Chapter 18.3 Suggestion for additional note to explain concept

[jhk0530]

In internal functions.

There's example code with separate shiny internal function.

before

server <- function(input, output, session) {
  switch_page <- function(i) {
    updateTabsetPanel(input = "wizard", selected = paste0("page_", i))
  }
  
  observeEvent(input$page_12, switch_page(2))
  observeEvent(input$page_21, switch_page(1))
  observeEvent(input$page_23, switch_page(3))
  observeEvent(input$page_32, switch_page(2))
}

after

switch_page <- function(i) {
  updateTabsetPanel(input = "wizard", selected = paste0("page_", i))
}

server <- function(input, output, session) {
  observeEvent(input$page_12, switch_page(2))
  observeEvent(input$page_21, switch_page(1))
  observeEvent(input$page_23, switch_page(3))
  observeEvent(input$page_32, switch_page(2))
}

Changed: switch_page function "moved out" of server function.

And there's explanation like below

"We could of course add session to the arguments of the function:"

However, the code is not much different as explained. (session is not mentioned in code)
Since shiny's updated version (updateTabesetPanel()) use session with getDefaultReactiveDomain() as default parameter.

So I suggest to mention like below and why session parameter didn't used to avoid confusion.

---

We could of course add session to the arguments of the function:

switch_page <- function(session, i){
  updateTabsetPanel(session, input = "wizard", selected = paste0("page_", i))
}

server <- function(input, output, session) {
  observeEvent(input$page_12, switch_page(session, 2))
  observeEvent(input$page_21, switch_page(session, 1))
  observeEvent(input$page_23, switch_page(session, 3))
  observeEvent(input$page_32, switch_page(session, 2))
}

However, since shiny > 1.6 uses update- function (like updateTabsetPanel) uses session as default pameter,
we can skip the session like below

switch_page <- function(i) {
  updateTabsetPanel(input = "wizard", selected = paste0("page_", i))
}

server <- function(input, output, session) {
  observeEvent(input$page_12, switch_page(2))
  observeEvent(input$page_21, switch_page(1))
  observeEvent(input$page_23, switch_page(3))
  observeEvent(input$page_32, switch_page(2))
}

But this feels weird as the function is still fundamentally coupled to this app because it only affects a control named “wizard” with a very specific set of tabs.

---

Thanks.

[jhk0530]

I found simillar issue #431 .