Default input values inside renderUI cause reactives to execute twice on startup

[jbowers]

When an input value is created using renderUI, the initial reactive graph completes as if the input value is NULL even if a default value is supplied. This results in all outputs dependent on the input value calculating twice at startup. Example below:

library(dplyr)
library(shiny)

ui <- fluidPage(
  uiOutput("iris_filter"),
  dataTableOutput('table')
)

server <- function(input, output, session) {
  
  output$iris_filter <- renderUI({

    selectInput(inputId = "species_filter",
                label = "Species: ",
                choices = iris %>% distinct(as.character(Species)) %>% pull(),
                multiple = T,
                selected = 'setosa')

  })
  
  output$table <- renderDataTable({
    
    iris %>%
      filter(Species %in% input$species_filter)
    
  })
  
}
  
options(shiny.reactlog = T)
shinyApp(ui, server)

Tracing through the reactive graph in this example shows the following sequence:

1. renderUI completes
2. input$species_filter evalutes to NULL
3. output$table renders
4. input$species_filter updates to 'setosa'
5. output$table renders

Our goal is to prevent output$table from rendering twice at startup, what is the best way to do this? Is there a way to force renderUI to set the right default value during init?

[nirgrahamuk]

add guards against NULL inputs to cause execution to stop, this is done with req()

 filter(Species %in% req(input$species_filter))

[jbowers]

This does not work because NULL is still a valid value for this input - it is just not the value that should be set at startup.

[nirgrahamuk]

hacky but it works :

library(dplyr)
library(shiny)

ui <- fluidPage(
  uiOutput("iris_filter"),
  dataTableOutput("table")
)

server <- function(input, output, session) {
  run_once <- reactiveVal(FALSE)

  output$iris_filter <- renderUI({
    shiny::onFlushed(fun = \(x){shiny::onFlushed(
      fun = \(x){run_once(TRUE)})})
    selectInput(
      inputId = "species_filter",
      label = "Species: ",
      choices = iris %>% distinct(as.character(Species)) %>% pull(),
      multiple = T,
      selected = "setosa"
    )
  })

  input_species_filter_wrapped <- reactive(
    {
      req(run_once())
   input$species_filter
    })
  
  output$table <- renderDataTable({
    cat("\nhere ", input_species_filter_wrapped())
    iris %>%
      filter(Species %in% input_species_filter_wrapped())
  })
}

options(shiny.reactlog = T)
shinyApp(ui, server)
```