Displaying an updated variable after clicking an action button in Shiny - r

Question

I'm new to shiny and trying to accomplish rather a simple task using an action button:
User clicks a button and a function is called
This function does some calculations using input variables and updates/creates several global variables (reactiveValues, probably inside an observe block?)
I'd like to display those values back on the UI (using render* function)
Whenever user changes input values, the UI is automatically updated
Relevant code bits are:
server.R
...
rv <- reactiveValues()
observe({
if(input$run){
rv$a <- someFunc(input$aa)
}
})
output$msg = renderText({ rv$a })
...
ui.R
...
selectInput("aa", ...)
...
actionButton("run", "Run")
...
textOutput("msg")
How can I change msg based on the input aa each time user clicks the button?

Answer 1

I am not convinced I understood what you want, but I imagine it to be something like this:
library(shiny)
u <- fluidPage(
titlePanel("Simple Selectable Reactive Function"),
sidebarLayout(
sidebarPanel(
sliderInput("vv", "Choose a value",min=-3.14,max=3.14,value=0),
selectInput("aa", "Choose a function", choices=c("sin","cos","exp")),
actionButton("run", "Change Function and Run")
),
mainPanel(
h2("Results"),
verbatimTextOutput("msg")
)))
s <- function(input,output){
rv <- reactiveValues(func=NULL)
observeEvent(input$run,{ rv$func <- input$aa })
funcval <- reactive({
v <- 0
if (rv$func=="sin") v <- sin(input$vv)
if (rv$func=="cos") v <- cos(input$vv)
if (rv$func=="exp") v <- exp(input$vv)
v
})
output$msg = renderPrint({
if (is.null(rv$func)) return("not running")
fv <- funcval()
sprintf("%s(%.3f)=%.3f",rv$func,input$vv,fv)
})
}
shinyApp(ui=u,server=s)
Yielding this:
Note that the slider input value formats its current value rather badly when the min and max values are not even. Not sure what one can do about this.