Shiny print reactive df to console (glimpse(myreactive_df) for debugging purposes?

Question

I'm trying to debug my Shiny app and would like to view a reactive dataframe with e.g. glimpse(df).

Initially I tried to create a breakpoint and then view the environment by my reactive df is a value not an object when used inside server.r. I also tried browser() but was not sure what it would do.

I did some searching on SO and tried various things using sink(), renderPrint() but had no success.

How can I print the contents of glimpse(some_reactive_df()) to the console when I run my app?

Answer 1

Calling print() from within the reactive({}) expression will do that.

library(shiny)

library(dplyr)

shinyApp(
    ui <- fluidPage(
        selectizeInput("cyl_select", "Choose ya mtcars$cyl ", choices = unique(mtcars$cyl)),

        tableOutput("checker") # a needed output in ui.R, doesn't have to be table
    ),
    server <- function(input, output) {

        d <- reactive({ 
            d <- dplyr::filter(mtcars, cyl %in% input$cyl_select)
            print(glimpse(d)) # print from within
            return(d)
            })

    output$checker <- renderTable({
        glimpse(d()) # something that relies on the reactive, same thing here for simplicty
    })
    })

Assuming you provide Shiny a reason to run (and re-run) your reactive of interest, by having it be involved with a rendering in server() and linked output in ui(). This is usually the case for my debugging scenarios but it won't work unless the reactive is being used elsewhere in app.R.