如何在闪亮的应用程序中在callr::r_bg中使用downloadHandler

Question

我在下面的最小示例中模拟的场景是允许用户使用闪亮的应用程序(单击numericInput控件并看到服务器端事件发生)，同时进行长时间运行的下载( Sys.sleep(10)在downloadHandler中进行模拟)。

在同步设置中，当单击"Download“按钮时，用户仍然可以与UI元素交互，但是其他闪亮的计算(在本例中是renderText)会被放到队列中。我想要异步设置，下载在后台进行，用户仍然可以与UI元素和获得所需的输出(例如renderText)进行交互。

我正在使用callr::r_bg()来实现闪亮内部的异步性，但问题是我当前的downloadHandler代码是不正确的(mtcars应该被下载，但是代码无法完成下载，404错误消息)，我相信这是由于downloadHandler期望content()函数编写的特定方式，而我编写callr::r_bg()的方式没有很好地处理这个问题。任何见解都将不胜感激！

参考：

https://www.r-bloggers.com/2020/04/asynchronous-background-execution-in-shiny-using-callr/

最小示例：

library(shiny)

ui <- fluidPage(
  downloadButton("download", "Download"),
  
  numericInput("count",
               NULL,
               1,
               step = 1),
  
  textOutput("text")
)

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

  long_download <- function(file) {
    Sys.sleep(10)
    write.csv(mtcars, file)
  }
  
  output$download <- downloadHandler(
    filename = "data.csv",
    content = function(file) {
      x <- callr::r_bg(
        func = long_download,
        args = list(file)
      )
      return(x)
    }
  )
  
  observeEvent(input$count, {
    output$text <- renderText({
      paste(input$count)
    })
  })
  
}

shinyApp(ui, server)

Answer 1

我想出了一个解决方案，并学到了以下几点：

• 因为downloadHandler没有传统的input$X，所以很难以传统的方式包含反应性。解决方法是将UI呈现为隐藏的downlodButton，由用户将看到的actionButton屏蔽。在以下过程中促进了反应性:用户单击actionButton -> Reactivity (当反应性完成(reactive()$is_alive() == FALSE)时)，使用shinyjs::click启动将callr函数放置在downloadHandler中的callr函数，我将文件保存在内容arg中。定义范围似乎有一些困难，因为文件需要在environment
• I'm函数中可用，使用一个反应性函数来跟踪后台作业(长期运行的计算)何时使用语法启动下载：reactive()$is_alive()
• The invalidateLater()和切换全局变量(download_once)对于防止反应性不断激活非常重要。没有它，你的浏览器就会无止境地下载文件--这种行为是可怕的，并且会出现病毒--就像你闪亮的应用程序用户一样！
• 注意到，设置全局变量不是闪亮应用程序的最佳实践(会想到一个更好的
• )。

代码解决方案：

library(shiny)
library(callr)
library(shinyjs)

ui <- fluidPage(
  shinyjs::useShinyjs(),
  #creating a hidden download button, since callr requires an input$,
  #but downloadButton does not natively have an input$
  actionButton("start", "Start Long Download", icon = icon("download")),
  downloadButton("download", "Download", style = "visibility:hidden;"),

  p("You can still interact with app during computation"),
  numericInput("count",
               NULL,
               1,
               step = 1),
  
  textOutput("text"),
  
  textOutput("did_it_work")
)

long_job <- function() { 
  Sys.sleep(5)
}

server <- function(input, output, session) {
  
  #start async task which waits 5 sec then virtually clicks download
  long_run <- eventReactive(input$start, {
    #r_bg by default sets env of function to .GlobalEnv
    x <- callr::r_bg(
      func = long_job,
      supervise = TRUE
    )
    return(x)
  })
  
  #desired output = download of mtcars file
  output$download <- downloadHandler(filename = "test.csv",
                                     content = function(file) {
                                       write.csv(mtcars, file)
                                     })
  
  #output that's meant to let user know they can still interact with app
  output$text <- renderText({
    paste(input$count)
  })
  
  download_once <- TRUE
  
  #output that tracks progress of background task
  check <- reactive({
    invalidateLater(millis = 1000, session = session)
    
    if (long_run()$is_alive()) {
      x <- "Job running in background"
    } else {
      x <- "Async job in background completed"
    
      if(isTRUE(download_once)) {
        shinyjs::click("download")
        download_once <<- FALSE
      }
      invalidateLater(millis = 1, session = session)
    }
    return(x)
  })
  
  output$did_it_work <- renderText({
    check()
  })
}

shinyApp(ui, server)

Answer 2

谢谢你的回答。我认为云很容易改进。invalidateLater应该非常小心地使用，并且只在需要的时候使用。我只使用invalidateLater一次，并将其移到等待结果的逻辑部分。因此，我们并不是无限地使反应性失效。

library(shiny)
library(callr)
library(shinyjs)

ui <- fluidPage(
  shinyjs::useShinyjs(),
  #creating a hidden download button, since callr requires an input$,
  #but downloadButton does not natively have an input$
  actionButton("start", "Start Long Download", icon = icon("download")),
  downloadButton("download", "Download", style = "visibility:hidden;"),
  
  p("You can still interact with app during computation"),
  numericInput("count",
               NULL,
               1,
               step = 1),
  
  textOutput("text"),
  
  textOutput("did_it_work")
)

long_job <- function() { 
  Sys.sleep(5)
}

server <- function(input, output, session) {
  
  #start async task which waits 5 sec then virtually clicks download
  long_run <- eventReactive(input$start, {
    #r_bg by default sets env of function to .GlobalEnv
    x <- callr::r_bg(
      func = long_job,
      supervise = TRUE
    )
    return(x)
  })
  
  #desired output = download of mtcars file
  output$download <- downloadHandler(filename = "test.csv",
                                     content = function(file) {
                                       write.csv(mtcars, file)
                                     })
  
  #output that's meant to let user know they can still interact with app
  output$text <- renderText({
    paste(input$count)
  })
  
  #output that tracks progress of background task
  check <- reactive({
    if (long_run()$is_alive()) {
      x <- "Job running in background"
      invalidateLater(millis = 1000, session = session)
    } else {
      x <- "Async job in background completed"
      shinyjs::click("download")
    }
    return(x)
  })
  
  output$did_it_work <- renderText({
    check()
  })
}

shinyApp(ui, server)