Shiny: create a selectInput after choosing a value of another selectInput - r

Question

I have a shiny application where I want to build a conditional query system on a data frame.
First, I want to have a selectInput showing all available tables. After the user has chosen a table, I want another box to appear where he can select the column name he wants to filter for. This is what I have until now:
ui.r:
library(shiny)
library(shinydashboard)
source("global_variables.r")
ui=dashboardPage(
dashboardHeader(title="Test"),
dashboardSidebar(
sidebarMenu(
menuItem("Conditionals",tabName = "conditionals")
)
),
dashboardBody(
tabItems(
tabItem(tabName="conditionals",
fluidRow(
box(
title = "Conditionals",
width = 4,
selectInput("Con_tableName",choices=c("NONE",tableNames),label = "Table Name"),
tags$div(id = 'placeholder')
)
)
)
)
)
)
server.r:
library(shiny)
source("global_variables.r", local = FALSE)
Table1=data.frame();
shinyServer(
function(input, output,session) {
observe({
reactive(
if(input$Con_tableName!="NONE"){
insertUI( selector="#placeholder",
ui={
selectInput("Con_colName",choices=c("NONE",colnames(dynGet(input$Con_tableName))),label = "Column Name")
}
)
}
)
})
}
)
global_variables.r:
tableNames=c("Table1","Table2","Table3")
The problem is, that if I choose a value in the selectInput, observe doesnt get fired.
EDIT:
According to BigDataScientists comment,changed insertUI to renderUI. Updated files:
ui.r:
library(shiny)
library(shinydashboard)
source("global_variables.r")
ui=dashboardPage(
dashboardHeader(title="Test"),
dashboardSidebar(
sidebarMenu(
menuItem("Conditionals",tabName = "conditionals")
)
),
dashboardBody(
tabItems(
tabItem(tabName="conditionals",
fluidRow(
box(
title = "Conditionals",
width = 4,
selectInput("Con_tableName",choices=c("NONE",tableNames),label = "Table Name"),
uiOutput("conditionalUI")
)
)
)
)
)
)
server.r:
library(shiny)
source("global_variables.r", local = FALSE)
Table1=data.frame();
shinyServer(
function(input, output,session) {
observeEvent(input$Con_tableName,{
reactive(
if(input$Con_tableName!="NONE"){
output$conditionalUI=renderUI({
selectInput("Con_colName",choices=c("NONE",colnames(input$Con_tableName)),label = "Column Name")
})
}
)
})
}
)

Answer 1

You can use conditionalPanel(). Below there is a small example which might work in your case.
library(shiny)
shinyApp(
ui <- fluidPage(
mainPanel(
selectInput("input1", "Select something", choices = c('','1','2','3')),
conditionalPanel("input.input1!=''",
selectInput('input2', "Select something else", choices = c('4','5')))
)
),
server <- function(input, output){}
)