How to update reactive element based on some condition and reassign ?

[AbhishekHP]

Please find the reprex below:
my_react is a reactive element and we want to update based on selectInput.

However, I tried to reassign it to the same reactive element but it gave an error.

Is there a way to reassign the reactive element to itself based on some condition user's select?

library(shiny)
library(tidyverse)

# Module UI to read content
mod_readUI <- function(id) {
  ns <- shiny::NS(id)
  shiny::tagList(
    fileInput(ns("file1"), 
              h3("Choose xlsx file"),
              accept=c(".xlsx")),
    actionButton(ns("ref"), "Refresh")
  )
}

# Server modules of reading content
mod_read <- function(input, output, session, choice){
  
  dropDown = reactive({
    choice() 
  })
  # Uploaded data as reactive element
  my_react <- reactive({
    req(input$file1) # Ensure file is uploaded
    if(!is.null(input$file1)){
      my_data <- readxl::read_excel(input$file1$datapath)
      my_data
    }
    else{
      my_data <- "nothing" %>% as.data.frame()
      my_data
    }
  })
  ### Basically, want to update the reactive element ONLY if dropdown  == "cyl"
  my_react <- reactive({
  # browser()
    if(dropDown() == "cyl"){
    my_react() %>% select(dropDown())
    } else{
      my_react()
    }
  })
  
  ### In order to pass data as reactive elements to other module:
  # Created list
  reactive({
    # browser()
    list("excl" = my_react()#,
         #"incl" = my_react_another()
         )
  })
}

# Module UI to display content
mod_displayUI <- function(id) {
  ns <- shiny::NS(id)
  shiny::tagList(
    DT::dataTableOutput(ns("contents"))
  )
}

# Server functions
mod_display <- function(input, output, session, file) {
  output$contents <- DT::renderDataTable({
    req(file()) # ensures rendertable waits until file is available
    DT::datatable(file()$excl,
                  options = list(pageLength = 7,scrollX = TRUE))
  })
}

ui <- 
  shinydashboard::dashboardPage(
    shinydashboard::dashboardHeader(),
    shinydashboard::dashboardSidebar(
      shinydashboard::sidebarMenu(id = "menu1",
                                  shinydashboard::menuItem('mod1',
                                                           tabName = 'mod1', 
                                                           icon = shiny::icon('file')),
                                  
                                  shinydashboard::menuItem('mod2',
                                                           tabName = 'mod2', 
                                                           icon = shiny::icon('file')),
                                  selectInput("sel","Choose",
                                              c("Cylinders" = "cyl",
                                                "Transmission" = "am",
                                                "Gears" = "gear"))
      )),
    shinydashboard::dashboardBody(
      shinydashboard::tabItems(
        shinydashboard::tabItem("mod1",
                                mod_readUI("sidemod1")),
        shinydashboard::tabItem("mod2",
                                mod_displayUI("bodymod2")
        )
      )))

server <- function(input, output) {
  
  choice <- reactive({
    input$sel})
  # storing mod_read in a variable
  readFile1 <- shiny::callModule(mod_read, "sidemod1", choice)
  # passing the output of readFile into mod_display module
  displayFile <- shiny::callModule(mod_display, "bodymod2", file = readFile1)
}

shinyApp(ui,server)

[nirgrahamuk]

no, you are giving two conflicting definitions of a reactive. There may be an edge case where that makes sense, but ...
Why not have a second reactive which decides if it wants to take the value of the first reactive or some alternative/transformation of it, based on additional criteria like a dropdown.

 my_react2 <- reactive({
  # browser()
    if(dropDown() == "cyl"){
    my_react() %>% select(dropDown())
    } else{
      my_react()
    }
  })

etc. then display my_react2