How to provide metadata values from server to ui method in shiny - shiny

Question

I'm very new to shiny and butting against something there must be a pattern for, but have had no luck Googling.
I have a simple shiny file (app.R below) that returns a plot for n observations. In addition to making the plot available to the ui method, I'd like to pass it n to help it size the plot to my liking
library(shiny)
server <- function(input, output) {
output$distPlot <- renderPlot({
firstNames <- c("Bob", "Jane", "Bob")
lastNames <- c("Builder", "Gorillas", "Weave")
ages <- c(25, 26, 27)
df <- data.frame(firstNames, lastNames, ages)
# I want to pass this to `ui`
numFirstNames <- length(unique(df$firstNames))
mapping <- aes(x = firstNames, y = ages)
ggplot(df, mapping) + geom_violin() + coord_flip()
})
}
getHeightInPx <- function(numFirstNames) {
paste(100 * numFirstNames, "px", sep="")
}
ui <- fluidPage(
# I want height to be a function of numFirstNames as calculated in the server definition
mainPanel(plotOutput("distPlot", height = getHeightInPx(2)))
)
shinyApp(ui, server)
Since ui isn't a function and doesn't have direct access to output (to me it's getting to distPlot by framework magic) how can I get to data prepared in server to help layout the page?
Thanks

Answer 1

https://github.com/rstudio/shiny/issues/650 tipped me off that you could use the height parameter in renderPlot. So I combined that with some use of reactive and observe to call a function that returns the plot AND gives me the number of elements to use.
I hardly get reactive and observe, so while it seems to work I would not be surprised to hear that I am abusing them or that there's an easier way.
library(shiny)
renderDistPlot <- function(input) {
firstNames <- c("Bob", "Jane", "Bob", "Carol")
lastNames <- c("Builder", "Gorillas", "Weave", "Xmasing")
ages <- c(25, 26, 27, 23)
df <- data.frame(firstNames, lastNames, ages)
# I want to pass this to `ui`
numFirstNames <- length(unique(df$firstNames))
mapping <- aes(x = firstNames, y = ages)
plot <- ggplot(df, mapping) + geom_violin() + coord_flip()
list(Plot = plot, NumFirstNames = numFirstNames)
}
server <- function(input, output) {
renderDistPloatResult <- reactive(renderDistPlot(input))
observe(output$distPlot <- renderPlot(renderDistPloatResult()$Plot, height = renderDistPloatResult()$NumFirstNames * 100))
}
ui <- fluidPage(
# I want height to be a function of numFirstNames as calculated in the server definition
mainPanel(plotOutput("distPlot"))
)
shinyApp(ui, server)