Prevent to read file multiple times from dynamic fileInput - r

Question

I've created a dynamic fileInput in shiny using lapply. When I want to read the file, I've also used lapply in an observer.
The problem of using lapply here is, it is triggered every time I upload a new file and thus, reads all files again and again if a new file is uploaded.
Here I provide a Hello World app. The lapply function depends on an input paramter which I abtracted from for simplicity.
library(shiny)
ui <- fluidPage(
titlePanel("Hello World"),
sidebarLayout(
sidebarPanel(),
mainPanel(
lapply(1:2, function(i) {
fileInput(
paste0("file", i),
label = NULL,
multiple = F,
accept = c(
"text/csv",
"text/comma-separated-values,text/plain",
".csv"
),
buttonLabel = paste("File", i)
)
}),
verbatimTextOutput("list")
)
)
)
server <- function(input, output) {
r <- reactiveValues()
observe({
lapply(1:2, function(i) {
file <- input[[paste0("file",i)]]
if(is.null(file)) return()
isolate({
r$file[[paste(i)]] <- readr::read_csv2(file = file$datapath)
})
})
})
output$list <- renderPrint(reactiveValuesToList(r))
}
shinyApp(ui = ui, server = server)
How to replace the loop or add a requirement to lapply?

Answer 1

While I started down the road of cache-invalidation in the comments, I think something else may work better for you since you have a fixed number of fileInput fields: swap the lapply and observe lines in your code (plus a couple of other tweaks).
server <- function(input, output) {
lapply(paste0("file", 1:2), function(nm) {
observeEvent(input[[ nm ]], {
req(input[[nm]], file.exists(input[[nm]]$datapath))
readr::read_csv2(file = input[[nm]]$datapath)
})
})
}
Explanation:
I'm creating a list of reactive blocks instead of a reactive block operating on a list. This means "file1" won't react to "file2".
I short-cutted the definition of the input names by putting paste0(...) in the data of the lapply instead of in the function, though it'd be just as easy to do
lapply(1:2, function(i) {
nm <- paste0("file", i)
# ...
})
It's important to have nm defined outside of the observeEvent, and it has to do with delayed evaluation and namespace search order. I fell prey to this a few years ago and was straightened out by Joe Cheng: you can't use a for loop, it must be some environment-preserving operation like this.
N.B.: this is a stub of code, and it is far from complete: having an observe or observeEvent read the data and then discard it is wrong ... it's missing something. Ideally, this should really be a reactive or eventReactive block, or the processed data should be stored in a reactiveValues or reactiveVal. For example:
server <- function(input, output) {
mydata <- lapply(paste0("file", 1:2), function(nm) {
observeEvent(input[[ nm ]], {
req(input[[nm]], file.exists(input[[nm]]$datapath))
readr::read_csv2(file = input[[nm]]$datapath)
})
})
observe({
# the following are identical, the latter more declarative
mydata[[1]]
mydata[["file1"]]
})
}
(And another note about defensive programming: you cannot control perfectly how readr::read_csv2 reacts to that file ... it may error out for some reason. One further step would be to wrap it in tryCatch(..., error = function(e) { errfun(e); NULL; }) where errfun(e) does something meaningful with the error message (logs it and/or gives it to the user in a modal popup) and then returns NULL so that reactive blocks downstream can use req(mydata[[1]]) and will not try to process the NULL.
server <- function(input, output) {
mydata <- lapply(paste0("file", 1:2), function(nm) {
observeEvent(input[[ nm ]], {
req(input[[nm]])
file <- input[[nm]]
tryCatch(
readr::read_csv2(file = input[[nm]]$datapath),
error = function(e) { errfun(e); NULL; })
})
})
observe({
# the following are identical, the latter more declarative
mydata[[1]]
mydata[["file1"]]
})
}