Shiny: How to prevent input duplicates when rendering outputs? - r

Question

In the following example, duplicate input elements seem to be created, although output$app is re-rendered with every login/logout.
library(shiny)
user <- reactiveValues(logged_on = 0)
ui <- fluidPage(
uiOutput("app")
)
server <- function(input, output) {
observe({
if (user$logged_on == 1) {
output$app <- renderUI({
list(
uiOutput("container1"),
uiOutput("container2")
)
})
output$container1 <- renderUI({
actionButton("logout", "Logout")
})
output$container2 <- renderUI({
actionButton("clickme", "Click Me")
})
observeEvent(input$clickme, {
print("got clicked!")
})
observeEvent(input$logout, ignoreInit = TRUE, {
user$logged_on <- 0
})
} else {
output$app <- renderUI({
actionButton("login", "Login")
})
observeEvent(input$login, ignoreInit = TRUE, {
user$logged_on <- 1
})
}
})
}
shinyApp(ui, server)
Clicking Login followed by Click Me prompts got clicked! once. However, after repeating the process by Logout>Login>Click Me prompts the message twice, and so on.
Examining this problem using...
x <- reactiveValuesToList(input)
print(x)
...confirms that duplicates of the input elements are created. Why does this happen and how do i prevent it?

Answer 1

I think it's because you have observeEvents within your observe. This isn't necessary and I've edited your code and now the duplicates seem to have been removed. (Also the ignoreInits were unnecessary here.)
library(shiny)
user <- reactiveValues(logged_on = 0)
ui <- fluidPage(
uiOutput("app")
)
server <- function(input, output) {
observe({
if (user$logged_on == 1) {
output$app <- renderUI({
list(
uiOutput("container1"),
uiOutput("container2")
)
})
output$container1 <- renderUI({
actionButton("logout", "Logout")
})
output$container2 <- renderUI({
actionButton("clickme", "Click Me")
})
} else {
output$app <- renderUI({
actionButton("login", "Login")
})
}
})
observeEvent(input$clickme, {
print("got clicked!")
})
observeEvent(input$logout, {
user$logged_on <- 0
})
observeEvent(input$login, {
user$logged_on <- 1
})
}
shinyApp(ui, server)

Answer 2

So I've found that nesting observeEvent() within each other, registers new observers and that caused the problem. A better example than my previous one is:
library(shiny)
ui <- fluidPage(
actionButton("clicks", "Click"),
actionButton("more_clicks", "Print")
)
server <- function(input, output) {
observeEvent(input$clicks,{
observeEvent(input$more_clicks, {
print("Hello") # you can see these piling up in the console when switching buttons
})
})
}
shinyApp(ui, server)
My guess is that this should be avoided when possible, although observeEvent()'s can be nested if the child observer is destroyed after it has gotten triggered. This can be accomplished by the argument observeEvent(eventExpr, handlerExpr, once = TRUE).