如何在文件不存在时退出downloadHandler

Question

我有一个闪亮的应用程序与一个downloadButton，它使用户能够下载一些日志文件。

由于日志文件也是由logrotate处理的，因此可能在某个时间不存在日志文件，这会在尝试下载应用程序时中断应用程序。

我怎样才能防止这种情况呢？或者，如何显示包含当前不存在日志文件的信息的modalDialog？

我尝试包含req(F)或return(FALSE)，但它们都不起作用。当前的方法是有效的，因为我创建了一个空的data.frame，然后将其导出，但这不是一个很好的解决方案。

library(shiny)
library(data.table)

## Write random log file. Uncomment the next line to make the example work.
#fwrite(x = iris, file = "logs.log")


ui <- fluidPage(
  downloadButton("showLogs", label="", title="Logs herunterladen", icon = icon("book-open"))
)

server <- function(input, output, session) {
  output$showLogs <- downloadHandler(
    filename = function() {
      paste('logs-', Sys.Date(), '.csv', sep='')
    },
    content = function(file) {
      logfile <- list.files(path = ".", pattern = basename("logs.log"))
      if (length(logfile) != 0) {
        logfile <- fread(logfile, sep = ";", header = F)
        fwrite(logfile, file, sep = ";", row.names = FALSE)
      } else {
        ## Problem is in here
        # req(F)
        # return(FALSE)
        fwrite(data.frame("No log-Files"), file, sep = ";", row.names = FALSE)
      }
    }
  )
}

shinyApp(ui, server)

Answer 1

这是一个没有reactiveTimer的解决方案。

library(shiny)
library(data.table)
library(shinyjs)

## Write random log file. Uncomment the next line to make the example work.
#fwrite(x = iris, file = "logs.log")

ui <- fluidPage(
  useShinyjs(),
  downloadButton("showLogs", label="", style = "display:none;"),
  actionButton("btn", "Download") 
)

server <- function(input, output, session) {

  observeEvent(input$btn, {
    logfile <- list.files(path = ".", pattern = basename("logs.log"))
    if(length(logfile)){
      runjs("$('#showLogs').click();")
    }
  })

  output$showLogs <- downloadHandler(
    filename = function() {
      paste('logs-', Sys.Date(), '.csv', sep='')
    },
    content = function(file) {
      logfile <- list.files(path = ".", pattern = basename("logs.log"))
      logfile <- fread(logfile, sep = ";", header = F)
      fwrite(logfile, file, sep = ";", row.names = FALSE)
    }
  )
}

shinyApp(ui, server)

Answer 2

如果在日志文件不存在且无法下载的情况下单击该按钮，此应用程序会抛出警报。

library(shiny)
library(data.table)
library(shinyjs)

## Write random log file. Uncomment the next line to make the example work.
#fwrite(x = iris, file = "logs.log")

ui <- fluidPage(
  useShinyjs(),
  downloadButton("showLogs", label="", title="Logs herunterladen", icon = icon("book-open"))
)

server <- function(input, output, session) {

  autoInvalidate <- reactiveTimer(1000)

  observe({
    autoInvalidate()
    logfile <- list.files(path = ".", pattern = basename("logs.log"))
    if(length(logfile)){
      runjs("$('#showLogs').off('click.x')")
    }else{
      runjs("$('#showLogs').off('click.x').on('click.x', function(e){alert('No log file'); e.preventDefault();})")
    }
  })

  output$showLogs <- downloadHandler(
    filename = function() {
      paste('logs-', Sys.Date(), '.csv', sep='')
    },
    content = function(file) {
      logfile <- list.files(path = ".", pattern = basename("logs.log"))
      logfile <- fread(logfile, sep = ";", header = F)
      fwrite(logfile, file, sep = ";", row.names = FALSE)
    }
  )
}

shinyApp(ui, server)