https://data-flair.training/blogs/exporting-data-from-r/
https://mastering-shiny.org/action-transfer.html

content 의 인수인 file은 temp폴더에 생성한 파일을 나타냄
즉, temp폴더에 temp파일에 data를 넣고, filename을 부여하여 파일을 생성함.

RStudio viewer에서는 filename=function() 의 결과가 제대로 나오지 않아,
항상 browser를 사용하도록 runApp(launch.browser=T) 를 사용했다.

pacman::p_load("tidyverse","vroom")

ui <- fluidPage(
  selectInput(inputId="dataset", "Pick a dataset", ls("package:datasets")),
  tableOutput(outputId="preview"),
  
  hr(),
  radioButtons(inputId="format", "",inline=T,
    c('CSV(comma)'="csv",
      'FST(rdata)'='fst')),
  downloadButton(outputId="download1", "DownButton", icon="download"),
  downloadLink(outputId="download2",   "DownLink",   class=("fa fa-download"))
)

server <- function(input, output, session) {
  data <- reactive({
    outDf <- get(input$dataset, "package:datasets")
    if (!is.data.frame(outDf)) {
      validate(paste0("'", input$dataset, "' is not a data frame"))
    }
    return(outDf)
  })
  
  output$preview <- renderTable({
    data() %>% head(6)
  })
  
  output$download1 <- downloadHandler(
  #output$download2 <- downloadHandler(
    filename =  function() {
      fileNm <- str_c(input$dataset,"_",Sys.Date(), switch(input$format, csv='.csv', fst='.fst') )
      return(fileNm)
    },
    content = function(file) {
      # out <- write.csv(data(), file)
      data <-  data()
      out <- switch(input$format,
         csv = write.csv(data, file),
         fst = write_fst(data, file)
      )
      ## print(data())    # for TEST whether data.frame
      return(out)
    }
  )
}

shinyApp(ui, server) %>% runApp(launch.browser=T)

https://shiny.rstudio.com/gallery/file-download.html
https://shiny.rstudio.com/gallery/radiant.html

https://shiny.rstudio.com/gallery/download-knitr-reports.html
https://shiny.rstudio.com/gallery/generating-reports.html

download file zip : https://stackoverflow.com/questions/26881368/shiny-download-zip-archive

서버(로컬) 파일 다운로드

https://stackoverflow.com/questions/37241747/r-shiny-detect-and-delete-local-files-after-download

ui <- fluidPage(
  downloadButton(outputId="download1", "DownButton", icon="download"),
)

server <- function(input, output, session) {
  output$download1 <- downloadHandler(
    filename =  function() { 
      fileNm <- glue("output","_",as.numeric(Sys.Date()),".csv")
      return(fileNm)
    },
    content = function(file) {
      data <- path.expand("~/Documents/test.csv")
      out <-  file.copy(data, file)
      return(out)
    }
  )
}

shinyApp(ui, server) %>% runApp(launch.browser=T)

다운로드가 오래걸리는 경우, 로딩메세지 활용

https://stackoverflow.com/questions/54277922/loading-message-for-slow-downloadhandler

  output$downloadData <- downloadHandler(
    filename = function() { str_c(input$dataset, ".csv", sep = "") },
    content = function(file) {
      write.csv(datasetInput(), file, row.names=FALSE)
    }
  )

 output$downloadData <- downloadHandler(
    filename = function() { str_c(input$dataset, ".csv", sep = "") },
    content = function(file) {
      shiny::withProgress( message=str_c("Downloading",input$dataset," Data"), value=0, {
        shiny::incProgress(1/10)
          Sys.sleep(1)
        shiny::incProgress(5/10)
\t\t  write.csv(datasetInput(), file, row.names=FALSE)
      })
    }
  )

데이터 NULL 인 경우, 경고창 띄우기

https://community.rstudio.com/t/how-to-stop-download-in-downloadhandler/8268/2

ui <- fluidPage(
  shinyjs::useShinyjs(),
  actionButton(inputId="btn_download", "Download", icon=icon("download")),
  downloadButton(outputId="download1", "", style="visibility: hidden;")
)

server <- function(input, output) {
  data <- mtcars
  #data <- NULL
  
  observeEvent(input$btn_download, {
    if (is.null(data)) {
      #showModal( modalDialog(title='Info', p('No data!!!')))
      alert("No data!!!")
    } else {
      shinyjs::runjs("document.getElementById('download1').click();")
    }
  })
  
  output$download1 <- downloadHandler(
    filename =  function() { glue("output","_",as.numeric(Sys.Date()),".csv") },
    content = function(file) {
      out  <- file.copy(path.expand("~/Documents/test.csv"), file)
      # ------ zip for network traffic ------
      # zipr(zipfile=file, files=path.expand("~/Documents/test.csv"), include_directories=F)
      # out  <- file
    }
  )
}

shinyApp(ui,server)