ShinyApp Function not returning Result - r

Question

I need help with the below shiny app server function. My problem is values$npv always comes out null, not even with a 0. and I think the fun function is not doing the right thing and i'm out of ideas.
If I hard-code the renderText with paste("Net Present Value:", isolate(input$val_inv)) i always have a result but not what i want and this makes me guess the fun function is not working as it should.
inline_numericInput=function(ni){
tags$div( class="form-inline",ni)
}
ui <- shinyUI(fluidPage(
tags$head(tags$style("#side_panel{
padding-left:10px;
}
.form-group {
margin-bottom: 15px !important;
}
.form-inline .form-control {
width:80%;
}
label{ width:30px;}
")),
titlePanel("Example"),
sidebarLayout(
sidebarPanel(width = 4,id="side_panel",
fluidRow(
column(6, inline_numericInput(numericInput("val_inv", label = "Inv:", value = 0))),
),
fluidRow(
column(6, inline_numericInput(numericInput("val_r", label = "R:", value = 0))),
),
fluidRow(
column(6, inline_numericInput(numericInput("val_n", label = "N:", min = 50,value = 50))),
column(6, inline_numericInput(actionButton("btn_calcnpv", label = "Compute NPV")))
)
),
mainPanel(
p('Results:'),
textOutput("val_npv")
)
)
))
server <- function(input, output) {
values <- reactiveValues()
values$npv <- 0
observe({
input$btn_calcnpv
fun <- function(n){
cf <- 0
for (i in 1:n){
cf <- cf + isolate(input$val_inv)/(1+input$var_r)**i
}
cf
}
values$npv <- fun(isolate(input$val_n))- isolate(input$val_inv)
#values$npv <- values$npv - isolate(input$val_inv)
})
output$val_npv <- renderText({
if(input$btn_calcnpv)
paste("Net Present Value:", values$npv)
else ""
})
}
shinyApp(ui, server)

Answer 1

Here is an answer using eventReactive and not so many isolations.
Furthermore, the inputs are coerced to numbers before the calculation takes place.
Using eventReactive, the calculation is started by pressing the compute-button.
server <- function(input, output) {
npv <- eventReactive(input$btn_calcnpv, {
val_inv <- as.numeric(input$val_inv)
val_r <- as.numeric(input$val_r)
val_n <- as.numeric(input$val_n)
fun <- function(n){
cf <- 0
for (i in 1:n){
cf <- cf + val_inv/(1+val_r)**i
}
cf
}
temp <- fun(val_n)- val_inv
temp
})
output$val_npv <- renderText({
req(npv())
paste("Net Present Value:", npv())
})
}