renderImage issue on shiny r

Question

I have an issue about this code.

observeEvent(input$next, {
output$mark1 <- renderImage({
      return(NULL)
    }, deleteFile = FALSE)
})

This code is from the server file on shiny. And the UI file, the code is

imageOutput(
       outputId = 'mark1',
       inline = TRUE
     )

My expectation is, if I click 'next', then the image 'mark1' should disappear.

But I got an error message saying:

Warning: Error in basename: a character vector argument expected

score 3 · Answer 1 · answered Jul 21 '20 at 12:01

Welcome to SO! Please provide a reprex the next time - this will help to get help.

renderImage needs to return a list with at least a src slot. From the help of renderImage

The expression expr must return a list containing the attributes for the img object on the client web page. For the image to display, properly, the list must have at least one entry, src, which is the path to the image file.

So you have to find a way how to tell shiny not to render the image at all when your button is pressed. A neat way to do so is to use req like in the example below. The button render shows the pic and whenever you press delete it goes away. This is due to the fact that req silently raises an error, which tells shiny not to proceed in the observer/render* function and thus not to show the picture in the first place.

library(shiny)

ui <- fluidPage(imageOutput("pic"), 
                actionButton("go", "Render Print"), 
                actionButton("delete", "Remove Pic"))

server <- function(input, output, session) {
   toggle_pic <- reactiveVal(FALSE)

   observeEvent(input$go, toggle_pic(TRUE), ignoreInit = TRUE)
   observeEvent(input$delete, toggle_pic(FALSE), ignoreInit = TRUE)

   output$pic <- renderImage({
      req(toggle_pic())
      outfile <- tempfile(fileext = ".png")
      
      # Generate a png
      png(outfile, width=400, height=400)
      hist(rnorm(100))
      dev.off()
      
      # Return a list
      list(src = outfile,
           alt = "This is alternate text")
   }, deleteFile = TRUE)
}

shinyApp(ui, server)

renderImage issue on shiny r

1 Answers1