Question

我正在使用renderUI创建一个“DisplayName”输入，我有一个名为“ReactiveTest”的反应函数，它使用输入$ DisplayName。如果运行下面的代码，您将看到“DisplayName”下拉列表调用NameAndID（），而NameAndID（）又调用global.r文件中的GetDisplayName（）。该调用填充了“DisplayName”下拉列表。

但是当ReactiveTest（）尝试访问输入$ DisplayName时，它表示输入$ DisplayName为NULL然后似乎第二次调用ReactiveTest（）然后它看到输入$ DisplayName

因此，当您运行代码时，您会看到时间如下：

[1] "Department dropdown list is being returned now"
[1] "Department dropdown list is being returned now"

Listening on http://155.0.0.1:555
[1] "DISPLAY NAME dropdown list is being returned now"
[1] "Now ReactiveTest() tries to access input$DisplayName"
[1] "ReactiveTest() says input$DisplayName Name is NULL" --> why is it NULL? if 2 lines above the dropdown was populated???
[1] "Now ReactiveTest() tries to access input$DisplayName"--> Where is this coming from? Shouldn't there only be  1 call to ReactiveTest()
[1] "ReactiveTest() says input$DisplayName Name is Bob"

我有3个问题：

（1）为什么ReactiveTest（）在调用DisplayName下拉列表后看不到输入$ DisplayName？

（2）为什么第二次调用ReactiveTest（）？

（3）如何让ReactiveTest（）在第一次调用的“DisplayName”下拉列表中看到什么？在我的真实代码中，我必须测试is.null（输入$ DisplayName）== TRUE然后不运行一些代码，但是没有更好的方法吗？

以下是我可以运行的代码：

library(shiny)

runApp(list(
  ui = basicPage(
    sidebarPanel(
      selectInput("Department", "Select a department", 
                  choices = as.character(GetDepartments()$Department), 
                  selected = as.character(GetDepartments()$Department[1])),
      uiOutput("DisplayName")
    ),
    mainPanel(textOutput("Text") )
  ),
  server = function(input, output, session) {

    NameAndID<- reactive({
      GetDisplayName(input$Department)
    })

    output$DisplayName<-renderUI({
      Department <- input$Department
      selectInput("DisplayName", 'DisplayName:', choices = as.character(NameAndID()$DisplayName), selected =as.character(NameAndID()$DisplayName[1] ))
    })

    ReactiveTest<- reactive({
      print("Now ReactiveTest() tries to access input$DisplayName")
      if(is.null(input$DisplayName) == TRUE)
      {
        print("ReactiveTest() says input$DisplayName Name is NULL")
        return("NULL")
      }else
      {
        print(paste0("ReactiveTest() says input$DisplayName Name is ", input$DisplayName))
        return(input$DisplayName)
      }
    })

    output$Text <- renderText({
      #myData <- GetDisplayName(input$Department)
      #code <- as.character(myData[myData$DisplayName == input$DisplayName,2])

      return(ReactiveTest())       
    })

  }
))

global.r

GetDepartments<- function(){
  df<- data.frame(Department= c("Dept A", "Dept B"), id = c(1,2))
  print("Department dropdown list is being returned now")
  return(df)
}

GetDisplayName<- function(Dept){

  if(Dept == "Dept A")
  {
    df<- data.frame(DisplayName= c("Bob", "Fred"), id = c(4,6))
    print("DISPLAY NAME dropdown list is being returned now")
    return(df)
  }else
  {
    df<- data.frame(DisplayName= c("George", "Mary"), id = c(10,20))
    print("DISPLAY NAME dropdown list is being returned now")
    return(df)
  }

}

Answer 1

你的问题基本上都是问同样的事情，为什么函数会被调用两次？

正如我在上一个问题中提到的，如果反应式表达式意识到输入中的某些内容发生了变化，它们将只会再次运行。使用反应式ui初始化闪亮的应用程序时，不会初始加载值（即空值）。之前提出过其他一些有待提问的问题，例如Google群组中的this one和SO上类似的问题，而且普遍的共识是您应该对代码进行某种检查（我会喜欢被证明是错误的）。因此，您添加is.null检查的直觉是正确的。因此，这是使用validate函数的好机会。虽然这是初始化问题有点烦人，但初始化过程中所花费的时间应该是微不足道的。

以下是使用validate的示例。但是，如果以某种方式传递空值并且未运行传递validate的任何内容，则可以很容易地提供用户友好的错误消息。更直接的是，在反应式表达式中，初始化仅打印然后离开函数。建议使用此类验证，并且可以找到文档here

runApp(list(
    ui = basicPage(
        sidebarPanel(
            selectInput("Department", "Select a department", 
                        choices = as.character(GetDepartments()$Department), 
                        selected = as.character(GetDepartments()$Department[1])),
            uiOutput("DisplayName")
        ),
        mainPanel(textOutput("Text") )
    ),
    server = function(input, output, session) {

        NameAndID<- reactive({
            GetDisplayName(input$Department)
        })

        output$DisplayName<-renderUI({
            Department <- input$Department
            selectInput("DisplayName", 'DisplayName:', choices = as.character(NameAndID()$DisplayName), selected =as.character(NameAndID()$DisplayName[1] ))
        })

        ReactiveTest<- reactive({
            print("Now ReactiveTest() tries to access input$DisplayName")
            validate(
                need(!is.null(input$DisplayName), "ReactiveTest() says input$DisplayName Name is NULL")
            )
            print(paste0("ReactiveTest() says input$DisplayName Name is ", input$DisplayName))
            return(input$DisplayName)
        })

        output$Text <- renderText({            
            return(ReactiveTest())       
        })  
    }
))

具有Reactive功能的动态UI - 时序如何工作？

1 个答案: