How to use reactiveFileReader with a reactive filePath?

[Ted Mendum]

Hi All,

I would like to read (and periodically poll) a log file that the user has selected through the gui (using, e.g., choose.files()).  Unfortunately, the filePath argument to reactiveFileReader does not allow the use of reactives.  So something like

file_contents <- reactiveFileReader(1000, session, filePath = myfile(), readFunc = scan, what = "character", sep = "\n")

where myfile() is some reactive that returns the file name as a  string will fail with the message:

Error in .getReactiveEnvironment()$currentContext() :
  Operation not allowed without an active reactive context. (You tried to do something that can only be done from inside a reactive expression or observer.) 

How can I get a value for the filePath which is chosen by the user but is not a reactive? 

Some constraints on acceptable solutions: 
1) The name of the log file will change every time the app is used. 
2) The user can't type the log file name in using textInput or the like because I am generating the file dynamically, nor do I want to deal with potential typos. 

A reproducible example is below. 

library(shiny)

ui <- fluidPage(
    headerPanel("Reactive File Reader"),
    sidebarPanel(
        actionButton("pick_file", "Pick File"),
        verbatimTextOutput("pick_file")
    ),
    mainPanel(
        textOutput("file_contents")
    )
)

server <- function(input, output, session){
    file_name <- eventReactive(
        input$pick_file,
        file.choose()
    )

   
    output$pick_file <- renderText(
        file_name()
    )


    file_contents <- reactiveFileReader(1000, session, filePath = file_name(),
                                         readFunc = scan, what = "character",sep = "\n")
   
    output$file_contents <- renderText(file_contents())
   
}

shinyApp(ui = ui, server = server)

Thanks,

Ted

[Peter Andrews]

I know this is very old but for those interested... the myfile does not need to a reactive but simply a function that returns the current filePath. Because the reactiveFileReader runs at an interval it will call the function every time so there is no need for myfile to be a reactive. Key idea is that rather than passing a value you pass a function to get that value. That level of indirectness makes it work. 

[Joe Cheng]

True! And in fact, the interval-based checking is itself done in an observer, so I believe that if your file-path-returning function does access reactive inputs/values/expressions, the checking will happen as soon as those reactives change, not waiting for the interval to elapse.

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[andrew.r...@accumula.com]

Is there anyway to stop the reactive file reader after its use is finished? I am having an issue stopping it as it triggers my conditional shiny-busy panel.

[Sanjay Mehrotra]

Hi Joe,

Just stumbled across this old response of yours that pleasantly surprised me. Whenever I tried a reactive file path with filereader in shiny it never worked. But as you explain it must work. 

Are you confirming the below statement or are there any qualifiers?

thanks a lot.

Rgds

Sanjay

(Shiny R developer)

Joe Cheng replied: 

True! And in fact, the interval-based checking is itself done in an observer, so I believe that if your file-path-returning function does access reactive inputs/values/expressions, the checking will happen as soon as those reactives change, not waiting for the interval to elapse.

[Joe Cheng]

Yes, should still be accurate!

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