Shiny技术问题：仅点击Action Button后显示Download Button

原代码中控制下载按钮显示的逻辑未生效，核心原因是actionButton的input$bttn1初始值为0，而非NULL，所以用is.null(input$bttn1)的判断条件永远不成立，导致下载按钮始终显示。

下面提供两种简洁的解决方案：

方案一：修正判断逻辑并初始隐藏按钮 #

直接基于input$bttn1的数值判断是否点击过按钮（初始为0，点击后递增），同时在应用启动时先隐藏下载按钮：

## downloadButton
output$ui_download <- renderUI({
  downloadButton("download", "Download")
})

# 控制按钮显示隐藏的逻辑
observe({
  if (input$bttn1 == 0) { # 未点击过Add按钮时
    shinyjs::hide("download")
  } else {
    shinyjs::show("download")
  }
})

# 初始启动时隐藏按钮，设置高优先级确保先执行
observe({
  shinyjs::hide("download")
}, priority = 10)

方案二：用req()控制按钮渲染（更推荐） #

利用Shiny的req()函数，只有当点击Add按钮后生成的rv1$values存在时，才渲染下载按钮。这种方式无需额外控制显示隐藏，更符合响应式编程逻辑：

## downloadButton - 仅当rv1$values存在时渲染按钮
output$ui_download <- renderUI({
  req(rv1$values)
  downloadButton("download", "Download")
})

完整修正后的代码（方案二版本） #

##### ui
ui <- dashboardPage(
  skin = "black",
  
  #### Upper navigation bar
  dashboardHeader(
    title = "title",
    titleWidth = 230
  ),
  
  
  #### Side bar
  dashboardSidebar(disable = T),
  
  
  #### Body
  dashboardBody(
    shinyjs::useShinyjs(),
    
    tabsetPanel(
      tabPanel(
        
        ### tab1
        tabName = "tab1", 
        h5("Tab 1"),
        
        fluidRow(
          
          reactableOutput("table1"),
          
          textInput("textinput1", "Initials:"),
          actionButton("bttn1", "Add", class = "btn-primary"),
          
          reactableOutput("table2"),
          uiOutput("ui_download")
          
        ) # fluidRow
      ), # tabPanel
      
      
      #### tab2
      tabPanel(
        tabName = "tab2", 
        h5("Tab 2"),
        
        fluidRow(
          
        ) # fluidRow
      ) # tabPanel
    ) # tabsetPanel
    
    
  ) # dashboardBody
) # dashboardPage


#### server 
server <- function(input, output, session) {
  
  ## df1
  # reactive
  df1 <- reactive({
    data.frame(
      "id" = c("A", "A", "A", "A"),
      "num1" = c(10, 11, 12, 13)
    )
  })
  
  # renderReactable
  output$table1 <- renderReactable({
    reactable(df1(), borderless = F, defaultColDef = colDef(align = "center"))
  })
  
  ## df2
  ## Text input 
  rv1 <- reactiveValues()
  
  observe({
    if (nrow(df1()) == 0) {shinyjs::hide("bttn1")} 
    else {shinyjs::show("bttn1")}
  })
  
  observe({
    if (nrow(df1()) == 0) {shinyjs::hide("textinput1")} 
    else {shinyjs::show("textinput1")}
  })
  
  observeEvent(input$bttn1, {
    rv1$values <- df1()
    rv1$values$name <- input$textinput1
  })
  
  rv1_text <- reactive({
    rv1$values
  })
  
  output$table2 <- renderReactable({
    req(rv1_text())
    reactable(rv1_text(), borderless = F, defaultColDef = colDef(align = "center"))
  })
  
  ## downloadButton
  # renderUI - 仅当rv1$values存在时渲染按钮
  output$ui_download <- renderUI({
    req(rv1$values)
    downloadButton("download", "Download")
  })
  
  # Download csv
  output$download <- downloadHandler(
    filename = function() {
      paste0('data', '.csv')
    },
    
    content = function(file) {
      write.csv(rv1_text(), file, row.names = F)
    }
  )
  
}

shinyApp(ui, server)

内容的提问来源于stack exchange，提问作者shsh