global.R

if(!require(shiny)) install.packages("shiny")
if(!require(shinythemes)) install.packages("shinythemes")

library(shiny)
library(shinythemes)

source("ui.R")
source("server.R")
shinyApp(ui, server)

server.R

is_logged <- FALSE;
auth_username <- 'username'
auth_password <- 'password'

server <- function(input, output, session) {
  ## check if user is logged in
  auth <- reactiveValues(is_logged = is_logged)
  
  observe({ 
    if (auth$is_logged == FALSE) {
      if (!is.null(input$login)) {
        if (input$login > 0) {
          username <- isolate(input$username)
          password <- isolate(input$password)
          is_same_username <- which(auth_username == username)
          is_same_password <- which(auth_password == password)
          if (length(is_same_username) > 0 & length(is_same_password) > 0) {
            auth$is_logged <- TRUE
          }
        } 
      }
    }    
  })
  
  ## render log-in or normal UI
  observe({
    if(!auth$is_logged) {
      output$page <- renderUI({
        div(class="outer", fluidPage(ui_login(), title = "Cadreon Dashboard - login", theme = shinytheme("cerulean")))
      })
    } else {
      output$page <- renderUI({
        div(class="outer", fluidPage(ui_main(), title = "Cadreon Dashboard - main", theme = shinytheme("cerulean")))
      })
    }
  })
}

ui.R

ui_login <- function() {
  fluidRow(
    tags$head(tags$style(HTML(".container-fluid {margin: 25px;}"))),
    column(2, offset = 5,
           div(id = "login_div",
               wellPanel(
                 h4("LOGIN"),
                 textInput("username", "User name"),
                 passwordInput("password", "Password"),
                 br(),
                 actionButton("login", "Log in", icon = icon("user"))
               )
           )
    )
  )
}

ui_main <- function() {
  fluidRow(
    column(2, offset = 5,
           wellPanel(h1("Hello!"))
    )
  )
}

ui <- (htmlOutput("page"))