Forcing render order in R Shiny

Question

I'm relatively new to R Shiny and reactive programming. From my understanding (and this tutorial), it seems like you are not supposed to tell Shiny "when" to do things (i.e. to enforce execution order) as it will figure that out itself. However, often I find myself wanting plots or other UI to render sequentially. Is there a good way to do this?

I've made up a minimal example below. I want to render header before plot, as plot requires a time-consuming computation.

library(shiny)

ui <- fluidPage(
    tags$h1("My app"),
    uiOutput("header"),
    plotOutput("plot")
)

server <- function(input, output) {
            output$header <- renderUI({
                tagList(tags$h2("Section header"), 
                        tags$p("Some information relevant to the plot below..."))
            })
            
            output$plot <- renderPlot({
                # hypothetical expensive computation
                Sys.sleep(2)
            
                # hypothetical plot
                hist(rnorm(20))
            })
}

shinyApp(ui = ui, server = server)

Here I could obviously replace uiOutput("header") in ui with its definition in server and that solves the problem; however, in practice I want header to be dynamic. A hacky solution I found was to include a hidden input inside header and then use req() inside plot. This is kind of like adding an action button that automatically clicks upon load.

server <- function(input, output) {
            output$header <- renderUI({
                tagList(tags$h2("Section header"), 
                        tags$p("Some information relevant to the plot below..."),
                        div(style = "display:none", textInput(inputId = "hidden", label = "", value = "x")))
            })
            
            output$plot <- renderPlot({
                req(input$hidden)
                
                ...
            })
}

However, if I want to repeat this in multiple situations or if I want to force a chain of more than two outputs to render sequentially, then this seems tedious. Can anyone suggest a more elegant solution?

question from:https://stackoverflow.com/questions/65839072/forcing-render-order-in-r-shiny

Answer 1

Waitting for answers