R/Shiny: can changing a value of a reactive trigger an event?

Question

Is it possible to trigger an event when a reactive changes value?

If you put an observeEvent on an input, changes in the input's value will trigger the event:

observeEvent(input$val1,{cat("triggered when input$val1 changes)")})

But if you put an observeEvent on a reactive, the event is only triggered during initialization:

  reactiveSum <- reactive({
    as.integer(input$val1) + as.integer(input$val2) + as.integer(input$val3)
  })
  observeEvent(reactiveSum,{cat("only triggers during initialization")})

Why do changes in the value of an input trigger an event, but changes in the value of a reactive won't?

Is there a way to trigger an event when the value of a reactive changes?

Answer 1

At the request of @starja and @Eric...

A reactive is simply a function

> shiny::reactive
function (x, env = parent.frame(), quoted = FALSE, ..., label = NULL, 
    domain = getDefaultReactiveDomain(), ..stacktraceon = TRUE) 
{
    check_dots_empty()
    x <- get_quosure(x, env, quoted)
    fun <- as_function(x)
    formals(fun) <- list()
    label <- exprToLabel(get_expr(x), "reactive", label)
    o <- Observable$new(fun, label, domain, ..stacktraceon = ..stacktraceon)
    structure(o$getValue, observable = o, cacheHint = list(userExpr = zap_srcref(get_expr(x))), 
        class = c("reactiveExpr", "reactive", "function"))
}
<bytecode: 0x7fdbfa6cb8d8>
<environment: namespace:shiny>

(as are observeEvent and the like). If you want observeEvent (or similar) to react to a change, you need to pass it something that changes. myReactive is simply the function itself, which does not change. myReactive() is the current value of the function, which does. So

observeEvent(myReactive(), { ... })

works, but

observeEvent(myReactive, { ... })

does not.

The only time (that I can think of) where you would use the reactive rather than its value is when passing additional parameters to a module server function: you pass the reactive (myReactive) as a parameter and call it (myReactive()) in the body of the server.