r - Shiny:如何使用默认值初始化 react 值

Question

考虑以下 actionButton 演示:
http://shiny.rstudio.com/gallery/actionbutton-demo.html

server.R:

shinyServer(function(input, output) {

  # builds a reactive expression that only invalidates 
  # when the value of input$goButton becomes out of date 
  # (i.e., when the button is pressed)
  ntext <- eventReactive(input$goButton, {
    input$n
  })

  output$nText <- renderText({
    ntext()
  })
})

ui.R:

shinyUI(pageWithSidebar(
  headerPanel("actionButton test"),
  sidebarPanel(
    numericInput("n", "N:", min = 0, max = 100, value = 50),
    br(),
    actionButton("goButton", "Go!"),
    p("Click the button to update the value displayed in the main panel.")
  ),
  mainPanel(
    verbatimTextOutput("nText")
  )
))

在此示例中，在按下操作按钮之前，右侧面板是空的。我希望默认情况下呈现默认值为“50”的文本。

如果尚未按下操作按钮，如何使用默认输入显示输出？

Answer 1

eventReactive还需要ignoreNULL如文档所示 here ，它允许您在没有 if 的情况下初始化对象陈述。

通过添加 ,ignoreNULL = FALSE到原始帖子(给出或采取一些格式)，verbatimTextOutput启动时显示 50。

我猜这在服务器端会带来一些经济上的好处。

ui <- fluidPage(titlePanel("actionButton test"),
                sidebarLayout(
                  sidebarPanel(
                    numericInput(
                      "n",
                      "N:",
                      min = 0,
                      max = 100,
                      value = 50
                    ),
                    br(),
                    actionButton("goButton", "Go!"),
                    p("Click the button to update the value displayed in the main panel.")
                  ),
                  mainPanel(verbatimTextOutput("nText"))
                ))

server <- function(input, output) {

  ntext <- eventReactive(input$goButton, {
    input$n
  }
  # Adding this parameter to the original example makes it work as intended
  # with 50 in the output field to begin with
  , ignoreNULL = FALSE
  )

  output$nText <- renderText({
    ntext()
  })
}

shinyApp(ui = ui, server = server)