Startup warning with reactive input in shiny module - r

Question

I am currently modularizing a Shiny app in different modules following the {golem} framework. For simplicity, let's say I have 3 main shiny modules:
mod_faith_plot: generates a scatterplot of a given dataset (I'll use faitfhul).
mod_points_select: decouples a dropdown menu to select how many points will be plotted. UI inputs have this dedicated module as I wanted to place the selector in the sidebarPanel instead of mainPanel (alongside the plot).
mod_data: provides a reactive dataframe depending on the n_points argument.
This modules talk to each other in the server function.
Now, when I start my app with a simple head(., n_points()) in mod_data I get the following warning:
Warning: Error in checkHT: invalid 'n' - must contain at least one non-missing element, got none.
The input in mod_points_select is clearly NULL before the selected_points argument gets assigned, is there a less hacky and more elegant way to avoid the warning at startup than my if condition?
library(shiny)
library(dplyr)
library(ggplot2)
# [Module] Plot faithful data -------------------------------------------------------
mod_faith_plot_ui <- function(id){
ns <- NS(id)
tagList(
plotOutput(ns("faith_plot"))
)
}
mod_faith_plot_server <- function(input, output, session, data){
ns <- session$ns
output$faith_plot <- renderPlot({
data() %>%
ggplot(aes(eruptions, waiting)) +
geom_point()
})
}
# [Module] Module for n_points dropdown ---------------------------------------------
mod_points_select_ui <- function(id){
ns <- NS(id)
uiOutput(ns("select_points"))
}
mod_points_select_server <- function(input, output, session){
ns <- session$ns
output$select_points <- renderUI({
selectInput(
ns("n_points"),
label = "Select how many points",
choices = seq(0, 200, by = 10),
selected = 50
)
})
reactive({input$n_points})
}
# [Module] Get filtered data -----------------------------------------------------------------
mod_data_server <- function(input, output, session, n_points){
ns <- session$ns
data <- reactive({
faithful %>%
# If condition used to avoid warnings at startup - switch lines to get warning
# head(., n_points())
head(., if(is.null(n_points())) { TRUE } else {n_points()})
})
}
ui <- fluidPage(
sidebarLayout(
sidebarPanel(
mod_points_select_ui(id = "selected_points")
),
mainPanel(
tabsetPanel(type = "tabs",
tabPanel("plot", mod_faith_plot_ui(id = "faith_plot"))
)
)
)
)
server <- function(input, output, session) {
data <- callModule(mod_data_server, id = "data", n_points = selected_points)
selected_points <- callModule(mod_points_select_server, id = "selected_points")
callModule(mod_faith_plot_server, id = "faith_plot", data = data)
}
shinyApp(ui, server)

Answer 1

You can use req() to ensure values are available:
data <- reactive({
req(n_points())
faithful %>%
head(., n_points())
})
When values are not available the call is silently canceled