testing answer to stackexchange question

app2.R

# Test answer to https://stackoverflow.com/questions/68473292/prevent-double-execution-of-shiny-reactive-with-two-dependency-pathways/68475515#68475515
library(shiny)
library(digest)

ui <- fluidPage(
  
  sidebarLayout(
    sidebarPanel(
      textInput("M", "M:", ""), 
      uiOutput("S_UI"),
      selectInput("A", "A:", c("A1","A2"))
    ),
    
    mainPanel(
      verbatimTextOutput("C")
    )
  )
)

server <- function(input, output, session) {
  
  # Count calculations of C
  count = 0
  
  # This value just exists to tell us whether M has changed
  # from its default of "". Doesn't need to be reactive
  changedM = FALSE
  
  S = reactive({
    input$S
  })
  
  # Create "content" from A and S
  C <- reactive({
    req(S())
    count <<- count + 1
    Sys.sleep(0.5)
    message("Count ", count)
    paste(
      "Count: ", count, "\n", digest::sha1( c(input$A, S() ))
    )
  })
  
  output$S_UI <- renderUI({
    val <- digest::sha1(c(input$M, input$A))
    curr_val <- isolate(input$S)
    changedM <<- changedM || input$M != ""
    
    if(is.null(curr_val) || (changedM & input$M == "")){
      val <- "testS"
    } else if (input$M == "" & !changedM) {
      val <- curr_val
    } else {
      freezeReactiveValue(input, "S")
    }
    
    return(textInput("S", "S:", val))
  })
  
  # "Content"
  output$C <- renderText({
    C()
  })
}

shinyApp(ui = ui, server = server)