Update variable created by eventReactive in another observeEvent - r

Question

I'm struggling to update a reactive variable, that is created with eventReactive(), in an observeEvent() with new data.
The background is following: I have a data.frame df with some variables (x and y) and number of observations depending on the selected city (created randomly for this example).
x and y are initialized with zeros.
Because I need to further process df, I pass df to city_df in an eventReactive().
So far, so good. Next, I want to add new data to city_df. The computation of this new data is dependent on the "compute" actionButton (input$compute), wherefore I update city_df in an observeEvent(). I manage to read the data stored in city_df, but I am struggling to overwrite its content.
Actually, I am a bit unsure if this is possible at all, but I hope that some of you could give me a hint on how to update the reactive variable city_df with the new data in this observeEvent() and have its output evaluated in the app(?).
library(shiny)
# global variables
cities <- c("Nairobi", "Kansas", "Uppsala", "Sangon", "Auckland", "Temuco")
# ui
ui <- fluidPage(
fluidPage(
fluidRow(
column(2,
selectInput("city", "Select city",
choices = cities,
selected = sample(cities,
size = 1)
),
actionButton("compute",
"Compute")),
column(8,
verbatimTextOutput("the_city"))
))
)
# server
server <- function(input, output, session) {
# create variable
city_df <- eventReactive(input$city, {
len <- round(runif(1, 20, 50), 0)
df <- data.frame(city = rep(input$city, len))
# initialize x and y with zeros
df <- cbind(df,
data.frame(x = rep.int(0, len),
y = rep.int(0, len)))
})
output$the_city <- renderText({
paste(city_df())
})
observeEvent(input$compute, {
# grab data
test <- city_df()
# compute new data
test$x <- runif(dim(test)[1], 11, 12)
test$y <- runif(dim(test)[1], 100, 1000)
# and how to send this values back to city_df?
})
}
# run app
shinyApp(ui, server)
The actual app is far more complex--so forgive me if this MWE app seems a bit overly complicated to achieve this usually simple task (I hope I managed to represent the more complex case in the MWE).
Instead of a data.frame, I am parsing layers of a GeoPackage and append some variables initialized with zeros. The selected layer is displayed in a Leaflet map. On pressing the "compute" button, a function computes new data that I wish to add to the layer to then have it displayed on the map.
The alternative solution I have on mind is to write the new data to the GeoPackage and then, reread the layer. However, I would appreciate if I could avoid this detour as loading the layer takes some time...
Many thanks :)

Answer 1

Rather than using an eventReactive, if you use a proper reactiveVal, then you can change the value whenever you like. Here's what that would look like
server <- function(input, output, session) {
# create variable
city_df <- reactiveVal(NULL)
observeEvent(input$city, {
len <- round(runif(1, 20, 50), 0)
df <- data.frame(city = rep(input$city, len))
# initialize x and y with zeros
df <- cbind(df,
data.frame(x = rep.int(0, len),
y = rep.int(0, len)))
city_df(df)
})
output$the_city <- renderText({
paste(city_df())
})
observeEvent(input$compute, {
# grab data
test <- city_df()
test$x <- runif(dim(test)[1], 11, 12)
test$y <- runif(dim(test)[1], 100, 1000)
city_df(test)
})
}
So calling city_df() get the current value and calling city_df(newval) updates the variable with a new value. We just swap out the eventReactive with observeEvent and do the updating ourselves.