Rshiny unexpected behaviour with images

Question

I am quite a newbie to Shiny apps but I managed to build an interface that allows me to choose from a list of image urls based on the images that can be found at the url. The app works fine when I do Run App on RStudio but as soon as I try to call it from an external function, the images are no longer shown (only thumbnails). Since the script is quite long, I managed after a bit of debbugging to recreate the effect on a MRE that is based on the shiny project example.

library(shiny)

app_ui <- fluidPage(
    
    # Application title
    titlePanel("Old Faithful Geyser Data"),
    
    # Sidebar with a slider input for number of bins and an image
    sidebarLayout(
        sidebarPanel(
            sliderInput("bins", "Number of bins:", min = 1, max = 50, value = 30),
            # Inserting the image
            img(src = "/prova/1.jpg", height = "300px", width = "100%")
        ),
        
        # Show a plot of the generated distribution
        mainPanel(
            plotOutput("distPlot")
        )
    )
)

# Define server logic required to draw a histogram
app_server <- function(input, output) {
    
    output$distPlot <- renderPlot({
        # generate bins based on input$bins from ui.R
        x    <- faithful[, 2]
        bins <- seq(min(x), max(x), length.out = input$bins + 1)
        
        # draw the histogram with the specified number of bins
        hist(x, breaks = bins, col = 'darkgray', border = 'white',
             xlab = 'Waiting time to next eruption (in mins)',
             main = 'Histogram of waiting times')
    })
}

# Run the application 
app <- shinyApp(ui = app_ui, server = app_server)

If I then have a different R script (external.R) where I call the app contained in file MainApp.R like so, the image breaks down.

source("path_to_app/MainApp.R")
app

In fact I noticed an even stranger behaviour. If I first Run App the app, the images are displayed correctly also when sourcing from an external file. If I clean the environment and try to source directly, the images are not shown. It seems almost like the images are cached somewhere.

My folder structure is

Shinyapp
   |- MainApp.R
   |- external.R
   |- www
    | |- prova
      | |- 1.jpg

Answer 1

Your folder structure and request path is correct. As the shiny docs note:

Static files under the www/ directory are automatically made available under a request path that begins with /

The odd thing is that the image is displayed at all when you source external.R. I suspect it might be some RStudio magic, and if you open the URL in a browser you will not see the image.

In any case, the issue is that you're using shiny::shinyApp(), which creates an app from a server and ui R object.

However, you need the context of the directory and subdirectory, so you want to use shiny::runApp(), to pass the current folder.

shiny::runApp() expects either a ui.R and server.R, or an app.R. So in your case, rename mainApp.R to app.R, and then the contents of external.R can simply be:

shiny::runApp()