具有反应功能的动态 UI - 计时如何工作？

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 input$DisplayName。如果您运行下面的代码，您将看到 "DisplayName" 下拉列表调用 NameAndID()，后者又调用 global.r 文件中的 GetDisplayName()。该调用会填充 "DisplayName" 下拉列表。

但是，当 ReactiveTest() 尝试访问 input$DisplayName 时，它说 input$DisplayName 为 NULL，然后似乎对 ReactiveTest() 进行了第二次调用，然后它看到了 input$DisplayName

所以当您运行代码时，您将看到时间如下所示：

[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) 为什么在调用 DisplayName 下拉菜单后 ReactiveTest() 看不到 input$DisplayName？

(2) 为什么要第二次调用 ReactiveTest()？

(3) 如何让 ReactiveTest() 在第一次调用时看到 "DisplayName" 下拉列表中的内容？在我的真实代码中，我必须测试 is.null(input$DisplayName)== TRUE 而不是运行一些代码，但是没有更好的方法吗？

这是我的代码，您可以运行:

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在问同样的事情，为什么这个函数被调用了两次？

正如我在你之前的问题中提到的，反应式表达式只有在意识到输入中发生了某些变化时才会再次运行。当您使用反应式 ui 初始化闪亮的应用程序时，最初不会加载这些值（即空值）。之前已经提出了其他闪亮的问题，例如 this one on google groups and a similar one on SO，并且普遍的共识是您应该对您的代码进行某种检查（希望被证明是错误的）。所以你添加 is.null 检查的直觉是正确的。因此，这是使用 validate 函数的好机会。虽然这个初始化问题有点烦人，但初始化过程中花费的时间应该是微不足道的。

这是一个使用 validate 的例子。然而，如果以某种方式传递了空值并且传递给 validate 的任何内容不是运行，这使得提供用户友好的错误消息变得非常容易。更直接的是，在反应式表达式中，初始化只打印然后离开函数。建议使用这种类型的验证，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())       
        })  
    }
))

具有反应功能的动态 UI - 计时如何工作？

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