input hierarchy

app.R

library(shiny)

file_structure <- list(
   LEVEL_1 = list(
      LEVEL_2a = list(
         LEVEL_3a1 = list(
            LEVEL_4a1 = list()
         ),
         LEVEL_3a2 = list(
            LEVEL_4a3 = list()
         ),
         LEVEL_3a3 = list(
            LEVEL_4a3 = list()
         )
         ),
      LEVEL_2b = list(
         LEVEL_3b1 = list(),
         LEVEL_3b1 = list(),
         LEVEL_3b2 = list()
      )
   )
)

ui <- fluidPage(
  selectInput("level1", "Level 1", choices = NULL),
  selectInput("level2", "Level 2", choices = NULL),
  selectInput("level3", "Level 3", choices = NULL),
  selectInput("level4", "Level 4", choices = NULL)
)

server <- function(input, output, session) {
  
   
   # Create L1 using box_config - this doesnt change 
   observe({
      updateSelectInput(session, 
                        "level1", 
                        choices = names(file_structure))
   })
   
   # L2 contingent on L1
   L2 <- reactive({
      req(input$level1)
      file_structure[[1]]
   })
   
   observeEvent(L2(), { 
      updateSelectInput(session, "level2", choices = names(L2())) 
   })
   
   # L3 contingent on L2
   L3 <- reactive({
      req(input$level1)
      # browser()
      
      if (input$level2 %in% names(L2())) {
         L2()[[input$level2]]
      } else {
         return(list())
      }
      
   })
   
   observeEvent(L3(), { 
      updateSelectInput(session, "level3", choices = names(L3())) 
   })
   
   # L4 contingent on L3
   L4 <- reactive({
      req(input$level3)
      
      if (input$level3 %in% names(L3())) {
         return(L3()[[input$level3]])
      } else {
         return(list())
      }
      
   })
   
   observeEvent(L4(), { 
      print(L4()) 
      updateSelectInput(session, inputId = "level4", choices = names(L4())) 
   })
   
}

shinyApp(ui, server)