Simple DT/reactive example

use_of_reactive.R

##### Use of reactive() #####

library(shiny)
library(dplyr)
library(DT)

dt <- data.frame(
  id = c(1, 1, 2),
  text = c("text1", "text2", "text1")
)

ui <- fluidPage(
  selectInput(inputId = "id", label = "Choose id", choices = unique(dt$id)),
  dataTableOutput("table"),
  actionButton(inputId = "add", label = "Add row"),
  textInput(inputId = "text", label = "Updated text", value = "")
)

server <- function(input, output, session) {

  df <- reactive({
    dt %>% filter(id == input$id)
  })

  output$table <- renderDataTable({
    datatable(
      data = df(),
      selection = "single",
      options = list(dom = 't')
    )
  })

  observeEvent(input$table_cell_clicked, {
    tab <- df() %>%
      slice(input$table_rows_selected)

    updateTextInput(session = session, inputId = "text", value = tab$text)
  })

  observeEvent(input$add, {
    old <- df()
    new <- data.frame(
      id = unique(old$id),
      text = paste0("text", nrow(old) + 1L)
    )

    output$table <- renderDataTable({
      datatable(
        data = rbind(new, old),
        selection = "single",
        options = list(dom = 't')
      )
    })

  })
}

shinyApp(ui, server)