R代码在发送到Javascript时会执行多次

我有这个应用程序：

    #
# This is a Shiny web application. You can run the application by clicking
# the 'Run App' button above.
#
# Find out more about building applications with Shiny here:
#
#    http://shiny.rstudio.com/
#

library(shiny)

# Define UI for application that draws a histogram
ui <- fluidPage(includeScript("www/script.js"),

   # Application title
   titlePanel("Old Faithful Geyser Data"),

   # Sidebar with a slider input for number of bins 
   sidebarLayout(
      sidebarPanel(
         sliderInput("bins",
                     "Number of bins:",
                     min = 1,
                     max = 50,
                     value = 30)
      ),

      # Show a plot of the generated distribution
      mainPanel(
         plotOutput("distPlot")
      )
   )
)

# Define server logic required to draw a histogram
server <- function(session, input, output) {

   output$distPlot <- renderPlot({
      # generate bins based on input$bins from ui.R
      x    <- faithful[, 2] 
      bins <- seq(min(x), max(x), length.out = input$bins + 1)

      observe({
          if(input$bins > 25) {
              Message1 = input$bins
              session$sendCustomMessage("bla", Message1)
          }
      })

      # draw the histogram with the specified number of bins
      hist(x, breaks = bins, col = 'darkgray', border = 'white')
   })
}

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

---

代码运行得非常完美，但每次我更改滑块时，代码的执行次数似乎比以前多了一次。更改2次后，我收到3个警报，例如。为什么会发生这种情况？我能做些什么来阻止它呢？

之所以会出现这种情况，是因为您的

observe（）

位于

renderPlot（）函数中。一般来说，观察者不应该在渲染函数中，这几乎总是导致发生非常奇怪的未定义行为

只需将观察者移到渲染函数之外即可解决问题。这还解决了您没有提到的另一个问题，即警报框实际上显示的是以前的编号，而不是当前编号

为完整起见，这是正确的服务器代码：

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

  output$distPlot <- renderPlot({
    # generate bins based on input$bins from ui.R
    x    <- faithful[, 2] 
    bins <- seq(min(x), max(x), length.out = input$bins + 1)

    # draw the histogram with the specified number of bins
    hist(x, breaks = bins, col = 'darkgray', border = 'white')
  })


  observe({
    if(input$bins > 25) {
      Message1 = input$bins
      session$sendCustomMessage("bla", Message1)
    }
  })
}

server如果此问题被标记为已解决，请不要忘记接受答案（单击我答案旁边的箭头）
server <- function(session, input, output) {

  output$distPlot <- renderPlot({
    # generate bins based on input$bins from ui.R
    x    <- faithful[, 2] 
    bins <- seq(min(x), max(x), length.out = input$bins + 1)

    # draw the histogram with the specified number of bins
    hist(x, breaks = bins, col = 'darkgray', border = 'white')
  })


  observe({
    if(input$bins > 25) {
      Message1 = input$bins
      session$sendCustomMessage("bla", Message1)
    }
  })
}