Render R Shiny output on start-up, prior to observeEvent acting

Question

I have plotOutput cyl in my Shiny app. It relies on observeEvent and selectizeInput to display data corresponding to whatever cyl in mtcars the user selects.

I am trying to render a plot on start-up, however, that displays data from all cyl, prior to user selection. In my attempt, I assumed that I could render the initial plot first, and then call observeEvent. This does not work. What am I doing wrong?

########
### ui
########

ui <- shiny::fluidPage(
  shiny::titlePanel("dummy app"),
  shiny::sidebarLayout(
    shiny::sidebarPanel(
      shiny::selectizeInput("cyl", "Cyl",
                            choices = c("Pick one" = "",
                                        unique(datasets::mtcars$cyl)))
    ),
    shiny::mainPanel(
      shiny::plotOutput(outputId = "cyl")
    )
  )
)

########
### server
########
server <- function(input, output, session) {

  ### make start-up plot prior to observeEvent
  ### doesn't do anything
  output$cyl <- shiny::renderPlot({
    ggplot2::ggplot(datasets::mtcars, ggplot2::aes(x = disp, y = hp)) +
      ggplot2::geom_point()
  })

  ### overwrite plot pending selectInput
  shiny::observeEvent(input$cyl, {
    output$cyl <- shiny::renderPlot({
      data <- datasets::mtcars[datasets::mtcars$cyl == input$cyl, ]
      ggplot2::ggplot(data, ggplot2::aes(x = disp, y = hp)) +
        ggplot2::geom_point()
    })
  })
}

# Run the application
shinyApp(ui = ui, server = server)

Answer 1

Use this:

########
### ui
########
library(shiny)
ui <- shiny::fluidPage(
    shiny::titlePanel("dummy app"),
    shiny::sidebarLayout(
        shiny::sidebarPanel(
            shiny::selectizeInput("cyl", "Cyl",
                                  choices = c("Pick one" = "",
                                              unique(datasets::mtcars$cyl)))
        ),
        shiny::mainPanel(
            shiny::plotOutput(outputId = "cyl")
        )
    )
)

########
### server
########
server <- function(input, output, session) {
    
    ### make start-up plot prior to observeEvent
    # ### doesn't do anything
    mydata <- reactive({
        if(input$cyl == "") return(mtcars)
        mtcars[mtcars$cyl == input$cyl, ]
    })
    
    output$cyl <- shiny::renderPlot({
        ggplot2::ggplot(mydata(), ggplot2::aes(x = disp, y = hp)) +
            ggplot2::geom_point()
    })
}

# Run the application
shinyApp(ui = ui, server = server)

short and clean.