Rendering dynamically loaded UI in a loop only shows last value - r

Question

I have a Shiny app which dynamically loads any number of outputPlot UIs. In the following example I am simply iteration over the first three letters of the alphabet.
To render the plot in the dynamically loaded UIs, I call renderPlot in a loop as such:
for (a in LETTERS[1:3]) {
output[[paste0('p',a)]] <- renderPlot(plot.df(df, a))
}
but the result is that all three outputPlots (pA, pB and pC) are all rendered with plot.df(df, 'C')). Seems to me renderPlot is rendered after the loop has completed and a = 'C'. Instead, the output UIs pA, pB and pC should have been rendered with plot.df(df, 'A'), plot.df(df, 'B') and plot.df(df, 'C'), respectively. But this is clearly not the case when viewing the output.
I have previous had success with this, if the output UI was a module and in the loop calling callModule, which somehow forced the evaluation of the arguments. But I for now am trying to avoid making a separate module for my output UI.
Full reproducible example
library(shiny)
library(dplyr)
# Define UI for application that draws a histogram
ui <- fluidPage(
plotOutput('pltA'),plotOutput('pltB'),plotOutput('pltC'),
tags$hr(),
tags$div(id='placeholder')
)
col <- c(A='#66bd63', B='#fdae61', C='#74add1')
plot.df <- function(df, a) {
#browser()
df <- filter(df, letter==a)
if (nrow(df) == 0) return()
plot(df$i, df$y, type='p', col=col[a], pch=19, main=a)
}
# Define server logic required to draw a histogram
server <- function(input, output, session) {
my_data <- reactiveVal(data.frame())
autoInvalidate <- reactiveTimer(2000)
# Generate some random data and assign to one of three different letters:
observe({
autoInvalidate()
a <- sample(LETTERS[1:3], 1)
data.frame(y=rnorm(5), letter=a) %>%
bind_rows(isolate(my_data())) %>%
group_by(letter) %>%
mutate(i=seq_along(y)) %>%
my_data
})
# Proof of function making a plot.
output$pltA <- renderPlot(plot.df(my_data(), 'A'))
output$pltB <- renderPlot(plot.df(my_data(), 'B'))
output$pltC <- renderPlot(plot.df(my_data(), 'C'))
# Dynamically load output UIs.
observe({
let <- unique(my_data()$letter)
if (is.null(let)) return()
for (l in let) {
if (is.null(session$clientData[[paste0('output_p',l,'_hidden')]])) {
insertUI('#placeholder', 'beforeEnd', ui=plotOutput(paste0('p',l)))
}
}
})
# Update dynamically loaded plots
observe({
df <- my_data()
if (nrow(df) == 0) return()
for (a in LETTERS[1:3]) {
cat('Updating ', a, '\n')
output[[paste0('p',a)]] <- renderPlot(plot.df(df, a))
}
})
}
# Run the application
shinyApp(ui = ui, server = server)

Answer 1

You have to use local (see here).
for (a in LETTERS[1:3]) {
local({
aa <- a
output[[paste0('p',aa)]] <- renderPlot(plot.df(df, aa))
})
}