Title: How to avoid wrapping everything in observeEvent() when passing URL parameters in shiny?
Tags: r;shiny
Question: I know that using something like this code:
library(shiny)
ui <- fluidPage(
mainPanel(
textOutput(&quot;Query_String&quot;)
server <- function(input, output, session) {
observeEvent(session$clientData$url_search,{
Query <- session$clientData$url_search
output$Query_String <- renderText(Query)
# Long list of operations dependant on the parameters passed in the URL
})
shinyApp(ui = ui, server = server)
Can enable me to read parameters into my shiny app via URL queries. However it seems like I basically have to wrap my whole server content into one big observe() or observeEvent() to create the needed reactive context.
Is there any way to avoid this?
Comment: If you want the application to be reactive, it's unclear how you intent to avoid an observer. You can separate code into the reactive parts and non-reactive parts. It doesn't really matter if you want them to change once or multiple times. If they can changed by the user in some way (via URL) then it needs to be reactive. I'm not sure I see how using an extra function really complicates the code.

Comment: @rbasa Well this is kind of the crux here: I want them to change exactly once, when the app is opened depending on the user ID that is passed via the URL. This is frustrating for me because, apart from the initial url_search, nothing in the pipeline is reactive. It's not an issue of efficiency, since the whole thing will execute only once, but it does seem to unnecessarily complicate the code. Sorry if im being unclear.

Comment: If you want the output of the "long list of operations" to change according to the parameters passed inside the url, yes. To avoid having a long `observe()` put your "long list of operations" inside a function.

Comment: You won't be able to output anything to the user if you don't put it in a reactive context.

Here is the accepted answer: To avoid recalculating everything if just a single item changed, one wants to create multiple separate observes instead of just a big one. Long code should be refactored in functions e.g. process_query keeping the server function small, allowing to read the overall structure. Important intermediate results can be refactored in their own reactive values (here query). output$Query_String doesn't need to be nested in another reactive context.
process_query <- function(url) {
# many steps to process the url
return(url)
server <- function(input, output, session) {
query <- reactive(process_query(session$clientData$url_search))
output$Query_String <- renderText(query())


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