Shiny text output to update at the end of SQL query - r

Question

I have a problem that has been discussed in different ways here but apparently not to face my quite simple need.
I have a simple app that makes a call to an SQL database. I use a button to launch the query.
I would simply need a text showing "click on button" to download at the very beginning.
Once a user clicks on the button, I would need this output text to show "Downloading the data, please wait".
Once the query is completed and the data has been fully received, I would need the output text to show "Data downloaded successfully."
I've seen some solutions based off the progress bar but I cannot use it since I'm not going through a data.frame. I query the database and I don't know how long this could take.
I've seen other solutions based off reactive values but the text output in this case should react based on the size of the dataframe (0 rows and button clicked -> still downloading the data; >0 rows and button clicked "data downloaded successfully").
Hence, I'm stuck here.
This is my simple code but that ideally does what I would need.
ui <- fluidPage(
fluidRow(actionButton("download_btn", "Download Data")),
fluidRow(textOutput(outputId = "load_data_status")),
fluidRow(dataTableOutput("output_table"))
)
server <- function(input, output) {
cat("\n output$output_table = \n", output$output_table)
data <- eventReactive(input$download_btn,{
output$load_data_status <- renderText({ "Downloading data from Server. Please wait..." })
# here I actually download the data from a database and this could take several seconds
df <- data.frame(mtcars)
output$load_data_status <- renderText({ "Data downloaded succesfully." })
df
})
output$output_table <- renderDataTable({
data()
})
}
shinyApp(ui, server)

Answer 1

Option A:
Here is a pretty good solution by Dean Attali: https://github.com/daattali/advanced-shiny/tree/master/busy-indicator
Option B:
You can listen to JavaScript events to:
Change text when the button is clicked
Change the text again when the output is rendered
I also added Sys.sleep() to simulate some loading time.
Code:
library(shiny)
ui <- fluidPage(
tags$head(tags$script(HTML('
// 1. Change text to "Downloading..." when button is clicked
$(document).on("shiny:inputchanged", function(event) {
if (event.name === "download_btn") {
$("#download_btn").html("Downloading data from Server. Please wait...");
}
});
//. 2. Change text to "Success" when output table is changed
$(document).on("shiny:value", function(event) {
if (event.name === "output_table") {
$("#download_btn").html("Data downloaded succesfully.");
}
});
'))),
fluidRow(actionButton("download_btn", "Download Data")),
fluidRow(DT::dataTableOutput("output_table"))
)
server <- function(input, output) {
data <- eventReactive(input$download_btn,{
df <- data.frame(mtcars)
Sys.sleep(3) # Simulate some loading time
df
})
output$output_table <- renderDataTable({
data()
})
}
shinyApp(ui, server)
Output: