Shiny conditionalpanel not evaluating at startup

Question

I'm using conditionalpanels to build up my Shiny app dynamically. It seems that the condition the panel is based on is not evaluated at startup. The panel is visible by default, and only disappears after some actions. The panel is visible, even though the condition is not met (input.select1.length = 0, which is less than 1...).

Minimal working example:

Server.R:

shinyServer(function(input, output,session){
  output$selectInput1 <- renderUI({
    selectInput(
      inputId = "select1",
      label = "Select",
      choices = c('1','2','3'),
      selected = NULL,
      multiple = TRUE
    )
  })
})

UI.R:

dashboardPage(
  title = "",

  ## Header content + dropdownMenu
  dashboardHeader(
    title = tags$b(""),
    titleWidth = 250
  ),

  ## Sidebar content
  dashboardSidebar(
    width = 250,
    sidebarMenu(
      id = "tabs",
      menuItem("tab1", tabName = "tab", icon = icon("table"))
    )
  ),

  ## Body content
  dashboardBody(
    tabItems(
      tabItem(tabName = "tab",
        div(
          div(
            style="float: left; padding-left: 20px; padding-right: 20px; width: 350px; background-color: #F4F4F4; height: calc(100vh - 50px) !important;",
            uiOutput('selectInput1')
          ),
          div(
            conditionalPanel(
              condition = "input.select1.length > 1",
              p('hi')
            )
          )
        )
      )
    )
  )
)

Answer 1

As @porkchop pointed out, you can create the condition in the renderUI function of server.R :

Server.R:

shinyServer(function(input, output,session){
  output$selectInput1 <- renderUI({
    selectInput(
      inputId = "select1",
      label = "Select",
      choices = c('1','2','3'),
      selected = NULL,
      multiple = TRUE
    )
  })
  output$panel1 <- renderUI({
    if(!is.null(input$select1) && length(input$select1) > 0){
      condition1 = 'true'
    } else {
      condition1 = 'false'
    }
    conditionalPanel(
      condition1,
      p('hi')
    )
  })
})

UI.R:

dashboardPage(
  title = "",

  ## Header content + dropdownMenu
  dashboardHeader(
    title = tags$b(""),
    titleWidth = 250
  ),

  ## Sidebar content
  dashboardSidebar(
    width = 250,
    sidebarMenu(
      id = "tabs",
      menuItem("tab1", tabName = "tab", icon = icon("table"))
    )
  ),

  ## Body content
  dashboardBody(
    tabItems(
      tabItem(tabName = "tab",
        div(
          div(
            style="float: left; padding-left: 20px; padding-right: 20px; width: 350px; background-color: #F4F4F4; height: calc(100vh - 50px) !important;",
            uiOutput('selectInput1')
          ),
          div(
            uiOutput('panel1')
          )
        )
      )
    )
  )
)