Feature request: add `ignoreIdentical` param to `observeEvent()`

[daattali]

I often use observeEvent(x, ...) when I want to tell shiny that a reactive code block should be executed whenever a specific value x changes. Usually observeEvent() does exactly what I want it to do, but because it gets fired when x gets invalidated, then it can also fire when x hasn't changed at all. I realize this is the correct behaviour based on how reactivity works, but in my opinion it's easier for people to think of it as only firing whenever the value changes rather than gets invalidated. So I was wondering if it'd be possible to add a parameter to observeEvent() that will do that?

Example:

library(shiny)

ui <- fluidPage(
  actionButton("btn", "Button")
)

server <- function(input, output, session) {
  num5 <- reactive({
    input$btn
    5
  })
  observeEvent(num5(), {
    cat("click\n")
  })
}

shinyApp(ui = ui, server = server)

It wouldn't be too far-fetched to think that the observer would only get run when num5() changes. It's wrong, but it can be useful to want to do that.

I realize this may be a strange request that doesn't really fit the reactivity model, and fully accept any incoming judgment if this is a dumb idea

[bborgesr]

Hmm, this seems like a reasonable request IMO. Thoughts, @wch, @jcheng5?

[jcheng5]

In the past, I've thought of this as a property of the reactive expression (does it invalidate its dependents if its new value is the same as the old?), not the observer. (This is actually how reactive expressions originally worked in Shiny, but I soon concluded that laziness was more frequently beneficial than this "de-dupe" behavior.)

You can achieve this yourself with a higher order reactive.

dedupe <- function(r) {
  makeReactiveBinding("val")
  observe(val <<- r(), priority = 10)
  reactive(val)
}

num5 <- dedupe(reactive({
  input$btn
  5
}))

Seems like we might want to start corralling some of these types of things into a shiny reactive extras package (or contribute to Hadley's shinySignals package)...

[daattali]

Your beautifully elegant solutions for these things always amaze me. I agree it'd be nice to have a place that aggregates a lot of these more complex constructs that achieve commonly needed results in shiny. I remember the reactiveTrigger you created for @gaborcsardi was also something I'd love to bookmark. This was sort of the idea behind my repo advanced-shiny...

[alandipert]

Closing because a workaround was accepted.

[sanjmeh]

I thought I understood shiny reactive exressions quite well till I saw this Joe Cheng's master piece snippet in response to @daattali 's equally valuable question.

dedupe <- function(r) {
  makeReactiveBinding("val")
  observe(val <<- r(), priority = 10)
  reactive(val)
}

num5 <- dedupe(reactive({
  input$btn
  5
}))

I am not sure how this works. How is the invalidation happening here? How does the assignment operator help?
@daattali : could you please please document this on your excellent advanced shiny website?

[daattali]

@sanjmeh it's documented https://deanattali.com/blog/advanced-shiny-tips/#reactive-dedupe