How to update UI on file change - r

Question

Hello I'm building a shinydashboard using several excel files.
I inserted links to these files in the footer of the box and I want to refresh the shinydashboard when changing something in my excel file.
I don't want to run the whole R code each time.
How can I re-render the Output once the file content changes?
Here an example:
sidebar <- dashboardSidebar(
sidebarMenu( menuItem("Hello", tabName = "Hello", icon = icon("dashboard"))
))
body <- dashboardBody(
tabItems(
tabItem(tabName = "Hello",
box(title = "my file",
footer = a("df.xlsx", href="df.xlsx" ) ,
DT::dataTableOutput("df1"),style = "font-size: 100%; overflow: auto;",
width = 12, hight = NULL, solidHeader = TRUE, collapsible = TRUE, collapsed = TRUE, status = "primary")
)))
ui <- dashboardPage(
dashboardHeader(title = "My Dashboard"),
sidebar,
body)
server <- function(input, output) {
output$df1 <- renderDataTable({
df <- read_excel("df.xlsx")
DT::datatable(df, escape = FALSE, rownames=FALSE,class = "cell-border",
options =list(bSort = FALSE, paging = FALSE, info = FALSE)
)
})
}
shinyApp(ui, server)

Answer 1

To monitor the change in a file you could use the cheksum of the file like this:
library(shiny)
library(digest)
# Create data to read
write.csv(file="~/iris.csv",iris)
shinyApp(ui=shinyUI(
fluidPage(
sidebarLayout(
sidebarPanel(
textInput("path","Enter path: "),
actionButton("readFile","Read File"),
tags$hr()
),
mainPanel(
tableOutput('contents')
)))
),
server = shinyServer(function(input,output,session){
file <- reactiveValues(path=NULL,md5=NULL,rendered=FALSE)
# Read file once button is pressed
observeEvent(input$readFile,{
if ( !file.exists(input$path) ){
print("No such file")
return(NULL)
}
tryCatch({
read.csv(input$path)
file$path <- input$path
file$md5 <- digest(file$path,algo="md5",file=TRUE)
file$rendered <- FALSE
},
error = function(e) print(paste0('Error: ',e)) )
})
observe({
invalidateLater(1000,session)
print('check')
if (is.null(file$path)) return(NULL)
f <- read.csv(file$path)
# Calculate ckeksum
md5 <- digest(file$path,algo="md5",file=TRUE)
# If no change in cheksum, do nothing
if (file$md5 == md5 && file$rendered == TRUE) return(NULL)
output$contents <- renderTable({
print('render')
file$rendered <- TRUE
f
})
})
}))

Answer 2

If I understand the question correctly, I'd say you need the reactiveFileReader function.
Description from the function's reference page:
Given a file path and read function, returns a reactive data source
for the contents of the file.
The file reader will poll the file for changes, and once a change is detected the UI gets updated reactively.
Using the gallery example as a guide, I updated the server function in your example to the following:
server <- function(input, output) {
fileReaderData <- reactiveFileReader(500,filePath="df.xlsx", readFunc=read_excel)
output$df1 <- renderDataTable({
DT::datatable(fileReaderData(), escape = FALSE, rownames=FALSE,class = "cell-border",
options =list(bSort = FALSE, paging = FALSE, info = FALSE)
)
})
}
With that, any changes I saved to 'df.xlsx' were propagated almost instantly to the UI.