Dynamic `updateTabItems()` in Shiny get executed later than it is supposed to - r

Question

In the following example, I use an actionLink() in "Tab1" to switch the display to "Tab2", then to update the text in the textInput() with "iris" and finally to click the button to display the actual data.
For an unknown reason, this switch is executed after the Sys.sleep() which is after. This lead to a non-desired behavior since shinyjs::click() for instance try to trigger a button that does not exist yet and thereby the 'iris' table is not displayed. I would have expected the tab to be switched first, then to get the Sys.sleep().
Can someone explain to me why Shiny is behaving like that and how to perform the expected behavior please?
library(shiny)
library(shinydashboard)
library(shinydashboardPlus) # shinydashboardPlus_0.7.5 // devtools::install_version("shinydashboardPlus", version = "0.7.5", repos = "http://cran.us.r-project.org")
library(shinyjs)
available_data_sets <- c("mtcars", "iris")
ui_welcome <- fluidPage(actionLink(inputId = "switch_tab", label = "Switch tab, update text with 'iris' & click button", icon = icon("hand-point-right")))
ui_search <- fluidPage(
textInput(inputId = "dataset_name", label = "Select data set", value = ""),
actionButton(inputId = "display_tab", label = "Display selected set"),
br(),
tableOutput('table'))
ui <- dashboardPagePlus(
collapse_sidebar = TRUE,
title = "Minimal",
header = dashboardHeaderPlus(
title = "Test",
enable_rightsidebar = FALSE,
rightSidebarIcon = "gears",
fixed = FALSE),
sidebar = dashboardSidebar(
sidebarMenu(
id = "left_sidebar",
menuItem("Tab1", tabName = "tab1", icon = icon("home")),
menuItem("Tab2", tabName = "tab2", icon = icon("search")))
),
body = dashboardBody(
useShinyjs(),
tabItems(tabItem(tabName = "tab1", ui_welcome),
tabItem(tabName = "tab2", ui_search)))
)
server <- function(input, output, session) {
# RENDER DATA
observeEvent(input$display_tab, {
print("Within observeEvent 'display_tab'")
selected <- input$dataset_name
print(paste0("Value of 'input$dataset_name' received: ", selected))
if(!isTruthy(selected)) return(NULL)
if(!selected %in% available_data_sets) return(NULL)
output$table <- renderTable(if(selected == "iris") iris else mtcars)
})
# SWITCH TAB & TRIGGER BTN
observeEvent(input$switch_tab, {
print("Within observeEvent 'switch_tab'")
print("updateTabItems"); updateTabItems(session, inputId = "left_sidebar", selected = "tab2")
print("updateTextInput 1"); updateTextInput(inputId = "dataset_name", value = "")
print("updateTextInput 2"); updateTextInput(inputId = "dataset_name", value = "iris")
print("Sys.sleep"); Sys.sleep(3)
print("shinyjs::click"); shinyjs::click(id = "display_tab")
})
}
shinyApp(ui, server)

Answer 1

Shiny reactivity is based on reaction on single events. You are basically trying to do two actions in reaction to a single event (reconfigure interface then act on it). It won't work as the reconfiguration will actually be done in between reaction code. You are trying to circumvent this by waiting using Sys.sleep function. This will only pause the whole application and it will wake up as if nothing happened. Try to put a longer sleep time and you will notice that the UI will be non responsive.
The solution is to refactor your code to be data centric and not user centric. What you want is to be able to select and display a dataset in two different ways. One "automatically" by clicking on a link the other manually by providing the dataset name and click on a button.
Here I give some flesh to the dataset that will be able to redisplay on change and provide two ways to set it's state.
server <- function(input, output, session) {
# reactive value to store selected dataset
state <- reactiveValues(selected=NULL)
# AUTO RENDER DATA on selected dataset change
output$table <- renderTable({
if(!is.null(state$selected)) {
if(state$selected == "iris") iris else mtcars
}
})
# manual select dataset
# already on tab so no need to switch
observeEvent(input$display_tab, {
if(!isTruthy(input$dataset_name) || !(input$dataset_name %in% available_data_sets)) {
state$selected <- NULL
} else {
state$selected <- input$dataset_name
}
})
# automatic selection through actionlink
# select dataset and switch tab
observeEvent(input$switch_tab, {
state$selected <- "iris"
updateTabItems(session, inputId = "left_sidebar", selected = "tab2")
})
}