A quick demo on how to render dynamic user interface in Shiny applications. In this case, a new text output element is shown when user enters a new line in the text input. The color toggles alternatively as well.

Techniques here involves using shiny::renderUI and R’s metaprogramming.

library(shiny)
library(purrr)
library(stringr)

## Demo: Render UI Function ----
shinyApp(
    
    ui <- fluidPage(
    
        # user's input 
        column(
            width = 11,
            offset = 1,
            textAreaInput(
                "text", label = "",
                placeholder = "Please insert name here. Use new line to break.",
                width = "1200px", height = "250px"
            )
        ),
        # dynamic print output here
        column(width = 7, offset = 1, uiOutput("renderNames")),
        column(width = 7, offset = 1, uiOutput("renderIDs"))
    ),

    server <- function(input, output, session) {
        
        # helper function
        split_text <- function(x) {
            
            # split text and remove empty 
            str_split(x, "\n")[[1]] %>% 
                str_remove("\n") %>% 
                keep(~ nchar(.x) > 1) %>% 
                unlist()
        }
    
        output$renderNames <- renderUI({
            
            # count number of lines
            n <- length(split_text(input$text))
            
            # render input according to n
            rn <- map_chr(1:n, ~ str_glue("verbatimTextOutput(outputId = 'id{.x}', placeholder = TRUE)"))
            map(rn, ~ eval(parse(text = .x)))
            
        })
        
        output$renderIDs <- renderUI({
            
            n <- length(split_text(input$text))
            
            # toggle color here
            m <- ifelse(1:n %% 2 == 0, "dodgerblue", "purple")
            
            # render color to respective id 
            rb <- map2(1:n, m, ~ paste0("tags$head(tags$style(HTML (' #id", .x, "{ color: ", .y, "}')))"))
            map(rb, ~ eval(parse(text = .x)))
            
        })
        
        observeEvent(input$text, {
            
            # return splitted texts
            txt <- split_text(input$text)
            
            # render output according to n
            res <- map2(1:length(txt), txt, ~ paste0("output$id", .x,  "<- renderText({ 'hello ", .y, "' })"))
            map(res, ~ eval(parse(text = .x)))
        })
      
    }
    
)