更改数据集时，在闪亮的应用程序中动态显示列名闪烁错误

Question

我有一个闪亮的应用程序，我希望允许用户根据一组上传的文件选择数据集，然后指定要从所选数据集中显示的列。如果我选择了某些列然后切换数据集，则会闪烁错误并输出到控制台，说明在应用程序切换数据集并正确显示之前，所选列是未知的。然而，在我的完整应用程序中，应用程序崩溃了，尽管我无法弄清楚如何重现崩溃。我认为这可能与一些预处理有关，这些预处理是为了添加跨数据集相同且保持选中状态的其他列，但如果没有该功能，错误是相同的。

library(shiny)
library(tidyverse)
library(DT)


ui <- fluidPage(
  checkboxGroupInput("select_var", label = "Select Variables"),
  selectInput("dataset", label = NULL, choices = c("mtcars", "rock")),
  DT::dataTableOutput("table")

)


server <- function(session, input, output) {
  # define the dataset
  data <- reactive({switch(input$dataset,"rock" = rock,"mtcars" = mtcars)})

  # add a common column name that is always selected
  dataprocessed <- reactive({data <- data()
                             data$num <- seq(1:nrow(data))
                             return(data)})

  # dynamically generate the variable names
  observe({
    vchoices <- names(dataprocessed())
    updateCheckboxGroupInput(session, "select_var", choices = vchoices, selected = c("num"))
  })

  # select the variables based on checkbox
  data_sel <- reactive({
    req(input$select_var)
    df_sel <- dataprocessed() %>% select(input$select_var) 
      })

  output$table <- DT::renderDataTable(data_sel())
}

# Run the application 
shinyApp(ui = ui, server = server)

Answer 1

我们可以在渲染前使用req()添加条件要求来测试列是否存在：

library(shiny)
library(tidyverse)
library(DT)

ui <- fluidPage(
  checkboxGroupInput("select_var", label = "Select Variables"),
  selectInput("dataset", label = NULL, choices = c("mtcars", "rock")),
  DT::dataTableOutput("table")

)

server <- function(session, input, output) {
  # define the dataset
  data <- reactive({
    switch(input$dataset,"rock" = rock,"mtcars" = mtcars)
  })

  # add a common column name that is always selected
  dataprocessed <- reactive({
    data <- data()
    data$num <- seq(1:nrow(data))
    return(data)
  })

  # dynamically generate the variable names
  observe({
    vchoices <- names(dataprocessed())
    updateCheckboxGroupInput(session, "select_var", choices = vchoices, selected = c("num"))
  })

  # select the variables based on checkbox
  data_sel <- reactive({
    req(input$select_var)
    req(names(dataprocessed()) %in% input$select_var)
    a <- names(dataprocessed())[names(dataprocessed()) %in% input$select_var]
    df_sel <- dataprocessed() %>% select(a) 
  })

  output$table <- DT::renderDataTable(data_sel())
}

# Run the application 
shinyApp(ui = ui, server = server)