
}
shinyApp(ui = ui, server = server)